avid understanding hd guide

Your comprehensive guide to High Definition on a budget

Part One

Understanding HD

Video formatsand sampling

Chapter 1

11Understanding HD with Avid

Video formats and sampling 2Understanding HD with Avid

4:1:1, used in some DV formats and DVCAM, makes Cr andCb samples at every fourth Y sample point on every line –but still carries more chrominance detail than PAL or NTSC.

4:2:2 sampling of luminance and colour difference signals

Perhaps one of the most baffling areas of HDand SD, is the shorthand jargon used todescribe sampling and colour space, such asRGB 4:4:4, and Y,Cr,Cb 4:2:2. Also the videoformats such as 1080/24P sound strange untilyou get to know them. For a quick initiation, orreminder, about sampling ratios, please readthe piece directly below.

4:2:2 etc (Chroma sub-sampling)The sampling rates used in digital television are describedby shorthand that has, in some ways, only a tenuousconnection to what it is used to describe. The numbersdenote ratios of sampling rates, not absolute numbers,and they need a little interpretation to understand themall. Sometimes these ratios are referred to as ‘chrominance(chroma) sub-sampling’.

In most instances the first number refers to luminance (Y),the last two refer to chrominance – the exceptions are4:4:4, or 4:4:4:4 (more later). The first number is nearlyalways a 4 and that means that the luminance is sampledonce for every pixel produced in the image. There are avery few instances where a lower sample rate is used forluminance. An example is HDCAM, which is generallyconsidered to use 3:1:1 sampling. Sampling at a lower ratethan the final pixel rate is known as sub-sampling

The second two numbers describe the samplingfrequencies of the two pure colour digitised componentsof (Red-Y) and (Blue-Y), called Cr and Cb. In line withtelevision’s practise of taking advantage of our eye’sresponse which is more acute for luminance than for purecolour, cuts to reduce data tend to be made in thechrominance sampling rather than luminance. The mostcommon studio sampling system is 4:2:2 where each of thetwo colour components is sampled coincidently with everysecond luminance sample along every line.

Line 1 • • • • • • • •Y Y Y Y Y Y Y Y

CrCb CrCb CrCb CrCb

Line 2 • • • • • • • •Y Y Y Y Y Y Y Y

CrCb CrCb CrCb CrCb

4:1:1 sampling

Line 1 Y Y Y Y Y Y Y Y YCrCb CrCb

Line 2 Y Y Y Y Y Y Y Y YCrCb CrCb

4:2:0 provides equal colour resolution verticallyand horizontally if using square pixels

Line 1 Y Y Y Y Y Y Y Y YCr Cr Cr Cr Cr

Line 2 Y Y Y Y Y Y Y Y YCb Cb Cb Cb Cb

Then another argument says that if the chrominance is sub-sampled horizontally, as in 4:1:1, why not do the same verticallyto give a more even distribution of colour information? Soinstead of sampling both Cr and Cb on every line, they aresampled on alternate lines, but more frequently on eachline (at every other Y). This is 4:2:0 sampling (4:2:0 on oneline and 4:0:2 on the next) and it is used in MPEG-2 andmost common JPEG compression schemes.


In many cases it is very useful to have a key (or alpha)signal associated with the pictures. A key is essentially afull image but in luminance only. So then it is logical toadd a fourth number 4, as in 4:2:2:4.

Technically 4:4:4 can denote full sampling of RGB or Y, Cr,Cb component signals – but it is rarely used for the latter.RGB may have an associated key channel, making 4:4:4:4.

Occasionally people go off-menu and do something else –like over-sampling which, with good processing canimprove picture quality. In this case you might seesomething like 8:8:8 mentioned. That would be makingtwo samples per pixel for RGB.

This sampling ratio system is used for both SD and HD.Even though the sampling is generally 5.5 times bigger,4:2:2 sampling is the standard for HD studios.

Why 4?Logic would dictate that the first number, representing a 1:1relationship with the pixels, should be 1 but, for many good(and some not so good) reasons, television standards aresteeped in legacy. Historically, in the early 1970s, the firsttelevision signals to be digitised were coded NTSC andPAL. In both cases it was necessary to lock the samplingfrequency to that of the colour subcarrier (SC), which itselfhas a fixed relationship to line and frame frequencies. NTSCsubcarrier is 3.579545MHz and PAL-I’s is 4.43361875MHzand the digital systems typically sampled at 4 x NTSC SCor 3 x PAL SC, making 14.3 and 13.3MHz respectively.

Then came the step to use component video Y, B-Y and R-Y (luminance and two pure colour components – known ascolour difference signals) that is much easier to process forre-sizing, smooth positioning, standards conversion,compression and all the other 1001 operations that can beapplied to pictures today. When a standard was developedfor sampling this component video it followed some of thesame logic as before, but this time also sort commonalitybetween the two SD scanning systems used around theworld: 525/60I and 625/50I. Putting all that together led to

what is now the ITU-R BT.601 standard for SD sampling.‘601’ defines luminance sampling at 13.5MHz (giving 720pixels per active line) and each of the colour differencesignals at half that rate – 6.75MHz.

The final twist in this tale is that someone then noticedthat 13.5MHz was nearly the same as 14.3MHz that was 4 xNTSC subcarrier. Had he looked a little further he mighthave seen a much nearer relationship to 3 x PAL SC and awhole swathe of today’s terminology would be that muchdifferent! But so it was that the number that might havebeen 3 and should have been 1, became 4.

As HD sampling rates are 5.5 times faster than those forSD, the commonly used studio 4:2:2 sampling actuallyrepresents 74.25MHz for Y and 37.125MHz for Cr and Cb.

1080IShort for 1080 lines, interlace scan. This is the very widelyused HD line format which is defined as 1080 lines, 1920pixels per line, interlace scan. The 1080I statement alonedoes not specify the frame rate which, as defined bySMPTE and ITU, can be 25 and 30Hz.

See also: Common Image Format, Interlace, ITU-R.BT 709, Table 3

1080PTV image size of 1080 lines by 1920, progressively scanned.Frame rates can be as for 1080I (25 and 30Hz) as well as 24,50, 60Hz.

See also: Common Image Format, Progressive, ITU-R.BT 709, Table 3

13.5MHzSampling frequency used in the 601 digital coding of SDvideo. The frequency was chosen to be a whole multiple ofthe 525 and 625-line television system frequencies tocreate some compatibility between the digital systems.The sampling is fast enough to faithfully portray the


highest frequency, 5.5MHz, luminance detail informationpresent in SD images. Digital sampling of most HDstandards samples luminance at 74.25MHz, which is 5.5times 13.5MHz.

See also: 2.25MHz, ITU-R BT.601

2.25MHzThis is the lowest common multiple of the 525/59.94 and625/50 television line frequencies, being 15.734265kHz and15.625kHz respectively. Although seldom mentioned, itsimportance is great as it is the basis for all digitalcomponent sampling frequencies both at SD and HD.

See also: 13.5MHz

24PShort for 24 frames, progressive scan. In most cases thisrefers to the HD picture format with 1080 lines and 1920pixels per line (1080 x 1920/24P). The frame rate is alsoused for SD at 480 and 576 lines with 720 pixels per line.This is often as an offline for an HD 24P edit, or to create apan-and-scan version of an HD down-conversion. Displaysworking at 24P usually use the double shuttering technique– like film projectors – to show each image twice andreduce flicker when viewing this low rate of images.

24PsF24P Segmented Frame. This blurs some of the boundariesbetween film/video as video is captured in a film-like way,formatted for digital recording and can pass throughexisting HD video infrastructure. Like film, entire imagesare captured at one instant rather than by the usual line-by-line TV scans down the image that means the bottomcan be scanned 1/24 of a second after the top. The imagesare then recorded to tape as two temporally coherentfields (segments), one with odd lines and the other witheven lines, that are well suited to TV recorders.

The images are a pure electronic equivalent of a film shootand telecine transfer – except the video recorder operatesat film rate (24 fps), not at television rates. The footage hasmore of a filmic look but with the low frame rate,movement portrayal can be poor.

25PsF and 30PsF rates are also included in the ITU-R BT.709-4 recommendation.

See also: ITU-R BT. 709

601See ITU-R BT. 601

709See ITU-R BT. 709

720PShort for 720 lines, progressive scan. Defined in SMPTE296M and a part of both ATSC and DVB television standards,the full format is 1280 pixels per line, 720 lines and 60progressively scanned pictures per second. It is mainly theparticular broadcasters who transmit 720P that use it. Its 60progressive scanned pictures per second offers the benefitsof progressive scan at a high enough picture refresh rate toportray action well. It has advantages for sporting events,smoother slow motion replays etc.

74.25MHzThe sampling frequency commonly used for luminance (Y)or RGB values of HD video. Being 33 x 2.25MHz, thefrequency is a part of the hierarchical structure used for SDand HD. It is a part of SMPTE 274M and ITU-R BT.709.

See also: 2.25MHz


Active pictureThe part of the picture that contains the image. With theanalogue 625 and 525-line systems only 575 and 487 linesactually contain the picture. Similarly, the total time perline is 64 and 63.5µS but only around 52 and 53.3µS containpicture information. As the signal is continuous the extratime allows for picture scans to reset to the top of theframe and the beginning of the line.

Digitally sampled SD formats contain 576 lines and 720pixels per line (625-line system), and 480 lines and 720 pixelsper line (525-line system) but only 702 contain pictureinformation. The 720 pixels are equivalent to 53.3µS.

The sampling process begins during line blanking of theanalogue signal, just before the left edge of active picture,and ends after the active analogue picture returns toblanking level. Thus, the digitised image includes the leftand right frame boundaries as part of the digital scan line.This allows a gentle roll-on and roll-off the between theblanking (black) and active picture.

HD systems are usually quoted just by their active linecount, so a 1080-line system has 1080 lines of active video,each of 1920 samples. This may be mapped onto a largerframe, such as 1125 lines, to fit with analogue connections.

AliasingArtefacts created as a result of inadequate or poor videosampling or processing. Spatial aliasing results from thepixel-based nature of digital images and leads to theclassic ‘jagged edge’ (a.k.a. ‘jaggies’) appearance ofcurved and diagonal detail and twinkling on detail. Thisresults from sampling rates or processing accuracy too lowfor the detail. Temporal aliasing occurs where the speed ofthe action is too fast for the frame rate, the classicexample being wagon wheels that appear to rotate thewrong way.

See also: Anti-aliasing

AnamorphicThis generally describes cases where vertical andhorizontal magnification is not equal. The mechanicalanamorphic process uses an additional lens to compressthe image by some added amount, often on the horizontalaxis. In this way a 1.85:1 or a 2.35:1 aspect ratio can besqueezed horizontally into a 1.33:1 (4:3) aspect film frame.When the anamorphic film is projected it passes throughanother anamorphic lens to stretch the image back to thewider aspect ratio. This is often used with SD widescreenimages which keep to the normal 720 pixel count butstretch them over a 33-percent wider display. It can alsoapply to camera lenses used to shoot 16:9 widescreenwhere the CCD chips are 4:3 aspect ratio.

See also: Aspect ratio

Anti-aliasingAttempts to reduce the visible effects of aliasing. This isparticularly the case with spatial anti-aliasing that typicallyuses filtering processes to smooth the effects of aliasingwhich may be noticeable as jaggedness on diagonal lines,or ‘twinkling’ on areas of fine detail. A better solution is toimprove the original sampling and processing and avoidaliasing in the first place.

See also: Aliasing

Aspect RatioFor pictures, this refers to the ratio of picture width toheight. HD pictures use a 16:9 aspect ratio, which also maybe noted as 1.77:1. This is a third wider than the traditional4:3 television aspect ratio (1.33:1) and is claimed toenhance the viewing experience as it retains more of ourconcentration by offering a wider field of view.

Pixel aspect ratio refers to the length versus height for apixel in an image. HD always uses square pixels as domost computer applications. SD does not. The matter is


further complicated by SD using 4:3 and 16:9 (widescreen)images which all use the same pixel and line counts. Careis needed to alter pixel aspect ratio when moving betweensystems using different pixel aspect ratios so that objectsretain their correct shape.

With both 4:3 and 16:9 images and displays in use, somethought is needed to ensure a shoot will suit its targetdisplays. All HD, and an increasing proportion of SD,shoots are 16:9 but many SD displays are 4:3. As most HDproductions will also be viewed on SD, clearly keeping themain action in the middle ‘4:3’ safe area would be a goodidea – unless the display is letterboxed.

See also: ARC

Chrominance (or Chroma) sub-samplingSee 4:2:2 etc.

CIFCommon Image Format. An image format that is widelyused and denoted ‘Common Image Format’ by the ITU.The idea is to promote the easy exchange of imageinformation nationally and internationally.

See HD-CIF

Colour spaceThe space encompassed by a colour system. Examplesare: RGB, YCrCb, HSL (hue, saturation and luminance) forvideo, CMYK for print and XYZ for film. Moving betweenmedia, platforms or applications can require a change ofcolour space. This involves complex image processing socare is needed to get the right result. Also, repeatedchanges of colour space can lead to colours drifting off.

It is important to note that when converting from YCrCb toRGB more bits are required in the RGB colour space to

maintain the dynamic range. For example, if the YCrCbcolour space video is 8 bits per component then the RGBcolour space video will need to be 10 bits.

Component videoMost traditional digital television equipment handles videoin the component form: as a combination of pure luminanceY, and the pure colour information carried in the two colourdifference signals R-Y and B-Y (analogue) or Cr, Cb (digital).The components are derived from the RGB delivered byimaging devices, cameras, telecines, computers etc.

Part of the reasoning for using components is that it allowscolour pictures to be compressed. The human eye can seemuch more detail in luminance than in the colour information(chrominance). The simple task of converting RGB to Y, (R-Y) and (B-Y) allows exclusive access to the chrominanceonly, so its bandwidth can be reduced with negligible impacton the viewed pictures. This is used in PAL and NTSCcolour coding systems and has been carried through tocomponent digital signals both at SD and HD.

For the professional digital video applications, the colourdifference signals are usually sampled at half the frequencyof the luminance - as in 4:2:2 sampling. There are also othertypes of component digital sampling such as 4:1:1 with lesscolour detail (used in DV), and 4:2:0 used in MPEG-2.

Co-sited samplingWhere samples of luminance and chrominance are alltaken at the same instant. This is designed so that therelative timing (phase) of all signal components issymmetrical and not skewed by the sampling system.Sampling is usually co-sited but there is a case of 4:2:0sampling being interstitial – with chrominance samplesmade between the luminance samples.

See also: 4:2:2 etc.


DTVDigital Television. This is a general term that covers bothSD and HD digital formats.

Gamut (colour)The range of possible colours available in an imagingsystem. The red, blue and green phosphors on televisionscreens and the RGB colour pick-up CCDs or CMOS chipsin cameras, define the limits of the colours that can bedisplayed – the colour gamut. Between the camera andviewer’s screen there are many processes, many usingcomponent 4:2:2 video. However, not all component valuecombinations relate to valid RGB colours (for example,combinations where Y is zero). Equipment that generatesimages directly in component colour space, such as somegraphics machines, can produce colours within thecomponent range but that are invalid in RGB, which canalso exceed the limits allowed for PAL and NTSC.

There is potential for overloading equipment – especiallytransmitters which may cut out to avoid damage! There isequipment that clearly shows many areas of out-of-gamutpictures, so that they can be adjusted before they causeproblems.

HDHigh Definition Television. This has been defined in theUSA by the ATSC and others as having a resolution ofapproximately twice that of conventional television (meaninganalogue NTSC – implying 486 visible lines) both horizontallyand vertically, a picture aspect ratio of 16:9 and a frame rateof 24fps and higher. This is not quite straightforward as the720-line x 1280 pixels per line, progressive scan format iswell accepted as HD. This is partly explained by the bettervertical resolution of its progressive scanning. Apart fromthe video format, another HD variation on SD is a slightlydifferent colorimetry where, for once the world agrees on acommon standard.

As HD’s 1080 x 1920 image size is close to the 2K used forfilm, there is a crossover between film and television. Thisis even more the case if using a 16:9 window of 2K as herethere is very little difference in size. It is generally agreedthat any format containing at least twice the standarddefinition format on both H and V axes is high definition.

After some initial debate about the formats available toprospective HD producers and television stations, theacceptance of 1080-HD video at various frame rates, as acommon image format by the ITU, has made matters farmore straightforward. While television stations may havesome latitude in their choice of format, translating, ifrequired, from the common image formats should beroutine and give high quality results.

See also: Common Image Format, Interlace Factor

2K, HD and SD images sizes

2K Film

1080-HD

720-HD

576&

480-SD

2048

1536

1080

720

576

480

1920

1280

720


PAL and NTSCPAL and NTSC do not exist in HD. They do not exist inmodern SD digital television either – although it wasdigitised in early digital VTR formats. PAL, means PhaseAlternating Line and is an analogue system for codingcolour that is still widely in use. Similarly NTSC (NationalTelevision Standards Committee) describes an analoguesystem. Confusingly PAL and NTSC are still used todescribe frame rates and formats that relate in some waywith their analogue world. So 1080 PAL might be 1080/50I.

QuantizationQuantization refers to sampling: the number of bits used inmaking digital samples of a signal. For video, 8 bits isquite common in consumer and prosumer products suchas DV. HDV also uses 8 bits. Note that the 8 bits can define28 or 256 numbers or levels that, for converting analoguevideo into digits, are assigned to levels of image brightness.

For more accuracy and to withstand multiple levels of complexpost production processing, studio video applicationsoften use 10-bit sampling – providing 1024 levels.

Usually the distribution of the levels between brightest anddarkest is linear (even) but in the case of scanning filmnegative for input to a digital intermediate chain, then alogarithmic distribution is often used that progressivelysquashes the levels into the darker areas of picture. This isbecause film negative has to carry a very wide range ofcontrast information from the original scene, and the levelsin the dark/shadow areas are more significant and visiblethan those in bright areas. The ‘log’ sampling suitablyredistributes the available digital levels – hence 10-bit log.This is considered to be as useful as 13-bit linear quantization.

NB: Quantization has another meaning.See section: Video Compression 1

RGBRed, Green and Blue. Cameras, telecines and mostcomputer equipment originate images in this colourspace. For digital sampling, all three colours are sampledin the same way at full bandwidth – hence 4:4:4. imagesmay offer better source material for the most criticalchroma keying, but they occupy 50 percent more dataspace than 4:2:2 and as no VTRs record 4:4:4, datarecorders or disks must be used to store them. Also, thereare no television means to connect them, so IT-basednetworking technology is used.

Often 4:4:4 is only used in post production areas and isconverted to 4:2:2 when material is more widely distributed.

See also: 4:4:4, Gamut

Segmented FrameSee: 24PsF

Square pixelsSquare pixels are the pixel aspect ratio where the pixelsdescribe a square area of the displayed image. This is thecase with HD broadcast standards, as the picture formatsdescribe line length (number of pixels per line) andnumber of lines, in exact 16:9 ratios – which is also thedisplay aspect ratio of the pictures.

There are places in HD where pixels are not square. Thevery widely used HDCAM sub-samples the 1920-pixel HDline lengths with 1440 luminance samples. This is only aninternal function of the recorder; the inputs and outputsuse square pixels. In a similar way the 1080I HDV(2) formatalso uses 1440 samples per line.

Generally, computers generate images with square pixelsbut digital SD television images are not square. Thismeans that any applications or equipment used needs to


take this into account when transferring between applications,or performing image manipulations to maintain correctimage aspect ratios (so circles remain circular).

See also: Anamorphic, Aspect ratio

Sub-samplingIn a digital sampling system, taking fewer samples of ananalogue signal than the number of pixels in the digitalimage is called sub-sampling. Generally sub-sampling isused to reduce the amount of data used for an image. Inthe widely used 4:2:2 sampling system for studio qualityvideo, each luminance sample corresponds to one pixel –denoted by the ‘4’. The two chrominance signals are eachsampled at half the rate, making one per two pixels. This isknown as chrominance sub-sampling – a term that issometimes more generally ascribed to the sampling ratios– such as 4:2:2, 4:1:1, etc.

See also: 4:2:2 etc

System nomenclatureA term used to describe television standards. Thestandards are mostly written in a self-explanatory form butthere is room for confusion concerning vertical scanningrates. For example, 1080/60I implies there are 60 interlacedfields per second that make up 30 frames. Then 1080/30Pdescribes 30 frames per second, progressively scanned.

The general rule appears to be that the final figure alwaysindicates the number of vertical refreshes per second.However, Table 3 (below) uses a different method. It definesframe rates (numbers of complete frames) and then defineswhether they are interlaced or progressive. So here the ‘framerate code 5’ is 30Hz which produces 30 vertical refresheswhen progressive, and 60 when interlaced. Be careful!

See also: Interlace, Progressive

This table lists no fewer than 18 DTV formats for SD and HD.Initially, this led to some confusion about which should beadopted for whatever circumstances. Now most HDproduction and operation is centred on the 1080-line formatseither with 24P, 25P or 60I vertical scanning, and 720-lineformats at 50P and 60P.

Table 3The video formats allowed for broadcast in the ATSC DTVstandard are listed in Table 3 of document Doc. A/53A.

Vertical_size_value

1080

720

480

1920

1280

704

640

1,3

1,3

2,3

1,2

1,2,4,5

4,5

1,2,4,5,7,8

1,2,4,5,7,8

4,5

1,2,4,5,7,8

4,5

1

0

1

1

0

1

0

Horizontal_size_value

aspect_ratio_

information

frame_rate_code

progressive_sequence

Table 3 Compression Format Constraints

Legend for MPEG-2 coded values in Table 3

aspect_ratio_information 1 = square samples 2 = 4:3 display aspect ratio 3 = 16:9 display aspect ratio

Frame_rate_code 1 = 23.976 Hz 2 = 24 Hz 4 = 29.97 Hz 5 = 30 Hz 7 = 59.94 Hz 8 = 60 Hz

Progressive_sequence 0 = interlaced scan 1 = progressive scan


Truncation (a.k.a. Rounding)Reducing the number of bits used to describe a value. This iseveryday practice; we may say 1,000 instead of 1024 in thesame way we leave off the cents/pence when talking aboutmoney. There is also the need to truncate the digits used indigital video systems. With due care, this can be invisible,without it degradation becomes visible.

Decimal: 186 x 203 = 37758Binary: 10111010 x 11001011 = 1001001101111110

It is the nature of binary mathematics that multiplication, whichis commonplace in video processing (e.g. mixing pictures),produces words of a length equal to the sum of the twonumbers. For instance, multiplying two 8-bit video valuesproduces a 16-bit result – which will grow again if anotherprocess is applied. Although hiways within equipment maycarry this, ultimately the result will have to be truncated to fitthe outside world which, for HD, may be a 10-bit HD-SDIinterface or 8-bit MPEG-2 encoder.

In the example, truncating by dropping the lower eight bitslowers its value by 01111110, or 126. Depending on videocontent, and any onward processing where the error iscompounded, this may, or may not be visible. Typically, flat (nodetail) areas of low brightness are prone to showing this typeof discrepancy as banding. This is, for example, sometimesvisible from computer generated images.

Inside equipment, it is a matter of design quality to truncatenumbers in an intelligent way that will not produce visibleerrors – even after further processing. Outside, plugging 10-bitequipment into 8-bit needs care. Intelligent truncation isreferred to as Rounding.

Universal Format1080/24P is sometimes referred to as the Universal Format fortelevision. The reason is its suitability for translation into allother formats to produce high quality results in all cases.

See also: HD-CIF, Universal Master

Universal MasterThe 1080/24P format has well defined and efficient paths to allmajor television formats and is capable of delivering highquality results to all. An edited master tape in this format issometimes referred to as a Universal Master.

See also: HD-CIF

Y, Cr, CbThis signifies video components in the digital form. Y, Cr, Cb isthe digitised form of Y, R-Y, B-Y.

Y, R-Y, B-YSee component video

YUVDe-facto shorthand for any standard involving componentvideo. This has been frequently, and incorrectly, used asshorthand for SD analogue component video – Y, R-Y, B-Y. Y iscorrect, but U and V are axes of the PAL colour subcarrierwhich are modulated by scaled and filtered versions of B-Yand R-Y respectively. Strangely, the term is still used todescribe component analogue HD. This is double folly.Although Y is still correct, all HD coding is digital and hasnothing to do with subcarriers or their axes. So forget it!

Chapter 2

Video Compression:Concepts


Video compression reduces the amount of dataor bandwidth used to describe moving pictures.Digital video needs vast amounts of data todescribe it and there have long been variousmethods used to reduce this for SD. And as HDhas up to a six times bigger requirement of1.2Gb/s and requiring 560GB per hour of storage,the need for compression is even more pressing.

Intro Compression – GeneralExactly which type and how much compression is useddepends on the application. Consumer delivery (DVD,transmission, etc) typically uses very high compression (lowdata rates) as the bandwidth of the channels is quite small.For production and online editing use much lightercompression (higher data rates) are used as good picturequality needs to be maintained though all the stagesleading to the final edited master.

Video compression methods are all based on the principleof removing information that we are least likely to miss –so-called ‘redundant’ picture detail. This applied to stillimages as well as video and cinema footage. This takes theform of several techniques that may be used together.Digital technology has allowed the use of very complexmethods which have been built into low cost massproduced chips.

First, our perception of colour (chroma) is not as sharp as itis for black and white (luminance), so the colour resolutionis reduced to half that of luminance (as in 4:2:2). This is usedin colour television (NTSC, PAL and digital). Similarly, finedetail with little contrast is less noticeable than biggerobjects with higher contrast. To access these a processcalled DCT resolves 8 x 8 pixel blocks of digital images intofrequencies and amplitudes to make it possible to scale(down), or ‘quantize’, the DCT coefficients (frequencies andamplitudes) and so reduce the data. This applies most of

Video Compression: Concepts 12Understanding HD with Avid

the digital video compression schemes in use todayincluding AVR, DV, HDV, JPEG (but not JPEG2000) and theI frames of MPEG-1, 2 and 4, and Windows Media 9. Afurther reduction is made using Huffman coding, a purelymathematical process that reduces repeated data.

MPEG-2 and the more recent MPEG-4 add another layerof compression by analysing what changes form frame toframe by analysing the movement of 16 x 16-pixel macroblocks of the pictures. Then it can send just the movementinformation, called motion vectors, that make up predictive(B and P) frames and contain much less data than I frames,for much of the time. Whole pictures (I frames, more data)are sent only a few times a second. MPEG-2 compressionis used in all forms of digital transmission and DVDs as wellas for HDV. The more refined and efficient MPEG-4 is nowbeing introduced for some HD services, and is set tobecome widely used for new television services.

Each of these techniques does a useful job but needs tobe applied with some care when used in the productionchain. Multiple compression (compress/de-compress)cycles may occur while moving along the chain, causing abuild-up of compression errors. Also, as many compressionschemes are designed around what looks good to us, theymay not be so good in production, post production andediting. This particularly applies in processes, such askeying and colour correction, that depend on greaterimage fidelity than we can see, so disappointing resultsmay ensue from otherwise good-looking compressedoriginals.

See also: AVR, Component video, DV, DNxHD, Huffman coding, JPEG,

JPEG2000, MPEG-2, MPEG-4

BlocksSee DCT


CodecCodec is short for coder/decoder – usually referring to acompression engine. Confusingly, the term is often misusedto describe just a coder or decoder.

Compression ratioThis is the ratio of the uncompressed (video or audio) datato the compressed data. It does not define the resultingpicture or sound quality, as the effectiveness of thecompression system needs to be taken into account. Evenso, if used in studio applications, compression is usuallybetween 2:1 and 7:1 for SD (and D1 and D5 uncompressedVTRs are also available), whereas compression for HD iscurrently approximately between 6:1 and 14:1 – as definedby VTR formats, and is I-frame only. For transmission, theactual values depend on the broadcaster’s use of theavailable bandwidth but around 40:1 is common for SDand somewhat higher, 50 or 60:1 for HD (also dependingon format). These use both I-frames and the predictiveframes to give the greater compression.

HDV records data to tape at 19-25 Mb/s – a ratecomparable with HD transmission and a compression ratioof around 40:1, depending on the standard used.

Transmission and video recorders in general work at aconstant bit rate so, as the original pictures may includevarying amounts of detail, the quality of the compressedimages varies. DVDs usually work on a constantquality/variable bit rate principle. So the compression ratioslides up and down according to the demands of thematerial, to give consistent results. This is part of thereason why DVDs can look so good while only averagingquite low bit rates – around 4 Mb/s.

Compression-friendlyMaterial that looks good after compression is sometimesreferred to as ‘compression friendly’. This can becomeimportant in transmission where very limited databandwidth is available and high compression ratios have tobe used. Footage with large areas of flat colour, little detailand little movement compress very well: for example,cartoons, head-and-shoulder close-ups and some dramas.As, MPEG-2 compression looks at spatial detail as well asmovement in pictures and an excess of both may show atthe output as poor picture quality. This often applies tofast-moving sports – for instance football.

Poor technical quality can be compression unfriendly.Random noise will be interpreted as movement by anMPEG-2 or MPEG-4 encoder, so it wastes valuable dataspace conveying unwanted movement information.Movement portrayal can also be upset by poor qualityframe-rate conversions that produce judder on movement,again increasing unwanted movement data to betransmitted at the expense of spatial detail. Suchcircumstances also increase the chance of movementgoing wrong – producing ‘blocking’ in the pictures.

Errors can be avoided by the use of good qualityequipment throughout the production chain. Also, thechoice of video format can help. For example, there is lessmovement in using 25 progressively scanned images thanin 50 interlaced fields, so the former compress more easily.The efficiency increase is typically 15-20 percent.

DCTDiscrete Cosine Transform is used as a first stage of manydigital video compression schemes including JPEG andMPEG-2 and –4. It converts 8 x 8 pixel blocks of pictures toexpress them as frequencies and amplitudes. This may notreduce the data but it does arrange the image informationso that it can. As the high frequency, low amplitude detailis least noticeable their coefficients are progressively


reduced, some often to zero, to fit the required file sizeper picture (constant bit rate) or to achieve a specifiedquality level. It is this reduction process, known asquantization, which actually reduces the data.

For VTR applications the file size is fixed and thecompression scheme’s efficiency is shown in its ability touse all the file space without overflowing it. This is onereason why a quoted compression ratio is not a completemeasure of picture quality.

DCT takes place within a single picture and so is intra-frame (I-frame) compression. It is a part of the currentlymost widely used compression in television.

See also: AVR, Compression ratio, DV, JPEG, MPEG-2, MPEG-4

GOPGroup Of Pictures – as in MPEG-2 and MPEG-4 videocompression. This is the number of frames to each integralI-frame: the frames between being predictive (types B andP). ‘Long GOP’ usually refers to MPEG-2 and 4 coding. Fortransmission the GOP is often as long as half a second, 13,or 15 frames (25 or 30fps), which helps to achieve therequired very high compression ratios.

Cutting long GOP MPEG is not straightforward as itsaccuracy is limited to the GOP length unless furtherprocessing is applied – typically decoding. HDV uses longGOP MPEG-2 of 6 or 15 frames for HDV1 or HDV2respectively making it editable at 1/4 or 1/2 secondintervals. A GOP of 1 indicates ‘I-frame only’ video, whichcan be cut at every frame without need of processing.

Studio applications of MPEG-2 have very short GOPs,Betacam SX has a GOP of 2, IMX has 1, (i.e. I-frame only –no predictive frames) which means cutting at any frame isstraightforward. Other formats such as DV, DVCPRO HDand HDCAM, D5-HD do not use MPEG but are also I-frame only.

See also: MPEG-2, MPEG-4

I-frame only (aka I-frame)Short for intra-frame only.

Inter-frame compressionVideo compression that uses information from severalsuccessive video frames to make up the data for itscompressed ‘predictive’ frames. The most commonexample is MPEG-2 with a GOP greater than 1. Such anMPEG-2 stream contains a mix of both I-frames andpredictive B and P (Bi-directional predictive and Predictive)frames. Predictive frames cannot be decoded in isolationfrom those in the rest of the GOP so the whole GOP mustbe decoded. This is an efficient coding system that is goodfor transmission but it does not offer the flexibility neededfor accurate editing as it can only be cut at the GOPboundaries. It also requires estimation of the movementfrom picture to picture, which is complex and not alwaysaccurate – leading to ‘blockiness’.

See also: GOP, MPEG-2, MPEG-4

InterlaceA method of ordering the lines of scanned images as two(or more) interlaced fields per frame. Most television uses2:1 interlacing; alternate fields of odd lines 1,3,5, etc.,followed by a field of even lines 2, 4, 6, etc. This doublesthe vertical refresh rate as there are twice as manyinterlaced fields as there would be whole frames. The

I B B P B B P B B P B B I

A typical group of pictures


result is better portrayal of movement and reduction offlicker without increasing the number of full frames orrequired signal bandwidth. There is an impact on verticalresolution and care is needed in image processing.

See also: Interlace factor, Progressive

Interlace FactorUse of interlaced, rather than progressive, scans has noeffect on the vertical resolution of still images. However, ifanything in the image moves the resolution is reduced bythe Interlace Factor, which may be 0.7 or less. This is dueto the time displacement between the two fields ofinterlace which will produce detail that is jagged, line-by-line, during the movement and it appears as an overallslight softening of vertical resolution.

Intra-frame compression (a.k.a. I-frame compression)Video compression which takes information from onevideo frame only. This way, all the information to re-createthe frame is contained within its own compressed data andis not dependent on other adjacent frames. This meansthat I-frame compressed video is easily edited as it cansimply be cut at any picture boundary without the need forany decoding and recoding. I-frame only video can beedited and the result output as first generation material.Any other operations such as wipes, dissolves, mixes, DVEmoves etc., can only be performed on the basebandsignal, requiring that the video is first decompressed.

See also: AVR, DV, JPEG, MPEG-2,

MacroblockA 16 x 16 pixel block, comprising four adjacent DCT blocks– macroblocks are used to generate motion vectors inMPEG-2 coding. Most coders use a ‘block matching’technique to establish where the block has moved and sogenerate motion vectors to describe the movement. Thisworks most of the time but also has its well-knownmoments of failure. For example, slow fades to black tendto defeat the technique, making the resulting misplacedblocks quite visible. Better technologies are available foruse in movement estimation, such as phase correlation.

Motion VectorsUsed in MPEG-2 and MPEG-4 compression systems, motionvectors describe the direction and distance that macroblocks(16 x 16 pixels) move between frames. Sending thismovement information requires much less data than sendingan I frame, and so further reduces the video data.

Progressive (scan)Sequence for scanning an image where the vertical scanprogresses from line 1 to the end in one sweep. In HDTVthere are a number of progressive vertical frame (refresh)rates allowed and used. 24Hz is popular for itscompatibility with motion pictures and its ability to beeasily translated into all of the world’s television formats.25 and 30Hz correspond with existing SD frame rates(although they use interlaced scans). 50 and 60Hz are alsoallowed for, but, due to bandwidth restrictions, these arelimited in picture size, e.g. 720/60P and 720/50P.

Today, progressive scanning is most commonly found incomputer displays and all the modern panel TV displays areprogressive. Progressive images are rock steady making thedetail easy to see. For the equipment designer progressiveimages are easier process as there is no difference betweenthe two fields of a frame to contend with.


Progressive scans have the disadvantage of a slow verticalrefresh rate. Thus, for the lower rates of 24, 25 and 30Hz,which can be used in HD television with the larger 1080-line formats, there would be considerable flicker ondisplays, unless there were some processing to show eachpicture twice (as in double shuttering in cinema projectors).Besides flicker, the other potential problem area is that offast action or pans, as the lower refresh rate means thatmovement will tend to stutter. Interlace, with its twovertical refreshes per frame, has advantages here.

See also: 24PsF, Interlace

QuantizationQuantizing is the process used in DCT-based compressionschemes, including AVC, JPEG, MPEG-2 and MPEG-4, toreduce the video data in an I frame. DCT allows quantizingto selectively reduce the DCT coefficients that representthe highest frequencies and lowest amplitudes that makeup the least noticeable elements of the image. As manyare reduced to zero significant data reduction is realised.

Using a fixed quantizing level will produce a constantquality of output with a data rate that varies according tothe amount of detail in the images. Alternativelyquantizing can be varied to produce a constant data rate,but variable quality, images. This is useful where the datamust be fitted into a defined size of store or data channel– such as a VTR or a transmission channel. The success innearly filling, but never overflowing, the storage is onemeasure of the efficiency of DCT compression schemes.

NB: Quantization has a second meaning. See Video Formats section

Chapter 3

Video Compression:Formats


Video Compression: Formats 18Understanding HD with Avid

AVCSee MPEG-4

AVRAVR is a range of Motion-JPEG video compressionschemes devised by Avid Technology for use in its ABVBhardware-based non-linear systems. An AVR is referred toas a constant quality M-JPEG resolution since the samequantization table (of coefficients) is applied to each frameof a video clip during digitization. For any given AVR, theactual compressed data rate will increase as thecomplexity of the imagery increases. For example, a headshot typically results in a low data rate while a crowd shotfrom a sporting event will yield a high data rate. To avoidsystem bandwidth problems, AVRs utilize a mode of ratecontrol called rollback which prevents the compresseddata rate from increasing beyond a preset limit for asustained period. So, when the data rate exceeds therollback limit on a given frame, high spatial frequencyinformation is simply discarded from subsequent framesuntil the rate returns to a tolerable level.

See also: DCT, JPEG

This is the practical side of compression showingthe systems and formats that are used. Some areproprietary, in which case the company involvedis mentioned.

DVCDVC is the compression used in DV equipment that isstandardised in IEC 61834. It is a DCT-based, intra-framescheme achieving 5:1 compression so that 8-bit videosampling of 720 x 480 at 4:1:1 (NTSC) or 720 x 576 4:2:0(PAL) produces a 25 Mb/s video data rate. The same isused for DV, DVCAM, Digital8 and DVCPRO (where PAL isPAL 4:1:1). It achieves good compression efficiency byapplying several quantizers at the same time, selecting thenearest result below 25Mb/s for recording to tape.

DNxHDAvid DNxHD encoding is designed to offer quality atsignificantly reduced data rate and file size and it issupported by the family of Avid editing systems.Engineered for editing, it allows any HD material to behandled on SD-original Avid systems. Any HD format canbe encoded edited, effects added, colour corrected andthe project finished.

There is a choice of compression image formats to suitrequirements. Some of the formats are:

Avid DNxHD maintains the full raster, is sampled at 4:2:2and uses highly optimised coding and decodingtechniques, so image quality is maintained over multiplegenerations and processes. When you’re ready, master toany format you need.

DNxHD efficiency enables collaborative HD workflow usingnetworks and storage designed to handle SD media. So,for example, Avid Unity shared media networks are HD-ready today! Cost-effective, real-time HD workflows can be

Format

Bit depth

Frame rate

Data rate

10 bit

29.92 fps

220 Mb/s

10 bit

25 fps

184 Mb/s

8 bit

25 fps

184 Mb/s

8 bit

25 fps

135 Mb/s

DNxHD220x

DNxHD185x

DNxHD185

DNxHD145

8 bit

25 fps

220 Mb/s

DNxHD120


designed with Media Composer Adrenaline HD and AvidDS Nitris systems. You can even edit HD on a laptop.

For more information see www.avid.com/dnxhd/index.asp

H.264See MPEG-4

Huffman codingA method of compressing data by recognizing repeatedpatterns and assigning short codes to those that occurfrequently, and longer codes to those that are lessfrequent. The codes are assigned according to a HuffmanTable. Sending the codes rather than all the original datacan achieve as much as a 2:1 lossless compression and themethod is often used as a part of video compressionschemes such as JPEG and MPEG.

JFIFJPEG File Interchange Format – a compression schemeused by Avid Technology in its Meridien hardware-basednon-linear systems. A JFIF M-JPEG resolution is termedconstant rate since compressing clips of varyingcomplexity results in a fixed data rate. Each JFIF resolutionis defined by a target data rate and a base quantizationtable. When digitizing, the quantization table is linearlyscaled (known as rolling Q) to conform the actualcompressed data rate to the target rate. Due to theflexibility of this approach, imagery compressed by a JFIFresolution generally looks better than that compressed byan AVR of comparable average data rate.

JPEGJoint (ISO and ITU-T) Photographic Experts Group; JPEG isa standard for compressing still picture data. It offerscompression ratios of between two and 100 times andthere are three levels of processing available: baseline,extended and lossless encoding.

JPEG baseline coding, which is the most common fortelevision and computer applications, starts by applyingDCT to 8 x 8 pixel blocks of the picture, transforming theminto frequency and amplitude data. This itself may notreduce data but then the generally less visible highfrequencies can be divided by a high quantizing factor(reducing many to zero), and the more visible lowfrequencies by a lower factor. The quantizing factor can beset according to data size (for constant bit rate) or picturequality (constant quality) requirements – effectivelyadjusting the compression ratio. The final stage is Huffmancoding which is a lossless mathematical treatment that canfurther reduce data by 2:1 or more.

Baseline JPEG coding creates .jpg files and is very similarto the I-frames of MPEG-1, -2 and -4, the main differencebeing they use slightly different Huffman tables.

See also: Compression, Compression ratio, DCT, DV, Huffman coding,

JFIF, M-JPEG,

JPEG 2000JPEG 2000 is an advanced image coding (compression)system from the Joint Photographic Experts Group. Like‘normal’ JPEG, this is intra-frame compression and it issuitable for a wide range of uses from portable digitalcameras, to scientific and industrial applications.

http://www.jpeg.orgwww


Rather than using the established DCT, it employs state-of-the-art techniques based on wavelet technology. Requiringmore processing than MPEG, JPEG 2000 has, until recentlybeen too costly for wide use in television applications.Now new chips have lowered the price barriers and JPEG2000’s use in TV and D-cinema is expected to rapidlyexpand as it has distinct advantages for the high qualitylarge images. It is already recommended for D-cinema andGrass Valley have adopted it for HD compression in theirnew Infinity range of cameras.

As it does not analyse images block-by-block but in acircular area-by-area pattern, there are no ‘blocky’artefacts, instead problem areas tend to become a littlesofter – which is much less noticeable. JPEG 2000continues to improve as more bits are used for the images.As a result, at high bit rates of 200-300Mb/s HD and D-cinema images are displayed with ‘visually lossess’ quality.It is also scalable, so image sizes different to the encodedsize, can be extracted directly without decoding.

MPEGMoving Pictures Expert Group. A group of industry expertsinvolved with setting standards for moving pictures andsound. These are not only those for the compression ofvideo and audio (such as MPEG-2 and MP3) but alsoinclude standards for indexing, filing and labelling material.

MPEG-2ISO/IEC 13818-1. This is a video compression systemprimarily designed for use in the transmission of digitalvideo and audio to viewers by use of very high compressionratios. Its importance is huge as it is currently used fornearly all DTV transmissions worldwide, SD and HD, as wellas for DVDs and many other applications where high videocompression ratios are needed.

The Profiles and Levels table (below) shows that it is not asingle standard but a whole family which uses similar toolsin different combinations for various applications. Althoughall profile and level combinations use MPEG-2, movingfrom one part of the table to another may be impossiblewithout decoding to baseband video and recoding.

http://www.jpeg.orgwww

http://www.mpeg.orgwww

M-JPEGMotion JPEG refers to JPEG compression applied tomoving pictures. As the detail contained within each framevaries, so some decision is required as to whether to use aconstant bitrate scheme or constant quality.

See also: AVR, JPEG

M-JPEG 2000JPEG 2000 used for moving pictures.

Profile

Level

High

High-1440

Main

Low

720x57015 Mb/s

1920x115280 Mb/s

1440x115260 Mb/s

720x57615 Mb/s

352x2884 Mb/s

720x60850 Mb/s

720x57615 Mb/s

352x2884 Mb/s

Simple4:2:0I, B

Main4:2:0I, B, P

422P4:2:2I, B, P

SNR*4:2:0I, B, P

1440x115260 Mb/s

Spatial*4:2:0I, B, P

1920x1152100 Mb/s

1440x115280 Mb/s

720x57620 Mb/s

High4:2:0,4:2:2I, B, P

MPEG-2 profiles and levels*SNR and Spatial are both scalable

MPEG-4MPEG-4 (ISO/IEC 14496) was developed by MPEG (MovingPicture Experts Group) and is a wide standard coveringmany subjects but its importance in television productionis mostly related to its video compression scheme. MPEG-4 Part 10, AVC (Advanced Video Coding) and H.264 allrefer to the same compression system. This is anotherDCT-based system that builds on MPEG-2 to produce amore efficient codec again using intra and inter-frametechniques. Coding is more complex than MPEG-2 but itcan produce extra data savings of around 30 percent – ormore. Some of the latest television services are planned touse MPEG-4. This is especially true with HD where morebandwidth is required. It will enable the delivery of betterimage quality to viewers, or more channels to be deliveredwithin a given bandwidth. It is said to be similar to, but notthe same as, WM 9.


Profiles outline the set of compression tools used. Levelsdescribe the picture format/quality from High Definition toVHS. There is a bit rate defined for each allocated profile/level combination. In all cases, the levels and bit ratesquoted are maximums so lower values may be used.Combinations applicable to modern HD are highlighted.

MPEG-2 is deliberately highly asymmetrical in that decodingis far simpler than the encoding – so millions of viewersenjoy reasonable prices while a few broadcasters incur thehigher unit costs. Coding has two parts. The first uses DCT-based intra-frame (I-frame) compression and application ofquantizing, to reduce the data – almost identically to JPEG.The second involves inter-frame compression – calculatingthe movement of macroblocks and then substituting justthat information for the pictures between successive I-frames – making a GOP. The movement is conveyed asmotion vectors, showing direction and distance, whichamounts to far less data than is needed for I-frames.Motion vector calculation is not an exact science so therecan be huge difference in quality between different MPEGcompressors. Decompression is deterministic so alldecompressors (decoders) should be the same.

The encoding process necessarily needs to look at severalframes at once and so introduces a considerable delay.Similarly, the decoder delays pictures. For transmissions thiscan add up to over a second. MPEG-2 is sometimes used onbroadcast contribution circuits, this becomes noticeablewhen news reporters appear to delay answering a question.

To fit HD video and audio down a transmission ‘data pipe’requires very high compression. Uncompressed 10-bit HDrequires up to 1244Mb/s. But this is 10-bit data andsampled at 4:2:2. MPEG-2 is 8-bit sampled at 4:2:0 –bringing the data down to 746Mb/s. However, the datapipes for ATSC (19.2Mb/s) or DVB (20Mb/s, depending onchannel width, parameters etc.) imply the need for around40:1 compression.

See also: DCT, GOP, Intra-frame compression, Inter-frame compression.

Macroblock

VC-1VC-1 is a video compression codec specification that iscurrently being standardised by SMPTE (SMPTE 421M) andimplemented by Microsoft as Windows Media Video(WMV) 9 Advanced Profile.

See: WMV 9

WMV 9Windows Media Video 9 is a video and audio compressionsystem (codec) developed by Microsoft. It is said to besimilar to MPEG-4 AVC and to have as good or slightlybetter performance giving lower data rates and claims tobe a less complex process. It’s applications are seen asbeing for content delivery such as HD DVD.

http://www.chiariglione.org/mpegwww

4

HD formats

Chapter 4


Tape formats for high definition television nowspan a wide range of qualities and prices. Thesecater for the recording needs of digitalcinematography, mainstream broadcast andprogramming and, most recently, the prosumermarket. The latter is addressed by HDV and hasenabled a huge expansion of HD use.

D6The D6 tape format uses a 19mm ‘D-1 like’ cassette torecord 64 minutes of uncompressed HD material in mostof the current HDTV standards. The recording rate is up to1020 Mb/s and uses 10-bit luminance and 8-bit chrominanceand records 12 channels of AES/EBU stereo digital audio.The only D6 VTR on the market is VooDoo from Thomsonand it has been used in film-to-tape applications.

D7-HDSee DVCPRO HD

DVCPRO HD (a.k.a. D7-HD and DVCPRO 100)This is the HD version of Panasonic’s DVCPRO VTRhierarchy. DV and DVCPRO record 25Mb/s; DVCPRO 50records 50Mb/s; and DVCPRO HD records 100Mb/s. Alluse the DVC intra-frame DCT-based digital compressionscheme and the 6.35mm (1/4-inch) DV tape cassette.

In the recording format, video sampling is 8-bit, 4:2:2 and1080I as well as 720P formats are supported. There areeight 16-bit 48kHz audio channels. The recording data ratemeans that considerable video compression must be usedto reduce around 1Gb/s video and audio data. Videocompression of 6.7:1 is quoted.

A feature of DVCPRO HD camcorder range is the VariCamthat offers variable progressive frame rates for shootingfrom 4-60Hz in one-frame increments.

http://www.panasonic.comwww

http://videoexpert.home.att.netwwwhttp://www.panasonic.com/pbds/index.htmlwww

D5-HDThis is an HD version of the D5 half-inch digital VTR formatfrom Panasonic and has been widely used for HD mastering.It records on a standard D-5 cassette shell for over two hourswith a wide selection of video formats: 1080/60I, 1035/60I,1080/24P, 720/60P, 1080/50I, 1080/25P and 480/60I. It canslew a 24Hz recording to use the material directly in 25/50Hzapplications – useful for European replay of movies. Thereare eight discrete channels of 24-bit 48kHz digital audio toallow for 5.1 and stereo mixes.

Panasonic uses a proprietary compression scheme to reducethe raw HD-SDI 4:2:2 component digital video data rate ofup to 1240Mb/s. The D5-HD compresses video 4:1 (8-bitmode) and 5:1 (10-bit mode).

Also see HD VCR formats at:

HD formats 23Understanding HD with Avid


in its pixel count (SD and HD), bit depth (10- or 12-bit), andcolour resolution (component or RGB). Its applicationsinclude high end HD recording, editing and as a masteringformat. HDCAM SR is probably the highest quality HD taperecording system available. Practical recorders at any higherbit rate use hard discs or flash memory.

Besides working at the 440Mb/s rate, called the SQ mode,HDCAM SR also offers an HQ mode with recording at880Mb/s to offer lower compression 4:4:4 RGB or two4:2:2 channels.

HDVHDV is a low cost system for shooting and recording HD. Itdefines video formats, a compression scheme and uses DVrecording and familiar DV, or MiniDV, cassettes. HDV isavailable in two standards HDV1 and HDV2 but, unlike DV,they use MPEG-2 long GOP compression to squeeze theHD video into DV-sized data. 4:2:0 colour 8-bit sampling iscommon to both standards. The two channels of 16-bit/48Hz audio are compressed (4:1) with MPEG-1 (Layer II)to 384 kb/s.

HDV1 is 1280x720 progressive scan format with frame ratesof 60, 50, 30 and 25Hz. JVC’s ProHD adds a 24Hz framerate. The luminance sampling rate is 74.25MHz. The videois compressed using MPEG-2 six-frame GOP compressionto produce a recording data rate of just 19 Mb/s. In thisstandard a 63-minute MiniDV cassette records 63 minutesof HDV and, with critical data interleaved over all therecorded tracks, dropouts are minimised.

HDV2 is a 1440x1080 interlaced scan format with framerates of 60 or 50Hz. The data rate is 25Mb/s after applyingMPEG-2 15-frame GOP compression. Note that the pixelcount is not in the usual 16:9 pixel/line ratio, but thepictures themselves are. So here luminance sampling rateis 55.7MHz and the pixels are not square but are stretchedto an aspect ratio of 1.33:1. This is the same luminancesampling as is used in HDCAM.

HDCAMSony’s HD camcorder version of the popular Digital Betacam.Introduced in 1997 at ‘near DigiBeta’ prices it was the firstmore affordable HD format. Now the expanded range includesstill lower priced models. HDCAM defines a half-inch taperecording format. There is also a range of studio recordersand players as well as options for down conversion to SD.

In the camcorder, the camera section includes 2/3-inch, 2.1million pixel CCDs to capture 1080 x 1920 images. Thelenses have compatibility with Digital Betacam products aswell as accepting HD lenses for the highest picture quality.The recorder offers up to 40-minutes’ time on a smallcassette making the package suitable for a wide range ofprogramme origination, including on location. A series ofsteps, including 4.4:1 intra-frame compression, reduces thebaseband video data rate to 140Mb/s. The format supportsfour channels of AES/EBU audio and the total recordingrate to tape is 185Mb/s. HDCAM effectively samples videoat 3:1:1 with the horizontal resolution sub-sampled to 1440pixels. It fulfils many HD needs but is not an ideal mediumfor Blue Screen work.

Video formats supported by HDCAM are: 1080 x 1920pixels at 24, 25 and 30 progressive fps and at 50 and 60Hzinterlace. Material shot at 24P can be directly played backinto 50Hz or 60Hz environments. Also, the ability toplayback at different frame rates can be used to speed upor slow down the action.

See also: CineAlta

HDCAM SRHDCAM SR can record either 4:4:4 RGB or component4:2:2 HD video at a net video rate of 440Mb/s. It uses mildMPEG-4 Studio Profile (ISO/IEC 14496-2:2001-1) ‘visuallylossless’ compression and records onto 1/2-inch tapecassettes. The Studio Profile addresses the need for highresolution; it is I-frame only and so easy to edit, and scalable


ProHDProHD is JVC’s adaptation of the HDV 720P recording modethat adds 24-frame progressive scan 24P – but not for the1080-line format. This is useful of productions seeking afilm look or wishing to output to film or D-cinema as itavoids the never-perfect process of deinterlacing. Apartfrom adding 24P, ProHD uses the same compression andbitstream format as HDV.

XDCAM HDSony’s XDCAM HD records 1080I 4:2:0 HD at bit rates of18, 25 and 35Mb/s onto Professional Disc media (Blu-ray).The 25Mb/s is a constant bit rate to give users a bridge toHDV, and the other two rates are variable. 18Mb/s allowsfor a recording time of two hours, and the other two allowfor 90 and 60 minutes. User can mix the different bit rateon the same disc. As with HDV, long GOP MPEG-2compression is used.

5

SD formats

Chapter 5


Standard definition has a wide variety of digitaltape formats to suit everyone from consumers to broadcast professionals. Recent trends includemore compact formats and lower costs. Many of the HD tape formats have their routes in SDincluding HDV that uses the widely used (SD) DV format.

D5Introduced by Panasonic in 1994 this records uncompressed625 and 525-line 4:2:2 10-bit component digital video ontothe same half-inch cassettes as D3. Being component it usesin post production and, as it has lower costs than D1, is stillin use today. The format also has provision for HDTVrecording by use of about 4 or 5:1 compression (see HD-D5).

Digital BetacamLaunched in 1993, ‘Digibeta’ superseded the analogueBetacam formats and costs much less D1. It provides goodvideo and audio quality and run time up to 124 minutes.720 x 576 or 720 x 480 4:2:2 component SD digital video isDCT-compressed to a bitrate of 90 Mb/s (about 2:1compression) plus 4 channels of uncompressed 48 kHzPCM audio.

DVLaunched in 1996, DV (IEC 61834) defines both the codec(video compression system) and the tape format for thefirst SD digital tape format for the consumer and prosumermarkets. Features include intra-frame compression forstraightforward editing, an IEEE 1394 interface for transferto non-linear editing systems, and good video qualitycompared to consumer analogue formats.

Variants include the DVCPRO series and DVCAM. Also,much of HDV has its roots in DV including the MiniDVtape, but not HDV’s use of MPEG-2 compression.

DVCAMIntroduced by Sony, DVCAM is a professional variant of theDV standard that uses the same cassettes as DV and MiniDV,the same compression scheme, but runs the tape through50 percent faster making it more robust with fewererrors/dropouts.

D1Digital tape format to record SD uncompressed 4:2:2component digital 625 and 525-line video onto 19mm (3/4-inch) cassettes. Introduced by Sony in 1987 it wasrelatively expensive and used for high-end work wheremulti-generation quality needed to be maintained. It is not widely used today.

D2Introduced in 1988 by Ampex, this records uncompresseddigitised composite PAL or NTSC video onto 19mm (3/4-inch) cassettes. Although it used less data, and so lesstape, than D1, and was good for analogue transmissionreplay, the signal suffered from all the original restrictions ofPAL and NTSC. It was little use in modern post productionand would have to be decoded for any digital transmission.The format is little used today.

D3Introduced by Panasonic D3 is similar to D2 in that itrecords composite PAL or NTSC video onto cassettes, theD3 ones being 1/2-inch. As it has the same benefits anddrawbacks as D2 and is not widely used today.

SD formats 27Understanding HD with Avid

SD formats 28Understanding HD with Avid

XDCAMSony’s camcorder that uses Professional Disc media. Itrecords Sony’s MPEG IMX (MXF compatible) format, 8-bitI-frame (only) MPEG-2 at 50, 40 or 30 Mb/s – claimingDigital Betacam quality with the highest bit rate. The ratesgive 45, 57, and 68 minutes recording time respectively.Some models can also record the 8-bit DVCAM formatwith 5:1 compression and 4:1:1 sampling for the 480/60Isystem (NTSC) and 4:2:0 for 576/50I (PAL) system. DVCAMrecording time is 85 minutes.

See also: MXF

DVCPRO (25 and 50)Panasonic created the DVCPRO range for professionalapplications of the root DV technology. Also known asDVCPRO 25, DVCPRO is identical to the DV format forrecording, and uses a 25Mb/s recording stream. There aretwo tracks of 16-bit, 48kHz audio and video is sampled at4:1:1 for both the 576/50I and 480/60I versions.

DVCPRO has a hierarchical structure that doubles the datarate. The next step up is DVCPRO 50 with 50Mb/s from thetape that allows reducing the video compression and theuse of 4:2:2 sampling to give the better image qualityrequired for studio production. Four 16-bit, 48kHz audiotracks are provided.

HD-CIFSee Common Image Format

P2Solid-state recording system from Panasonic that recordsDV, DVCPRO and DVCPRO HD video onto flash memoryto offer advantages of speed and reliability over tape, butat a high cost and with shorter run times. Currentlyavailable P2 cards offer up to 8GB storage – enough forabout 40 minutes of DV, 20 minutes of DVCPRO 50, and 10minutes of DVCPRO HD. But the random access and ‘loop’recording possibilities mean this space is more useful thanthe equivalent length of tape. The workflow may includein-camera shot selection and very fast data dumping tohard disc storage for editing.

Your comprehensive guide to High Definition on a budget

Part Three

Understanding HD

6

Digital Film

Chapter 6


Digital film 30Understanding HD with Avid

In years gone by, many TV dramas, documentaries and‘soaps’ were produced on film. Today, not only is that isbecoming increasingly rare as HD and digital technologyshows many benefits in these areas, but the moviesthemselves are going digital. Shoots and cinemas may stilluse film but all the processes between increasingly aredigital. A number of movies have been shot on digitalcameras, including blockbusters such as Sin City and thelater Star Wars episodes, and the installation of digitalcinemas is gathering pace. Digital film has many crossoverswith television as well as its own standards and terminology.

So a television image and a film negative carry verydifferent information. While 10 bits (linear) is usually plentyto smoothly resolve all the contrast levels in TV, a filmnegative needs about 13 bits (linear). However as we candetect small brightness differences in darker areas andonly larger ones in bright areas, assigning more digitallevels to low light, and fewer to the highlights is a moreefficient way to use the available digital levels. This is whatthe ‘log’ sampling does.

See also: Quantizing (Video formats, colour space and sampling)

2KThis is a picture format generally used with imagesscanned from 35mm motion picture film, as well as aslightly different format for cinema exhibition. For theproduction side, it refers to 1536 lines each with 2048pixels and describes a 4 x 3 aspect ratio picture. Thesampling is 4:4:4 RGB with 10-bit log accuracy to carry thefull sharpness and contrast detail of 35mm negatives. Thisis not a television format but 35mm film is commonlyscanned to this resolution for use as ‘digital film’ for effectswork and, increasingly, to input to DI for grading, cuttingand mastering.

For publishing in television, a 16:9 (1080 x 1920), and a4 x 3 aspect ratio window can be selected from the 2Kmaterial for HD and SD distribution. The format is alsosuitable to support high quality transfers back to film orfor direct D-cinema exhibition. Just as with film, not allthe original image is shown on the screen. For digitalprojection 2K refers to a size or 2048 x 1080 lines, givinga wide aspect ratio display.

10-bit logWidely used for digitising film material, this usually refersto 10-bit sampling of an image, that describes 210 or 1024discrete numbers or brightness levels which havelogarithmic scaling – rather than the linear scale that isalways used in television. This highlights a major differencein the way that film and television material is shot. In film,the camera negative is designed to pick up as much detailas possible over a very wide brightness range of up to 11stops – equivalent to a contrast ratio of over 2000:1 andcapturing all detail from bright sunlit objects to down inthe shadows. This gives latitude for later adjustments andgrading before selecting the much more limited contrastrange used for the release print that gives a punchypresentation at the cinema.

In television it is always possible to see exactly what theimages look like and so any adjustments and selectionsare be made live while the camera is shooting. What yourecord is, essentially, what viewers see and this may be8 stops – a contrast range of 256:1 – but it looks greatat home.


D-cinema and E-cinemaD-cinema or Digital Cinema may involve the whole scene-to-screen production chain but it is usually refers to thedistribution and exhibition of cinema material, movies, bydigital means. There are no hard-and-fast rules about whatconstitutes D or E-cinema but some say D-cinema, imagesshould be 2K size or bigger. Smaller HD or SD formatsthen fall into the E-cinema category. Nonethelessaudiences have been generally impressed with the resultsfrom HD projections.

Digital presentations lack film weave, scratches, sparklesetc., to deliver a new standard of technical excellence tothe cinema screen and, unlike film, quality is maintainedregardless of the number of replays. Digital movies aredistributed by disks or over networks rather than on 35mmfilm that costs around $1000-2000 per copy which lasts onlyabout 200 passes through the projector. Copying anddistribution of film prints cost an estimated $800 millionper year, spent by studios.

E-cinema is currently further developed than D-cinema andalready has proven viable in a support role to the mainfeatures. It allows low cost production of local advertisingand promotions as well as the flexibility to easily add anyother TV-based content.

Among the necessary technologies, the recent rapiddevelopment of high-resolution, large screen digitalprojectors has made digital cinema exhibition possible.These are based on three technologies: D-ILA, DLPand SXRD.

D-cinema standards have recently been recommended byDigital Cinema Initiatives.

See also: DCI, DLP, D-ILA, SXRD

4KThis is a digital film production image format of 3072lines by 4096 pixels – four times the area of 2K. With eachimage producing about 32MB of data it requires apowerful workstation to play and process 4K footage inreal time. Also the storage requirement is massive.Despite the current technical challenges, a small butincreasing number prefer to work at 4K partly as it is seenas more future proof than 2K. Also some effects shots thathave to be seamlessly re-inserted back into a 2K moviemay be created at 4K. As the onward march of technologymakes 4K easier and less costly to use, so it will becomemore widely used as a digital film mastering formatalongside 2K.

CineAltaSony’s name for its family of products that bridgecinematography and HDTV and includes HDCAM-basedcamcorders and studio VTRs as well as extending to wholeproduction and post production systems. The more recentHDCAM SR series offers a more refined cine package withhigher recording data rates and direct access to theoriginal RGB images, rather than the ‘gamma corrected’images used for television.

See also: 24PsF

Dark chipSee DLP Cinema


DCIDigital Cinema Initiatives was set up in 2002 by a group ofmajor Hollywood studios to establish an open digitalcinema set of standards that ensures a uniform high levelof technical performance, reliability and quality control.The standard was completed in 2005 and is beingimplemented by various suppliers. Among a host of detailincluding security, its recommendations include 2K and 4Kimage formats and JPEG 2000 compression.

Digital Intermediate (DI)Digital Intermediate is a digital alternative to the traditionalphotochemical process that accepts original cameranegative (OCN) and produces the internegatives that makethe release prints of a movie. This has always included manystages of colour grading to match up all the shots seen inthe final release print. DI is increasingly accepted as thepreferable and path as, depending on the system used, itcan be instant, interactive, presented on a big screen, canhave audio and allows any grading changes right up to theoutputting the internegative film from the graded and editeddigital internegative. This way the grades are made on theedited material, complete with all effects shots, rather thanlooking at isolated individual shots. It is also possible tooutput fully graded whole reels, rather than applying furtherfinal when making the release prints.

DI starts with scanning the 35mm film. This is usually made at2K size using 10-bit log RGB (4:4:4) sampling to carry all thesharpness and contrast detail from highlights to deepshadows, from the film into the digits. The contrast latitude isneeded to allow headroom for onward grading. If usingfootage from a digital cinematography camera, the scanningoperation, which can be quite costly, is not needed.

www.dcimovies.comwww

Digital CinematographyDigital Cinematography refers to the use of electroniccameras in shooting material for movies. A number ofcameras have been designed specifically for this asalternatives to 35mm, including Viper (Thomson), CineAltarange (Sony) and DVCPRO HD (Panasonic). These produceHD formats, can run at 24P, capture a wider contrast rangethan TV cameras and do not use TV’s gamma correctioncurves. Origin (Dalsa) and D20 (ARRI) provide largerD-cinema sized images: Origin offers up to 4K and D20 3018x 2200 active pixels. The D20 also offers frame rates from 1-60fps. These cameras are designed as an alternative to 35mmmovie cameras however any video camera could be used.


D-ILADirect-Drive Image Light Amplifier. A technology thatuses a liquid crystal reflective CMOS chip for lightmodulation in a digital projector. In a drive for higherresolutions, the latest developments by JVC haveproduced a 2K (2,048 x 1,536) array, which is said tomeet the SMPTE DC 28.8 recommendation for 2000 linesof resolution for digital cinema.

The 1.3-inch diagonal, 3.1 million-pixel chip is addresseddigitally by the source signal. The tiny 13.5-micron pitchbetween pixels is intended to help eliminate stripe noiseto produce bright, clear, high-contrast images. This is anefficient reflective structure, bouncing more than 93percent (aperture) of the used light off the pixels.

See also D-cinema

DLPDigital Light Processing: Texas Instruments Inc digitalprojection technology that involves the application ofdigital micromirror devices (DMD) for television, includingHD, as well as cinema (see DLP cinema below). DMD chipshave an array of minute mirrors which can be angled by +/-10 degrees so as to reflect projection lamp light throughthe projection lens, or not. Since mirror response time isfast (~10 microseconds), rapidly varying the time ofthrough-the-lens reflection allows greyscales to beperceived. For video, each video field is subdivided intotime intervals, or bit times. So, for 8-bit video, 256 greylevels are produced and, with suitable pre-processing,digital images are directly projected.

The array, which is created by micomachining technology,is built up over conventional CMOS SRAM addresscircuitry. Array sizes for video started with 768 x 576 pixels– 442,368 mirrors, for SD. The later 1280 x 1024 DMD hasbeen widely seen in HD and D-cinema presentations. Mostagree it is at least as good as projected film. TI expect tooffer an ‘over 2000-pixel wide’ chip in the near future.

While much interest focuses on the DMD chipsthemselves, some processing is required to drive thechips. One aspect is ‘degamma’: the removal of gammacorrection from the signal to suit the linear nature of theDMD-based display. Typically this involves a LUT (LookUp Table) to convert one given range of signal valuesto another.

See also: Gamma

www.jvc.com/profwww

www.dlp.comwww


DLP cinemaThis refers to the application of Texas Instruments’ DLPtechnology to the specific area of film exhibition. Hereparticular care is taken to achieve high contrast ratios anddeliver high brightness to large screens. The developmentof ‘Dark chips’ has played an important part by very muchreducing spurious reflected light from the digitalmicromirror devices. This has been achieved by makingthe chip’s substrate, and everything except the mirrorfaces, non-reflective. In addition, the use of normalprojection lamp power produces up to 12 ft/l light levelon a 60-foot screen.

See also: D-cinema, DLP

HD RGBTelevision usually uses 4:2:2 component video (Y,Cr,Cb).Slightly higher quality can be achieved through using RGBsampled at 4:4:4. Many of the digital cinematographycameras offer this type of output that can use linear or logsample scaling. The 1080 x 1920 HDTV image format isvery close to the 2K projected image size, so RGB HD canbe considered as a TV/film crossover format, able to takeadvantage of many of the economies and speed of TVequipment to produce ‘film’ quality results.

ILASee D-ILA

OCNOriginal Camera Negative has very high value and isdesigned to hold a very wide contrast range. It is alwayshandled with great care and, to avoid damage, as little aspossible. The onward path toward making a programmeinvolves either scanning the OCN and proceeding alongthe DI route, or copying to make an interpositive film, andso on into the photochemical intermediate chain.

SXRDSilicon X-tal Reflective Display (X-tal is short for crystal) isdigital projector display technology developed by Sony. Itsfirst claim to fame was that it provided the first viable 4K(4096 x 2160 pixels) size as incorporated in Sony SXRDprojectors. The design of this reflective liquid crystalmicrodisplay is also aimed to provide for enhancedcontrast, speed allowing up to 200 fps and minimisesimage smear, and offering extended service life.

7Post production

and editing

Chapter 7


Post production and editing 36Understanding HD with Avid

Shot selection and editing for film and video arenow undertaken using nonlinear editing systems.Post production has grown immensely inimportance with the advent of highly-capableonline digital equipment and nonlinear editing.Now it is often cheaper to ‘fix it in post’ ratherthan spend extra time on another take on the set.

Blue screenShooting items against a blue background or screen allowsthem to be cut out and keyed onto other backgrounds.The blue is normally chosen as being unique in the pictureand not present in the foreground item to be keyed. Thisshould enable easy and accurate derivation into a keysignal used to cut out the object. Consideration may alsobe given to the colour spill onto the object’s edges. So, forexample, if the object is set into a forest, maybe a greenscreen would be preferred. Modern colour correction andkey processing allow a wider choice of colour and thepossibility of correcting for less-than-perfect shoots.However, this will increase post production time and effort.

The accuracy of the key signal derived from blue screenshots depends on the accuracy and resolution of colourinformation. Unlike SD, where the popular Digital Betacamor DVCPRO 50 records 4:2:2 sampled video using only 2:1or 3:1 compression, most HD recorders do not offerequivalent quality with the 100-140Mb/s camcorders,where restrictions in chrominance bandwidth can limit theeffectiveness of HD key. The notable exception is HDCAMSR, offering up to 440Mb/s with 10-bit 4:2:2 (4:4:4 possibletoo) sampling with ‘lossless’ compression.

ContentAny material completed and ready for delivery to viewers.Content is the product of applying metadata to essence(for TV, video and audio).

See also: Metadata

http://www.panasonic.comwww

AAFAdvanced Authoring Format. This is an industry-driven,open standard for the multimedia authoring and postproduction industries which is supported by manycompanies, including Avid. It is intended to enable contentcreators to easily exchange complete digital media –video, audio and metadata – across platforms andbetween applications. It simplifies project management,saves time and preserves valuable metadata that was oftenlost in the past during media transfers.

It is in editing and post production areas that the metadataload is greatest and individual systems and applicationshave become isolated by incompatibilities,: so limitingtheir interaction, interoperability and usefulness. Use of theAAF file format allows the passage of full informationbetween AAF-enabled applications. Thus video, audio andmetadata, with the decisions about how material has beenmanipulated (cuts, DVE, colour correction etc.) andassembled – a complete, modern-day EDL – can always beavailable and, where needed accessed. The metadata alsopasses on existing, original information such as timecodeor edgecode, ownership, previous editing etc. that helpswith any later archive retrieval and versioning.

See also: EDL, MXF, OMFI


CSOColour Separation Overlay. Another name for chromakeying.

See also: Keying

DS NitrisDS Nitris is Avid Technology’s flagship effects and editingsolution for HD and film resolutions. It was launched inSeptember 2000 and based on the successful V4 releaseof DS (Digital Studio) code. The original version had nohardware acceleration and was entirely software basedwith the exception of input/output operations, but theNitris DNA hardware offers powerful hardwareacceleration, while still benefiting from the continuingdevelopment of faster CPUs.

The system is well supported by nearly all plug-inmanufacturers and is resolution-independent. It alsosupports the transparent import of multi-layered effect-based OMF files from products such as Avid MediaComposer and Digidesign’s ProTools to provide anefficient link between off-line and on-line operations.

DTF/DTF2Name for Sony’s half-inch Digital Tape Format which offershigh data storage capacity (up to 200GB) on half-inch tapecartridges. Such stores are often used for storing digitalvideo – such as HD – in post production areas, where theymay be available to clients on a network.

Chroma KeyingThe process of deriving and using a key signal formedfrom areas of a particular colour in a picture (often blue,sometimes green).

See also: Keying

Colour correctionHistorically this is the process of adjusting the colours in apicture so that they match those from other shots or createa particular look.

Colour correction in HD and SD television has becomehighly sophisticated. This can include secondary colourcorrection that can be targeted at specific areas of picturesor ranges of colour. So, for example, a blue car in acommercial can be changed to red. Depending onequipment, operation can be real-time and interactive;enabling fine adjustments to achieve precise results in ashort time.

Compositing(a.k.a. Vertical Editing)The process of adding layers of moving (or still) video toassemble a scene. This involves many tools such as DVE(sizing and positioning), colour correction and keying. As theoperation frequently entails adding many layers, the work isbest suited to nonlinear equipment using uncompressedvideo to avoid generation losses. Techniques are now highlydeveloped and are a key part of modern production forboth film and television – cutting production costs andbringing new possibilities and new effects.


EDLEdit Decision List. This is data that describes how material isto be edited, e.g. from offline to online, or a record of whathappened in the editing process.

EDLs were devised before the days of nonlinear editing andwere never updated to take on board any of the digitalenhancements such as DVEs and advanced colour correctionand keying. Even so, they remain in wide use as a well-recognised means of conveying the more basic editingdecisions, cuts, dissolves, wipes, slo-mo, etc. Popular formatsare CMX 3400 and 3600.

More recently, new initiatives such as AAF and OMF offer thefar wider capabilities needed for today’s production needs.OMF has become a de facto standard for transferring fulldecision data between offline and online operations.

See also: AAF, OMF

EssenceTerm used to describe essential material which, fortelevision, is what appears on the screen and comes outof the speakers – video, audio and text. Essence consistsof those recorded elements that may be incorporated bymeans of editing, mixing or effects compositing into afinished programme (content).

See also: Content, Metadata

Gamma (correction)Gamma describes the difference in the brightness transfercurve characteristics between video source devices, suchas the CCDs in cameras, and the response of the displaydevices – usually considered to be cathode ray tubes.Gamma correction is normally applied early to the sourcevideo R, G, B signals as part of the processing in cameras.It is imposed here as it makes the video signal moreimpervious to atmospheric noise during ‘over-the-air’analogue transmissions. However, the more recent use ofother display devices – plasmas, LCDs and DLPs – withvery different technologies and gammas means that theymust again adjust gamma to match their transfercharacteristics. For example, DLP technology uses DigitalMicromirror Devices (DMDs) – millions of mirrors that areactually time-modulated. The amount of light they reflectonto the screen is a function of a duty cycle for time ‘on’.Thus, DLP systems program the display gamma for anygiven luminance level by adjusting the exposure time forthat level through a Look Up Table (LUT).

Gamma corrected colours or components are annotatedwith a prime, e.g., R´, G´, B´, and Y´, Cr´, Cb´. As virtually allmentions in this document involve gamma correctedsignals, the primes have not been included, for simplicity.

See also: DLP


GradingColour grading, also called colour correction, involvesadjusting the colour of recorded footage. This is highlyskilled work and depends on sensitive and very accurateadjustments. Traditionally, television has not had a use forgrading as all cameras are matched to make a TVprogramme, but when shooting over several days, withisolated (iso) cameras, or simply using footage from anumber of sources, grading becomes necessary so that allshots have the same colour look.

Primary grading is applied to whole frames. Secondarygrading involves adjusting the colour of a specific area of apicture. This could be to grade an object or to affect aspecified range of colours – perhaps to change seasons bymodifying the green leaves of spring to look like the huestones of autumn. Defining the area to be changed maywell involve using a key (see below)

KeyingA general term for the process of placing an object orsection of picture over another – as in keying text overvideo. This is a video version of matting in film but mayuse interactive tools and feature live operation.

Operation splits into two areas, deriving the key signal andapplying it to produce the keyed result. In HD’s high quality,big picture environment it is essential that keyed resultsare accurate and look convincing. Increasing use ofcompositing to add scenery, objects and actors to makefootage that the camera never saw, requires excellence inkeying so that the keyed items look ‘photo-real’ – like apart of the original image.

Keying tools have developed rapidly with the introductionof digital technology and online nonlinear editing. If workingwith electronically generated material, such as graphics orcaptions, the key signal is supplied along with the video.Otherwise sophisticated means are available to derive the

key signal. Typically, objects are shot against a blue or greenscreen and that colour then defines the key signal. In realitythe key colour spills onto the object so de-spill techniquesare applied. The boundary between the object andbackground is often the subject of much effort. It is rarelya hard cut (hard key), which tends to look jagged and false,but a carefully set up dissolve to render a smooth, natural-looking edge (shaped or linear key).

Further techniques are used to key semi-transparent materialsuch as smoke, fog, and glass. Often this uses a non-additivemix technique which apportions foreground and backgroundaccording to its luminance.

The availability of highly developed digital keying techniqueshas been a large factor in swinging motion picture effects intothe digital domain. Their excellence and efficiency haschanged the way many are made, cutting costs by simplifyingthe shoot and avoiding some expensive location work.

In digital systems, the key is a full-bandwidth signal (like Y,luminance), and is often associated with its foreground videowhen stored. Disk-based nonlinear systems can store andreplay this video-with-key combination in one operation,but it would take two VTRs.

See also: Blue Screen, 4:2:2:4, 4:4:4:4


Media ComposerThis series of non-linear editing systems has formed thecore part of Avid’s business over recent years. There aremany permutations of hardware platforms, video cards andbreakout boxes on both Apple Mac and PC formats. Seenas the de facto standard in editing for both on-line andoff-line, Media Composer has tens of thousands of usersworldwide and touches the vast majority of mainstreamfilm and television production.

See also: AVR

MetadataMetadata is data about data. Essence, or video and audio,is of little use without rights and editing details. Thisinformation also adds long-term value to archives.

Metadata is any information about the essence, forinstance how, when (timecode) and where it was shot,who owns the rights, what processes it has been, or shouldbe, subjected to in post production and editing, andwhere it should be sent next. Uses with audio aloneinclude AES/EBU with metadata to describe sample rate,also metadata in AC3 helps the management of lowfrequencies and creating stereo down-mixes.

Typically the audio and video essence is preserved as itpasses along a production chain, but the metadata isoften lost. Avid with OMF and the AAF Association haveboth done much to rectify this for the area of editing andpost production.

See also: AAF, Essence, OMF

MXFMaterial eXchange Format is standardised in SMPTE 377Mand supported by the Pro-MPEG Forum. It is aimed at theexchange of programme material between file servers,tape streamers and digital archives. It usually contains onecomplete sequence but this may comprise a sequence ofclips and programme segments.

MXF is derived from the AAF data model, integratesclosely with its files and so bridges the worlds of file-basedand streaming transfers. It helps to move material betweenAAF file-based post production and streaming programmereply over standard networks. This set-up extends thereliable essence and metadata pathways so that bothformats together reach from content creation to playout.

The MXF body carries content, which can include MPEG,DV and uncompressed video, and contains an interleavedsequence of picture frames, each with audio and dataessence, plus frame-based metadata.

www.pro-mpeg.orgwww

Non-additive mixSee Keying


OMFIOpen Media Framework (OMF) or Open Media FrameworkInterchange (OMFI) is a platform-independent file formatintended for transfer of digital media between differentsoftware applications and equipment. Besides sending thevideo and audio, the transfers can include metadata aboutthe content and what editing and other processes it hasbeen through. It is used by Avid products, Final Cut Pro,Pro Tools and others. It is the basis for the AAF.

Photo realTerm to describe effects-generated material that looks as ifit originated from a camera. This may apply to computer-generated objects or to items shot on camera andcomposed into the picture. Here, attention to detail suchas shadows and reflections as well as keying are neededto maintain the illusion. Achieving such quality at HD andfilm resolutions is all the more demanding as their bigger,sharper displays make detail, including errors, easier to see.

See also: Keying

Plug-insA generic term for software applications that can beadded to existing applications to enhance theirfunctionality. Nonlinear video and audio systems are oftenexpanded with new effects or functionality via plug-ins.

SymphonyAvid’s Symphony is a pure editing and finishing tool withreal-time effects processing which offers advanced primaryand secondary colour correction, captioning and titles.Initially working only at SD, its universal mastering allowsusers to generate both 525/50 and 625/50 version of anedit in real-time from a 24P master.

Symphony’s real-time uncompressed performance isextended to HD with Avid Nitris DNA hardware. SymphonyNitris systems combine Symphony’s full finishing toolset toprovide real-time uncompressed HD and SD performanceusing Avid Nitris DNA hardware.

TimecodeTimecode is a 24-hour frame-accurate reference of hours,minutes, seconds and frames and fields designed fortelevision production use. For example 10:32:24:16

Typically it is recorded with the video and is the firstreference when logging and editing. EDLs run ontimecode. It is relatively straightforward in the 25/50Hzframe-rate world but gets a lot more complicated in the30/60Hz world where, for historic reasons, the wholenumber frame frequencies was offset by a factor of1000/1001 – hence 29.97 and 59.94Hz. To make up thetime to that of a whole 30 or 60Hz rate, one frame isdropped in every 1000. This ‘drop-frame’ is accountedfor in drop-frame timecode.

The Lifecycle of a Project:ShootingShooting HD on a budget – in practice

Shooting HD on a budget 2The Lifecycle of a Project: Shooting

By Chris Jones

HD is here. In fact, it’s been here for some time, invarious guises. But the big difference now is thatit’s affordable, and the distribution technologies(such as digital cinema and HD screens in thehome) are also appearing in the marketplace. Sowhat does HD actually mean? What does it mean toyou? How can you get the best balance betweenyour needs and your cashflow?

Let’s look at formats first, as this is the first HD quagmire.First off let’s be clear. HD is a video format. It’s just higherresolution than what we have previously been exposed to.The cameras are now starting to carry onboard processingthat produce aesthetically pleasing images that are closerto film, but it’s still different. At the higher end of the scaleis HDCAM, which does not use a Firewire interface to getfrom the tape to an Avid Xpress Pro. So if you are on abudget, HDCAM is out of the equation. As we slide downthe scale there is DVCProHD, an excellent format that sadly,has failed to capture the consumer’s eye, partly because of itsprice tag. As we slide even lower, we reach HDV. If there wasa prosumer HD format war, (between DVCProHD and HDV)then HDV has won, mainly due to Sony and its aggressivepricing. I just bought myself an HDV Camcorder for £900 -now all my home movies are shot in HD! That is very cool. (Inreality, DVCProHD is a much higher spec format that is moresuited to higher end TV drama and low budget features).

Given that HDV (HD) cameras can also shoot DV (SD, shortfor standard definition, which for us is PAL), in my view buyinga DV camera now is a bad idea. This stance is strengthenedwhen you consider that most cameras can shoot HDV andthen internally down-convert to DV when playing out. Soshooting everything in HDV makes sense.

So what are the pro’s and con’s of HDV? Firstly, the camerasare newer technology, so it should be that little bit better.Everything from the audio encoding circuitry to the lensquality is improved over older versions, not to mentionbatteries being smaller and longer lasting. But what of theimage technology? HDV has a resolution of 1080 by 1920pixels, opposed to the SD PAL resolution of 768 by 576pixels. So you can see it produces roughly four times moreimage. That’s an awful lot more detail. And when you see itfor the first time on an HD monitor, it’s quite staggering.So the advantages are clear.

The disadvantages are less apparent though, and rumourand myth don’t help either. Looking at the facts, HDV usesthe same data rate as DV; that’s 25 Mb per second. Theinternal circuitry of the cameras have very fast processorsthat can handle that kind of data compression on the fly, andit also reduces the amount of colour information in the signal.Most importantly it encodes using MPEG technology, whichis something of a dark art as it doesn’t actually have definedfields and frames like standard DV. This means that it’sslightly more complicated in postproduction, though Avidnow supports HDV even from frame-accurate edits.

Such high compression can cause problems when shooting.It’s possible that when shooting material containing a lot ofdetail and movement, for instance panning around a stadiumfull of cheering people, that you may see some imagecompression artefacts. It’s also possible that when panningaround quickly, due to the way MPEG works, you may getsome kicks or juddering in the image. It’s also been said thatdrop-out on the tape can be disastrous. This may be thecase, but if you take care of your tapes, and use only newtapes, then this is unlikely to happen. I come from the daysof film when all sorts of problems could occur, so for everysingle shot we took, we would also do a second take, whetherwe needed it or not. We considered the job mission criticaland the technology NEVER 100% reliable. In fact, the moreI think about it, film and old TV cameras were extremelyrestrictive and limited, and the complaints I hear about


HDV are insignificant in comparison. All formats require careand attention when shooting, and HDV is no different.

All this brings me on to the primary problem with HDV. It’snothing to do with Avid, Sony, Panasonic, the format... It’sthe users. Years ago, film and TV was a very expensive andtechnical business. To be successful, film and programmemakers needed to be diligent, professional and educated.Now, anyone can pick up an HDV camcorder and be shootingin moments, with quite good results. Switch everything toauto and point the camera. But this approach, for anythingother than news coverage, can lead to disaster. In fact, theway the image is captured is less forgiving than film, and soeven greater care and attention must be taken unless youwant your footage to look like corporate video, or worse,home videos

Not all cameras are the same, either. The newer HDV andDVCProHD cameras all have true 16:9 CCD’s (the chips thatcapture the light from the lens) which frankly, is about time!So shooting SD footage in 16:9 is now better than ever.However, there are a number of other issues that are lessclear cut. First is progressive scan, or one of the aspectsthat yields a ‘film look’. Some cameras can handle a trueprogressive scan, where others can’t. Ideally, if you want thatsingle frame (as opposed to dual field look), get a camerawith true progressive scan. If not, you will need to shootinterlaced and create the look in postproduction.

The second issue is the camera and how it is set up torespond to light. Not all cameras have a great deal of controlover how they interpret light coming in through the lens -in general, the cheaper the camera, the less control it has.The result is that, all too often, HDV tends to look a lot likehi-res news material, or at best a TV soap opera. Somecameras do offer control over the image, and with these youneed to apply a set-up that captures quite flat, low-contrastimages. Detail in both the dark areas and the highlights arewhat will make your project look better once you get it intopost. You can always add contrast, but it’s hard to removeit without seeing crushed blacks and burnt out whites.

Remember, what you shoot on set is not how it will lookafter you have edited and graded on Avid. The image canbe graded and colour corrected hugely, as long as whatyou shoot contains the visual information in the first place.

The next step is lighting and what you actually choose toshoot. If your frame contains very dark areas and very lightareas, you may start to have problems - after all, you wantto keep the detail in the light areas and dark areas as well.The upshot is that to shoot HDV properly takes more timeand experience than it does to shoot film! Exposure iscritical. It’s also best to avoiding shots where the contrast istoo high. The nightmare situation is where an actor is inshadow, but a light is pointing right into camera and burningout to 100% white. The actor in shadow is so underexposedthat when it’s pulled up in post, the image has horrible milkyblacks with image compression artefacts, and the whites lookelectronic and harsh. In short, it looks awful. The biggestgiveaway to my eye is burnt out whites. So avoid brightskies, car headlights, film noir-like lighting (unless you canachieve the look with less contrast) etc. Of course, this isn’talways possible.

As the definition increased to HD, so did the need for higherquality lenses. The lens that comes with any HDV camera isalready pretty good, but you can make huge aestheticimprovements by using high quality prime lenses hookedup to a P + S Technic Mini 35 (of course you will need acamera that you can actually change lenses with). The Mini35 works by mimicking the depth of field look you get on35mm. Couple that with a prime lens that gives you a nicelong shot, and you can shoot quite startling images wherethe subject is beautifully cut out from a soft background.Very filmy and not at all like video. This look is one of themain differences between an amateur video look and profilm look. Video tends to hold everything in focus, and itcan give a cluttered and unattractive image. The Mini 35goes some way to reducing this.

Another bizarre problem with HDV is that the cameras tendto be small, and some actors who see the ‘big camera’ as


their audience can be put off. Or worse, without noticing,they may play the scene at a lower energy level. The sameis true of the crew. Of course, everyone will deny this, but Ihave seen it repeatedly. So get yourself some accessoriessuch as a big tripod and head, a big matte box and followfocus, so that your small camera looks worthy of the effortof the cast and crew.

A component of the project that is often forgotten is thesound. The capabilities of the digital audio tracks on an HDVcamera are impressive. The problem isn’t the format, it’s themicrophone (mic): how you choose to mic up for sound,and who is in charge of monitoring levels. You will need acamera with manual control over the recording level, thenideally (for drama) an external mixer like an SQN, calibratedto the levels on the camera, so that a separate soundrecordist and boom swinger can take care of sound withoutinterfering with the camera department. Of course this willmean your camera is permanently and umbilically connectedto the sound recordist, but this is the most cost effective wayof doing it. Your choice of mic is vital, as the mic on thecamera should only really be used for grabbing guide tracks.It is not good enough to capture good dialogue unless theperson is speaking directly to camera and projecting theirvoice. So while your camera can quite easily record excellentsound, getting that excellent sound to the camera in thefirst place is a much bigger job than you would imagine.Of all the things that are overlooked, sound is the biggest.

Considering all the work that goes into a film project - theplanning, the script writing, the casting etc - it’s astonishingto see filmmakers just throw around the tapes onto whichEVERYTHING is committed! Tapes are not indestructible.Treat them with care and attention and label them clearly.Ideally make a backup of every tape on the day that youshoot (ideally in the cutting room every night). It will onlytake one tape to get trodden on, dropped in coffee, getdamp, left in the sun on a dashboard etc., to spell completedisaster. Make backups, and store those backups somewhereelse, so that if your building burns down you still haveyour movie.

Don’t fall into the trap that somehow shooting HDV is as goodas, or the same as, film. It is not. It is different and requiresa different way of working. It has many advantages over film- less noise (grain), immediate playback, cheap running costs,sync sound, stable and sharp image (over Super 16mm) andso on - but on the downside, it has less latitude (that is theamount information between absolute black and absolutewhite), it does not have the same aesthetic as film (thoughthis can be mimicked to some degree in post), and mostsubtly, it often does not command the same ‘presence’ on set.

One last point about grading: To view HD properly, certainlyfor grading the image, you need an HD monitor capableof displaying exactly what you are getting in the image. Ofcourse, most people cannot afford one, nor the hardwareinterface to drive it. Some people choose to hook up a 16:9TFT monitor, such as the Apple Cinema display, but thisdoesn’t give a true and exact representation of the imagein terms of colour, hues and contrast. You would be betteradvised to down-convert all your material to SD on the fly(for example using the Avid Mojo DNA box), hooking up agraded monitor and doing all your grading in thatenvironment. Viewing SD while working in HD: It’s clearthat ‘offline’ and ‘online’ is becoming a thing of the past!

Chris Jones is a director/producer and co-founder of Living SpiritPictures Ltd, a UK-based film company which produces commercialfeature films for the international market place, writes the GuerillaFilm Makers series of books, and runs courses for film makers.www.livingspirit.com

The Lifecycle of a Project:The CameraShooting HD on a budget

Shooting HD on a budget 2The Lifecycle of a Project: The Camera

By Christina Fox and David Fox

HDV is to HDTV what DV is to StandardDefinition – the cheapest way to producereasonably high-quality pictures. If you use DValready, and want to move to high definition,HDV is the obvious upgrade path (there is analternative, as we'll mention later).

Even if you don't expect to need HD soon or are using anolder SD format, such as one of the Betacam variants, ifyou need a new camera then HDV is worth a look. Thecameras are widescreen native (16:9) and can record verygood pictures in SD, to DV or DVCAM (or downconvertHDV to SD), so there is no need for anamorphic adaptersor aspect ratio converters.

There are essentially three professional three-chip HDVcamcorders: the JVC GY-HD100/101, the Sony HVR-Z1 andCanon's XLH1. There are also a few single-chip HDVmodels worth considering (especially as a back-up camerathat can also be used as a low-cost play-in deck besideyour editing system).

There are also two varieties of HDV: interlaced orprogressive, with 1080 or 720 lines. Interlaced is thetraditional way of scanning video (it is how cathode raytube TV sets work), which displays each TV frame as twofields made up of the odd-numbered and even-numberedlines. Repeating an image 50 times per second, or 50i,gives a very natural-looking motion blur, because that isclose to how the eye works. PAL is 576 active lines at 50i,while NTSC is 480/60i. Progressive displays a full frame at atime, like film, and is how LCD screens work. If you want tomake your video look like film, then cameras that can do24p or 25p will have an advantage. The ideal, and whatEuropean broadcasters want for HD, is 50p, but this isn'tpart of the HDV standard. With HDV you currently get

either 1080 lines at 50i (Sony and Canon), or 720 lines at24/25 or 30p (JVC). Both 1080i and 720p are being used forHD transmission by broadcasters; so all HD equipmentshould cope with either, although most HDTV-ready setswill initially be 720 native rather than 1080.

HDV (and its problems) explainedHDV is an HD version of DV, but uses a different form ofcompression. In DV, each frame is individually compressed,making it easy to edit. To fit more picture information ontothe same miniDV tape, HDV uses MPEG-2 long GoPencoding. This means that the camera does not record everyframe of video as a full frame. It records occasional key framesand just enough other information to enable it to recreatethe rest. The video between one key frame and the next iscalled a Group of Pictures (or GoP), which consists of threetypes of frames - the I-frame, P-frame and B-frame (Intraframe, predictive frame and backwardly predictive frame).

A typical HDV GoP of 12 frames will be: I B B P B B P B B PB B, where only the I-frame holds the full picture information.In DV, a speck of dirt on the heads when recording couldresult in dropout problems on a single frame only. WithHDV, dirt causing dropout on an I-frame could affect half asecond of video, ie the whole GoP. To counter this, eitheruse fresh, new tapes (particularly the higher quality tapessold for HDV), or buy second-hand tapes from somewherethat cleans and assesses each one. We buy used tapes forabout £1.50 for a 60-minute DV tape that has been erased,cleaned (removing any loose iron oxide) and evaluated fordefects, including a printout of where they are – almostinvariably within the first or last minute, as that is wheretapes are fixed to the spool, so, whatever tape you buy,don't record anything valuable at the beginning or end.

Long GoP was designed for transmission rather thanediting, so it is not easy to edit it frame accurately.Fortunately the latest software, such as Avid Xpress Proand Avid Liquid, can now edit natively in HDV with frame


accurate edits. HDV also has a further problem, in that itisn't great at dealing with fast-moving detail where everypixel can change from frame to frame. This can result invisible artefacts around the fine detail (typically blockiness).If the camera and/or the fine detail is moving, this can belost in the motion blur, and might look OK.

We haven’t noticed any nasty artefacts on anything we'veshot on our Z1 or the other cameras we've used, and thereare a few techniques to avoid them:

1. Keep your lens angle wide if going handheld, becausetight angles accentuate any shakes, provoking artefacts.Otherwise use a tripod, monopod, Steadicam or othercamera support.

2. Limit depth of field to throw the background out offocus (so there is less detail for the compression to dealwith). To do this, you might need to invest in someexternal neutral density filters and/or graduated filters(alongside the camera's built-in filters), to reduce thelight entering the lens so that you can widen the irisand increase the depth of field. Cameras with smallCCDs (these all have one-third inch chips), deliver moredepth of field than those with large CCDs or 35mmfilm, so defocusing the background may require movingcloser to the subject, where a wide-angle adapter mayalso prove useful.

3. Do any important camera moves at several speeds, tosee which delivers the best results. Or do movesagainst less detailed backgrounds, such as blue skies orpainted walls, so that the compression has fewermoving details changing between each frame.

4. Shoot more cutaways, so you have room to manoeuvrein the edit suite.

5. The various "film" modes (especially that on the Z1)seem to exacerbate any artefacts, so don't use them -particularly if you are not shooting true film style (alllong, slow, fairly static shots).

The two versions of HDV also differ: HDV1's 720 linesrecord at 19Megabits per second, while HDV2's 1080 linesrecord at 25Mbps. Both are 4:2:0, which means that thereisn't as much colour information as some other formats.For comparison, DV, DVCAM and DVCPRO 25 record at25Mbps in 4:2:0 (or 4:1:1 for DVCPRO), DVCPRO 50 at50Mbps 4:2:2, and DVCPRO HD at 100Mbps 4:2:2.

The HDV image is 1440x1080 pixels (the same as the CCDsused in many other HD cameras), but the pixels arerectangular with an aspect ratio of 1.333:1, which makesthe 1440 equivalent to 1920 so you get 16:9 output.

The contendersLet’s look at the three main cameras in the professional HDVrange. They cost from approximately £3,000 to almost £6,000each, although extras such as batteries will increase this.

SONY HVR-Z1

JVC was first to bringout an HDV camcorder,releasing its single-chipJY-HD10 in 2004. Itfound favour with somecorporate producerswanting to display

material on a large screen, but it wasn't until the end of2004 that Sony brought out the first 3CCD model, theHDR-FX1. It delivers excellent pictures, but lacks the XLRsockets required of a professional camcorder, and isrelatively light on features. If you shoot sync sound on anexternal recording device, it will save a few hundredpounds compared to Sony's XLR-equipped model, theHVR-Z1. Since shipping in Spring 2005, it has become themost popular HDV camcorder, and is now widely used bythe BBC (although mainly as a widescreen DV camcorder).

Documentary evidence:Sony's HVR-Z1 camcorder


Anyone who has used a PD150 or PD170 will find the Z1 tobe a natural progression. This is primarily a documentarymaker's camera and records in HDV (1080i), DVCAM andDV formats.

Pros: Very nice lens, with good wide angle (4.5mm); abilityto creatively manipulate white balance; easilyswitched from PAL to NTSC, useful if you haveclients all over the world; and very good battery life.

Cons: No variable shutter, only stepped; can’t displaypeaking and zebra simultaneously; expanded focusonly works in standby, not in record mode; not asgood in low light as PD150/170 was; not designedfor use on your shoulder; and no interchangeablelenses - although Italian producer/cinematographer,Matteo Ricchetti (www.eidomedia.com), bravelydismantled his FX1 to fit a universal lens mount, so itcan be overcome. It's not worth using its CineFramefilm-look mode as you can probably achieve betterresults in post.

JVC GY-HD100/101

JVC's 3CCD HDV camera, theGY-HD100 went on sale inAugust 2005. The Europeanversion, the GY-HD101, isslightly more expensivebecause it has full FireWirei/o which attracts an EU levy.However, it is worth payingthe extra if you also want to

use the camera as an editing deck. The HD100 and 101 takeinterchangeable lenses, a desirable addition - assumingthere is money left in the budget to buy them.

It is made from die cast aluminium and is pretty robust.Uniquely in this group, it has a proper adjustable shouldermount, so, from a handling point of view, it should bemore comfortable to operate. It can be a bit front-heavy,but if you opt for an IDX or Anton/Bauer battery option, it

Getting the cine look: HD100 with cine lens and matte box

Steady state: JVC's HD100 in use on a Sachtler Artemis stabilisation system

A Mini35 adapter can allow the HD100 (or the other low-budget HD cameras) to work with 35mm lenses.

will help balance things out. If you are used to ENG-stylecameras like the DSR500/570 then you'll feel at home withthis camera and its lenses.

Pros: Ergonomic; good for the film look (as the CCDs areprogressive); lots of lens options; and you can storeyour menu settings on an SD card. For better qualityyou can access the camera's analogue HDcomponent 720p output signal, but need to put thisthrough an analogue HD to HD-SDI converter (whichcosts from around £750 to £2,000).

Cons: Standard (Fujinon16x) lens (5.5mm) not as wideangle as Z1 and exhibits some colour aberrations atthe edge of the frame; needs an upgraded batteryas the standard battery lasts only about an hour.


Canon XLH1

Canon's XLH1 would be an ideal choice for anyoneequipping an HD studio or OB on a budget, as it is thelowest cost way of getting full 1080i HD-SDI 4:2:2 output.This bypasses the HDV compression, and can be pluggedstraight into an HD vision mixer. It also does SDI (SerialDigital Interface) for SD. It will deliver high quality picturesfor live big screen presentations at concerts orconferences. The H1 can also be genlocked and havetimecode synched with other cameras, so it should behavenicely in multicamera set ups.

Its other big attraction is its interchangeable lens (althoughvery few XL1/XL2 owners ever took advantage of thatpreviously). It started shipping late 2005 in the US, and isdue to arrive in Europe in early 2006.

Pros: PAL/NTSC switchable (via option, which also gives60i, 24f and 30f); can shoot (and save on SD card)1920x1080 still images; very good standard 20x lens;lots of other lenses available; reportedly good in lowlight; useful black stretch function compared toXL1/2 (although that can be done in post); creativewhite balance; four audio channels. It can alsodeliver 24 or 25 frames per second images, althoughits isn't true progressive - this is why it is called 24for 25f as it uses a Frame mode that, in the case of24f, runs the CCDs at 48Hz, then de-interlaces usingCanon's own method. From the few examples we'veseen, it is certainly better than Sony's CineFrame,but may need further post-processing to achieve afilm look.

Cons: Has a 2.4-inch LCD combining the job of viewfinderand LCD screen - not as easy to see as its two rivalswhen handholding camera; doesn't sit particularlywell on the shoulder (front heavy); standard lens onlymarginally wider (5.4mm) than standard HD101 lens;and the 24/25f images will only playback properlyfrom the H1. A good deal more expensive than Z1.

One chip wonderIf you are on a morerestricted budget, Sonyhas two smaller, singleCMOS chipped HDVcameras, the"professional" HVR-A1E(with XLR inputs andcosting as little as £1,300plus VAT) and the HDR-

HC1E (a consumer version, without XLRs). The A1 wouldalso be ideal as both a back-up camera and to use as anedit deck as it costs less than dedicated HDV VTRs.

Single CCD (Charge Coupled Device) cameras are notusually recommended for professional use because individual

Back in black: the XLH1 is the most stylish HDV camera

Even lower budget HD:Sony's single-chip HVR-A1E


CCDs don't deal well with multiple colours. They aredesigned to convert light into an electrical signal and youideally need three – one each for red, green and blue lightto give both good colour accuracy and high resolution.

However, CMOS (Complementary Metal Oxide Semiconductor)sensors work differently. They allow more individual lightsensors per square centimeter than CCD, and offer a widerdynamic light range, for better detail in both shadows andhighlights, and are less susceptible to vertical smear. Theyoffer higher resolution (and good multi-resolution handling)at lower costs, which is why they have become the mainstayof many high-resolution digital stills cameras, shooting atresolutions way above HDTV. In the A1, having a single chipremoves the need for a bulky beam splitter (to split the lightinto red, green and blue), enabling it to be a lot smallerwithout compromising picture quality significantly. Certainly,if you have a limited budget and had been consideringbuying a second-hand DV camcorder, such as the Sony PD-150or 170, the A1 is a better option, as it is widescreen, can recordDV and DVCAM and produces surprisingly good results.

However, even if you are on a tight budget, if you require anHDV camera for a specific project you can save money bybuying a camera and then selling it upon completion; thedepreciation of a Sony Z1, for instance, over two months willalmost certainly be less than the cost of hiring it for the samelength of time (especially if you buy it on a 0% credit card).

HD but not HDVA budget alternative to HDV is Panasonic's new AG-HVX200,which is potentially better as it records using the DVCPROHD format (as well as in DVCPRO 25 and 50). Its otherinteresting feature is that it records onto P2 cards. These areremovable flash memory - which makes for a quicker workflowbecause there is no need to transfer from tape to computerhard drive for editing and they work very well with Avid.Unfortunately, P2 cards are very expensive (approx £1,000 foran 8GB card compared to a £2.75 miniDV tape). As P2 cards

David Fox is a freelance journalist, scriptwriter, producer and director, and

associate editor of TVB Europe magazine. Christina Fox is a broadcast

consultant and trainer, andmaintains thewww.UrbanFox.tv website, which

hasmore information on buying a low-budget broadcast camera, as well

as a guide to all the other camera kit you might need to go with it.

Card sharp: the HVX200 will record to P2 card, tape or add-on disk

come down in price, and go up in capacity, this could be avery interesting camera. However, anyone using it seriouslyfor HD should buy the upcoming Focus Enhancements'FireStore FS-100 hard disk recorder, which clips on theback and has native Pinnacle AVI and Avid OMF support,so you can plug it straight into your editor and startediting. There is also an FS-4 Pro HD for HDV camcorders.

The HVX200 records 1080i and 720p (including, notably,720/50p and 1080/25p). If it lives up to its promise, this willbe a strong alternative to HDV. It will probably be March2006, or later, before they go on sale - at least for theEuropean version.

Pros: DVCPRO HD is a more robust, less compressedformat than HDV, and gives a full 4:2:2 signal, so itrecords more colour information, making it easier touse for effects work, such as bluescreen composites.The camera also promises the widest-angle standardlens (4.2mm) and four-channel 16-bit PCM audio. Italso records DV to tape.

Cons: As expensive as the XLH1 if you buy a bundle withtwo 8GB P2 cards. Has only been seen as aprototype so far.

The Lifecycle of a Project:Editing with HDV

Shooting HD on a budget 2The Lifecycle of a Project: Editing with HDV

By Kevin Hilton

The fundamental form and principles of pictureediting have remained pretty much the same sincethe technique was first developed during the earlydays of film. However material is cut, whether onfilm, linear video or a nonlinear digital system, thenarrative and artistic principle is the same. Footageis assembled in order to tell the story, usingspecific cuts and devices for effect and to elicitan emotional response from the audience.

Like any other new acquisition technology HDV, the highdefinition format that is recorded onto mini digital videotapes, does not change the basic philosophy of editingthat was laid down in 1903 by Edwin Porter and was thenbuilt upon and turned into an art form by DW Griffith andEisenstein. What today's filmmakers and editors have to bearin mind is that a format such as HDV will cause problems inthe post-production process unless it is handled in a suitableform and domain.

HDV does not lend itself well to the post-production processdue to the technology that has made it attractive to lowbudget filmmakers, videographers, corporate productionsand the education sector: the Long-GOP (Group of Pictures)MPEG2 compression that enables HD 24p pictures to geton the DV tape in the first place. Multiple copying andcompositing contribute to image degradation, thus damaginga major selling point of HDV, the quality of its images.

A long-term user of Avid editing systems is Swedishdocumentary maker Loui Bernal, who sums up HDV well:"It doesn't have the clarity of other formats, but it doeshave the detail." Avid has worked to make HDV a properpost-production tool. The general opinion is that the bestway to edit HDV material is in its native form. Avid editingsystems support native HDV editing at 1080i 50/59,94

(Sony/Cannon) and at 720p 30 (JVC ProHD). That meansreal-time editing with no transcoding, real-time HD effects,and real-time Avid multicamera editing in HD.

Working in native HDV avoids the need for lengthytranscoding to intermediate formats, which takes up largeamounts of disk space. It also means no rendering isnecessary, with editing, effects creation and compositingbeing done in real-time.

Avid systems also allow HDV to be edited on the sametimeline with other video formats, both HD and non-HD.Again, there is no need for rendering or transcoding to getmaterial into the system. Clips are dragged and droppedonto the editing timeline in the usual way: it is possible tohave two kinds of HD, for example native HDV and DVCProHD, in the same file and edit the footage together. It is alsopossible to combine multiple HDV sources without firstconverting material to a common format.

As ever, opinions differ as to the best way to deal with HDV,and which forms of the format give the best results for thedifferent stages of the post-production process. A commonview is that native HDV is fine for straightforward offlinecontinuity editing, but when it comes to online functionssuch as colour correction, other formats are better suitedto avoid generational loss.

In such cases the commonly used argument is to convertHDV and any other acquisition format being used, be it P2or HDCAM, to a common platform. Whatever the editingformat chosen, there are many in the business who feel thatit should be uncompressed. Despite modern broadcastand post-production being increasingly digital and movingtowards high definition, standard definition SDI is stillfavoured as a solid platform on which to edit and finishmaterial drawn from different sources. Among the usefullittle boxes available for this conversion process isConvergent Design's HD-Connect LE unit. This takes HDVand converts it to SDI for use on any NLE editing system.

Avid's position is that the HDV format is fine for simplecutting, but the format can hinder efficient working due to


the Long-GOP compression used to reduce bandwidth onthe tape. Because of this the intervals between individualframes can be obscured, with the common problem of adecrease in image quality over a number of generations.Avid DNxHD encoding has been developed to keep theimage quality as it was shot, and can be used in thosesections of the programme where the post-productionprocess involves multi-layer and multiple generationcompositing, titles and graphics.

Mark Dyson, founder and head of factual programmes atCreative Touch Films, is now shooting in HD and HDV tosatisfy the demands of American broadcasters. CreativeTouch Films produces documentary series in which travellersand explorers journey to remote locations, very oftenenduring extreme cold or heat. Dyson is a long-term userof Avid systems and now works on Avid Xpress Pro. "In theearly days of HD and HDV I felt it was necessary to onlineedit using Avid Media Composer Adrenaline," he says."Now the software for Xpress Pro HD has been finalisedthe whole process can be kept within the desktop domain."

Problems continue to exist, even though HD and HDV givethe impression of being mature technologies. HDV isbarely three years old and is continuing to develop fromits consumer/prosumer beginnings and offer an even moreconvincing high-end product. Monitoring of HD in generalis a particular stumbling block, as Loui Bernal points out:"We've been able to work online with other formats but withHDV, nothing worked. When you're using high definitionyou can't watch it back, which is a problem when you'redoing colour correction.”

Wailing Banshee is a UK production company that has beenworking with HDV for approximately eight months, and istherefore familiar with all the arguments. Founded in 1997by video producer and director David Baumber, WailingBanshee specialises in digital production, with an animationdivision in New York. In the UK approximately 80 per cent ofthe company's work is in corporate production for blue chipclients such as Unilever, Lipton and BT. The remaining 20 percent comprises commercials, largely for theatrical release.

Both the corporate and commercials sectors are demandinghigher quality and resolution, with the latest effects andprocessing. Baumber comments that the requirement is tomaintain as high a quality as possible; DigiBeta has been afavoured format and now Wailing Banshee is making atransition to HD and HDV. "Corporate clients are nowbeginning to hire cinemas for their presentations, it's notalways in a hotel function room these days," Baumberobserves. "DigiBeta is good but higher resolution formatsare so much better, partly because it means we can use alot of chromakeying."

In the relatively short time that Wailing Banshee has beenusing HDV, Baumber says clients have been "amazed" atthe quality of the finished work. “When we put footage onthe internet it looks as though we went out with a 35mmfilm camera," he comments. "The cameras are very flexible.We've been using the JVC, and for TV and presentations itlooks like 16mm."

Baumber trained at the BBC as an Avid editor and so haslong experience of the business of editing and gettingmaterial in a form that can be cut. For some time Baumberhas worked with Avid Liquid Chrome, following it throughfrom its early days as a Pinnacle product to its presentposition as part of the Avid editing family. Material is fedfrom the camcorder into Liquid Chrome through the TARGA2000, and now the 3000, video capture card, using MotionJPEG encoding.

In Baumber's experience, having the right interface to getmaterial into the editing system is crucial. "Pinnacle had itsown interface and it was nice but it was never quite right,"he comments. "But TARGA is so powerful and when LiquidChrome was packaged with TARGA everything went to anew level. We're now using Chrome HD and in terms of thegraphical interface there's no difference between Chromeand Chrome HD. We're using it for both offline and online,sometimes three to four projects at the same time."

Material is fed into the editing systems over FireWire links,which Baumber sees as very easy to set up. "The nice thing


about the Liquid interface is that it is self-explanatory abouthow to get things in and out of the system," he says. "Fromthe control panel you can set up the disks to receive whateverinput you're working with." As for loading in HDV Baumbersays, "It's no different from any DVcam - there's nothingmysterious about it, although a lot more data is capturedat the same speed."

When it comes to the edit Baumber also says there is verylittle difference between working with HDV and DV. "Thereare more megabits in the signal, so storage is something tobear in mind, but there is no real difference in the editing,"he says. Wailing Banshee uses either eight Ultra 320 SCSIdrives in an EonStor cabinet striped as a single drive, or eightSATA drives. The eight Ultra 320s give 2TB of storage and areused for commercials work as that demands uncompressedvideo, while the SATAs give 250GB of space and tend tobe used more for corporate projects, where the use ofcompressed DV is common and not as much an issue as in TV.

Baumber says that once the footage is in the system, HDVgives "extra space to play with" if the final output is to be inSD. "That's largely why we're using HD at the moment," heexplains. "It's not because we want to create HD projects butbecause the technology gives us more flexibility for what wedo in SD. For example, a shot of a person might have beenframed from the waist up but in the edit we may want justthe face. With HDV we can zoom in and not lose definition."

Editing on HDV takes place in native format and Baumberis looking ahead to working with it on a Media ComposerAdrenaline system. "As Media Composer Adrenaline is a10-bit system we'll be able to get even better results,particularly for our TV work." As for editing HDV in general,Baumber observes, "It's a strange medium. The situation issimilar to when we moved from Betacam to Digicam. It'sbetter quality and because of that you have to cope with ahigher data rate and use high specification software forediting and outputting."

Baumber agrees that the fundamental principles and formsof editing remain the same whether material is on film,

DigiBeta, DV, HDCAM or HDV. "We let Avid decide whatgoes into the editing machine," he says. "What we and otherusers have to worry about is our own creativity." WhereBaumber sees the new HD formats as coming into theirown is for the processes involved in online editing. "Whenit comes to compositing, using HD technology for SDproduction is a godsend - it's really beautiful at the moment"he says. "Selfishly, I don't want anyone to buy HD televisionsbecause we will eventually lose the opportunity and abilityto use HD for SD, and we'll have to learn new tricks!"

As new, faster and higher resolution film stock came intouse, followed by the transition to video tape and then thesuccessive development of analogue and digital formats, itis likely that editors and directors over the years have hadsimilar feelings - that their knowledge base will have todevelop and change. HD transmission will bring an end tosome things, but the fundamentals remain and editing isone of them, regardless of format.

Contact Kevin Hilton at [email protected]

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