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Multimedia Systems
Video I
(Basics of Analog and Digital Video)
Mahdi Amiri
April 2011
Sharif University of Technology
Course Presentation
Page 1
VideoVisual Effect of Motion
The visual effect of motion is due to biological phenomenons
1. Persistence of vision
An object seen by the human eye remains mapped
on the eye’s retina for a brief time after viewing
(approximately 25 ms)
2. Phi phenomenon
When two light sources are close by and they are
illuminated in quick succession, what we see is not two
lights but a single light moving between the two points
(perceiving movement)
Due to the above two phenomena of our vision system, a
discrete sequence of individual pictures can be perceived as a
continuous sequence
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Page 2
VideoDefinition
A sequence of still images representing scenes in motion.
Frame Rate
Number of still images/pictures per unit time
Frames / Second (fps)
Frame rate of video ranges from 6 to 8 fps
for old mechanical cameras to 120 or
more fps for new professional cameras.
The minimum frame rate to achieve
illusion of a moving image is 15 fps.
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Frame N-1
Frame 0Time
Page 3
Video RepresentationVideo Display
In conventional TV sets or monitors, the video signal is displayed using a
CRT (Cathode Ray Tube).
An electron beam sweeps the screen from top to bottom beam carrying the
corresponding pattern information, such as intensity in a viewed scene.
Video can be interlaced or progressive ( Are described in the following slides ).
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Page 4
Video DisplayProgressive Scanning
Flicker free at around 50 frames per second (fps)
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Flicker is a visible fading between
cycles displayed on video displays,
especially the refresh interval on
cathode ray tube (CRT) based
computer screens.
Flicker and Bandwidth
Page 5
Video DisplayInterlaced Scanning
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
First the solid (odd) lines are traced, P to Q,
then R to S, etc., ending at T; then the even
Field starts at U and ends at V.
The jump from Q to R, etc. is called the
horizontal retrace, during which the
electronic beam in the CRT is blank.
The jump from T to U or V to P is called
the vertical retrace.
Electronic signal for
one NTSC scan line.
Flicker free at around 25 frames per second (fps)
Page 6
Video DisplayOdd and Even Fields
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Odd
Field
Even
Field
Difference
of Fields
A Video Frame
Because of interlacing, the odd and even lines are displaced in
time from each other - generally not noticeable except when
very fast action is taking place on screen, when blurring may
occur.
Initially the odd-numbered lines are scanned and then the
process is repeated for even-numbered lines - this time
starting at the second row.
Page 7
Video DisplayDeinterlacing
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
When interlaced video is watched on a
progressive monitor with very poor
deinterlacing, it exhibits combing when there
is movement between two fields of one frame.
Deinterlacing is the process of converting interlaced video, such as
common analog television signals into a non-interlaced form.
Method 1: Capturing one field and combining it with the next field
Problem: "combing" effect
Method 2: Line doublerThe most basic and literal way to double lines is to repeat each
scanline, though the results of this are generally very crude.
Most line doublers use digital interpolation to recreate the
missing lines in an interlaced signal, and the resulting quality
depends on the technique used. Generally a line doubler will
only interpolate within a single field, rather than merging
information from adjacent fields, to preserve the smoothness
of motion, resulting in a frame rate equal to the field rate.
Serrated image
Page 8
Analog Broadcast TV SystemsNTSC
NTSC (National Television System Committee)
Mostly used in North America and Japan
Aspect Ratio: 4:3
525 scan lines at 30 fps
Interlaced scanning (262.5 lines/field)
Color Space: YIQ
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Page 9
Analog Broadcast TV SystemsYIQ Color Space
In the YIQ color system, the I axis runs from cyan to
orange, and the Q axis runs from green to violet.
Eye is most sensitive to Y, next to I, next to Q.
Bandwidth allocation for color components
4 MHz is allocated to Y,
1.5 MHz to I,
0.6 MHz to Q.
When compared to PAL in particular, NTSC color
accuracy and consistency is sometimes considered
inferior, leading to video professionals and television
engineers jokingly referring to NTSC as Never The
Same Color, Never Twice the Same Color, or No True
Skin Colors,[16] while for the more expensive PAL
system it was necessary to Pay for Additional Luxury.
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Page 10
Analog Broadcast TV SystemsPAL
PAL (Phase Alternating Line)
Mostly used in Western Europe, China and India
Aspect Ratio: 4:3
625 scan lines at 25 fps
Interlaced scanning (312.5 lines/field)
Color Space: YUV
5.5 MHz is allocated to Y, 1.8 MHz each to U and V.
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Page 11
Analog Broadcast TV SystemsSupplementary Materials
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The horizontal sweep frequency is
525×30 ≈ 15,750 lines/sec, so that each
line is swept out in 1/15,750 sec ≈
63.5μsec.
A typical waveform of a NTSC composite video signal
Vertical retrace takes place during 20
lines reserved for control information at
the beginning of each field. Hence, the
number of active video lines per frame is
only 485.
Similarly, almost 1/6 of the raster at the
left side is blanked for horizontal retrace
and sync. The nonblanking pixels are
called active pixels.
NTSC video is an analog signal with no
fixed horizontal resolution. Therefore one
must decide how many times to sample
the signal for display: each sample
corresponds to one pixel output.
NTSC Video Signal
Page 12
Analog Broadcast TV SystemsSupplementary Materials
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Spectrum of a System M television channel with NTSC color.
An NTSC television channel as transmitted occupies a
total bandwidth of 6 MHz
Modulation of NTSC
The actual video signal, which is amplitude-modulated, is
transmitted between 500 kHz and 5.45 MHz above the
lower bound of the channel. The video carrier is 1.25 MHz
above the lower bound of the channel. Like most AM
signals, the video carrier generates two sidebands, one
above the carrier and one below. The sidebands are each
4.2 MHz wide. The entire upper sideband is transmitted,
but only 1.25 MHz of the lower sideband, known as a
vestigial sideband, is transmitted.
The color subcarrier, as noted above, is 3.579545 MHz
above the video carrier, and is quadrature-amplitude-
modulated with a suppressed carrier.
The audio signal is frequency-modulated.
Page 13
Analog Broadcast TV SystemsSupplementary Materials
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Chroma Modulation of NTSC
In NTSC, chrominance (C) is encoded using two
3.579545 MHz signals that are 90 degrees out of phase,
known as I (in-phase) and Q (quadrature) QAM. These
two signals are each amplitude modulated and then
added together.
In the ideal case I(t) is demodulated by multiplying the
transmitted signal with a cosine signal.
0 0cos 2 sin 2C t I t f t Q t f t
0 is the carrier frequencyf
Chroma Demodulation of NTSC
0
2
0 0 0
0 0
0 0
2cos 2
2 cos 2 2 sin 2 cos 2
1 cos 4 sin 4
cos 4 sin 4
i
i
i
i
r t C t f t
r t I t f t Q t f t f t
r t I t f t Q t f t
r t I t I t f t Q t f t
Low-pass filtering ri(t) removes the high frequency terms
(containing 4πf0t), leaving only the I(t) term.
Similarly, Q(t) can be extracted by first multiplying C(t)
by 2sin(2πf0t) and then low-pass filtering.
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Analog Broadcast TV SystemsSupplementary Materials
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Spectrum of a System I television channel with PAL color.
The basics of PAL and the NTSC system are very similar;
a quadrature amplitude modulated subcarrier carrying the
chrominance information is added to the luminance video
signal to form a composite video baseband signal.
Modulation of PAL
In order to improve picture quality, chroma signals have
alternate signs (e.g., +U and -U) in successive scan lines,
hence the name “Phase Alternating Line”. This facilitates
the use of a (line rate) comb filter at the receiver —the
signals in consecutive lines are averaged so as to cancel
the chroma signals (that always carry opposite signs) for
separating Y and C (chroma) and obtaining high quality
Y signals;
However, this resulted in a comblike effect known as
Hanover bars on larger phase errors. Thus, most receivers
now use a chrominance delay line, which stores the
received color information on each line of display; an
average of the color information from the previous line
and the current line is then used to drive the picture tube.Hanover bars Cancellation of Hanover bars
through a chroma delay line
Page 15
Analog Broadcast TV SystemsSupplementary Materials
SECAM stands for Système Electronique Couleur Avec
Mémoire, the third major broadcast TV standard.
Aspect Ratio: 4:3
625 scan lines at 25 fps, Interlaced scanning
SECAM and PAL are very similar. They differ
slightly in their color coding scheme:
In SECAM, U and V signals are modulated using separate
color subcarriers at 4.25 MHz and 4.41 MHz respectively.
They are sent in alternate lines, i.e., only one of the U or V
signals will be sent on each scan line.
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Analog Broadcast TV SystemsSupplementary Materials
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.
More at
http://en.wikipedia.org/wiki/Broadcast_television_systems
Page 17
Analog Color VideoSignal Protocols
Composite video
1956
1 wire
Color (chrominance) and luminance
signals are mixed into a single carrier wave.
Since color and intensity are wrapped
into the same signal, some interference
between the luminance and chrominance
signals is inevitable.
Composite video jacks are often grouped
with corresponding stereo audio jacks (the
composite video jack is usually yellow)
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Page 18
Analog Color VideoSignal Protocols
S-Video (Separated video, e.g., in S-VHS)1979
2 wires, one for luminance and another for composite
chrominance signal
As a result, there is less crosstalk between the color information
and the crucial gray-scale information.
A compromise between component analog video and the
composite video.
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
A standard 4-pin S-Video cable
connector, with each signal pin (3, 4)
paired with its own ground pin (1,2)
Page 19
Analog Color VideoSignal Protocols
Component video1990
3 wires
Each primary is sent as a separate video signal. The primaries
can either be RGB or a luminance-chrominance transformation of
them (e.g., YPbPr, YIQ, YUV).
Y: Green, Blue: Pb, Red: Pr
Best color reproduction
Requires more bandwidth and good
synchronization of the three components
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More about signal protocols at
http://en.wikipedia.org/wiki/List_of_display_interfaces
YPbPr is analog
form of YCbCr
Page 20
Digital VideoAdvantages
Video can be stored on digital devices or in memory,
ready to be processed (noise removal, cut and paste, etc.),
and integrated to various multimedia applications.
Direct access is possible, which makes nonlinear video
editing achievable as a simple, rather than a complex, task.
Repeated recording does not degrade image quality.
Ease of encryption and better tolerance to channel noise.
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Page 21
Digital VideoHDTV vs. Conventional TV
HDTV has higher resolution
1280 × 720 or 1920 × 1080.
HDTV has a much wider aspect ratio of 16:9 instead of
4:3.
16:9 is closer to aspect ratio of the human eye sight
HDTV moves toward progressive (non-interlaced) scan.
The rationale is that interlacing introduces serrated
edges to moving objects and flickers along horizontal
edges.
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Page 22
Digital VideoHDTV Display Resolutions
720p
Referred to in marketing materials as “HD”
1280×720, progressive scan
0.9 megapixels
1080p
Referred to in marketing materials as “Full HD”
1920×1080, progressive scan
2.0 megapixels
1080i
1920×1080, interlaced scan
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I
Aspect Ratio
for all is 16:9
(W:H)
Page 23
Digital Color VideoSignal Protocols
DVI (Digital Visual Interface), 1999
Analog and Digital
2560 × 1600 @ 60 fps
3840 × 2400 @ 33 fps
Max. 1.65 Gbit/sec
HDMI (High-Definition Multimedia Interface), 2003
Uses a pixel based data stream
2560 × 1600 @ 75 fps
4096 × 2160 @ 24 fps
Max. 1.65 Gbit/sec
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Page 24
Digital Color VideoSignal Protocols
DisplayPort, 2007
Uses a packetized data protocol often used in high-speed data communications.
This provides a faster data rate over the same number of wires.
2560 × 1600 @ 75
1.6 or 2.7 Gbit/sec
Multimedia Systems, Spring 2011, Mahdi Amiri, Video I