1FUNDAMENTALS OF

MONOCHROME AND COLOUR TV SYSTEM

Picture formation

A picture can be considered to contain a number of small elementary areas of light or shade which are called PICTURE ELEMENTS. The elements thus contain the visual image of the scene.

In the case of a TV camera the scene is focused on the photosensitive surface of pick up device and a optical image is formed. The photoelectric properties of the pick up device convert the optical image to a electric charge image depending on the light and shade of the scene (picture elements). Now it is necessary to pick up this information and transmit it. For this purpose scanning is employed. Electron beam scans the charge image and produces optical image. The electron beam scans the image line by line and field by field to provide signal variations in a successive order.

The scanning is both in horizontal and vertical direction simultaneously.

The horizontal scanning frequency is 15,625 Hertz.

The vertical scanning frequency is 50 Hz.

The frame is divided in two fields. Odd lines are scanned first and then the even lines. The odd and even lines are interlaced. Since the frame is divided into 2 fields the flicker reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the same as power supply frequency).

Number of TV Lines per Frame

If the number of TV lines is high larger bandwidth of video and hence larger R.F. channel width is required. If we go for larger RF channel width the number of channels in the R.F. spectrum will be reduced. However, with more no. of TV lines on the screen the clarity of the picture i.e. resolution improves. With lesser number of TV lines per frame the clarity (quality) is poor.

A compromise between quality and conservation of r.f. spectrum led to the selection of 625 lines in CCIR system B. Odd number is preferred for ease of sync pulse generator (SPG) circuitary to enable interlace of fields.

The capability of the system to resolve maximum number of picture elements along scanning lines determines the horizontal resolution. It means how many alternate black and white elements can be there in a line. Let us also take another factor. It is realistic to aim at equal vertical and horizontal resolution. We have seen earlier that the vertical resolution is limited by the number of active lines. We have already seen that the number of active lines are 575. so for getting the same resolution in both vertical

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and horizontal directions the number of alternate black and white elements on a line can be 575 multiplied by Kell factor and aspect ratio. Therefore, the number of alternate black and white dots on line can be 575 x 0.69 x 4/3 which is equal to 528.

It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264 cycles are there during 52 micro seconds. Hence the highest frequency is 5 MHz.

Therefore the horizontal resolution of the system is 5 MHz.

A similar calculation for 525 lines system limits the highest frequency to 4 MHz and hence the horizontal resolution of same value.

In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz.

Viewing Distance

Optimum viewing distance from TV set is about 4 to 8 times the height of the TV screen. While viewing TV screen one has to ensure that no direct light falls on the TV screen.

Composite Video Signal (CVS)Composite Video Signal is formed with Video, sync and blanking signals. The

level is standardized to 1.0 V peak to peak (0.7 volts of Video and 0.3 volts of sync pulse). The composite video signal (CVS) has been shown in figure 1.

Fig. 1 Composite Video Signal (CVS)RF Transmission of Vision and Sound Signals

TV Transmission takes place in VHF Bands I and III and UHF Bands IV and V.

Picture is amplitude modulated and sound is frequency modulated on different carriers separated by 5.5 MHz.

Also for video amplitude modulation negative modulation is employed because of the following main advantages.

Pictures contain more information towards white than black and hence the average power is lower resulting in energy saving. (Bright picture points

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correspond to a low carrier amplitude and sync pulse to maximum carrier amplitude).

Interference such as car ignition interfering signals appear as black which is less objectionable.

Picture information is in linear portion of modulation characteristic and hence does not suffer compression. Any compression that may take place is confined to sync pulse only.

The design of AGC circuit for TV Receiver is simpler.

AM produces double side bands. The information is the same in both side bands. It is enough to transmit single side band only. Carrier also need not be transmitted in full and a pilot carrier can help. However, suppressing the carrier and one complete side band and transmitting a pilot carrier leads to costly TV sets. A compromise to save RF channel capacity is to resort to vestigial side band system in which one side band in full, carrier and a part of other side band are transmitted.

Fig. 5 Theoretical representation of the side bands in VSB transmission.

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THE PAL COLOUR TELEVISION SYSTEM

The Colour Television

It is possible to obtain any desired colour by mixing three primary colours i.e. Red, Blue and green in a suitable proportion.

Additive Colour Mixing

The figure 10 shows the effect of projecting red, green, blue beams of light so that they overlap on screen.

Y= 0.3 Red + 0.59 Green + 0.11 Blue

Fig. 10 Additive Colour Mixing

The Colour Television

It is possible to obtain any desired colour by mixing three primary colours i.e., red, blue and green in suitable proportion. Thus it is only required to convert optical information of these three colours to electrical signals and transmit it on different carriers to be decoded by the receiver. This can then be converted back to the optical image at the picture tube. The phosphors for all the three colours i.e. R, G and B are easily available to the manufacturers of the picture tube. So the pick up from the cameras and output for the picture tube should consists of three signals i.e. R, G and B. It is only in between the camera and the picture tube of the receiver we need a system to transmit this information.

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Colour television has the constraint of compatibility and reverse compatibility with the monochrome television system which makes it slightly complicated. Compatibility means that when colour TV signal is radiated the monochrome TV sets should also display Black & White pictures. This is achieved by sending Y as monochrome information along with the chroma signal. Y is obtained by mixing R,G & B as per the well known equation :

Y = 0.3 R + 0.59 G + 0.11 B

Reverse compatibility means that when Black & White TV signal is radiated the colour TV sets should display the Black & White pictures.

If we transmit R, G, B, the reverse compatibility cannot be achieved. Let us see how :

If we transmit Y, R & B and derive G then :Since Y = 0.3R + 0.59G + 0.11 B

G = 1.7Y - 0.51 R - 0.19 B

In such a case what happens with a colour TV set when we transmit black and white signal. R and B are zero, but G gun gets 1.7 Y. The net result is black & white pictures on a colour TV screen appear as Green pictures. So reverse compatibility is not achieved.

Colour Difference Signals

To achieve reverse compatibility, when we transmit Y, R-Y and B-Y instead of Y, R & B, we do not take G-Y as this will always be much lower than R-Y and B-Y and hence will needs more amplification and will cause more noise into the system. G-Y can be derived electronically in the TV receiver.In the previous paragraph we have seen

G = 1.7 Y - 0.51 R - 0.19 BSo G-Y = -0.51 (R-Y) - 0.19 (B-Y)Thus, colour difference signals fulfill the compatibility and reverse compatibility. Because in this case the colour difference signals are zero if the original signal is monochrome (i.e. R = B = G)So if we take R - Y

R - Y = R - (0.3 R + 0.59 R + 0.11 R) = 0Similarly B - Y = 0As such colour difference signals are zero for white or any shade of gray whereas, Y carries the entire Luminance information.

It is to be noted while R, G, B signals always have positive value R-Y, B-Y and G-Y signals can either be positive or negative or even zero.

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The R-Y and B - Y chrominance signals may be recovered at the television receiver by suitable synchronous demodulation. But sub-carrier is to be generated by a local oscillator. This generated sub-carrier in the receiver must have same frequency as that of transmitted sub-carrier and also the same phase. This is achieved by transmitting 10 cycles of sub-carrier frequency on the back porch of H synchronizing pulse. This 10 cycles sub-carrier signal is known as BURST or colour BURST.

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2VIDEO CHAIN IN A TYPICAL

DOORDARSHAN STUDIO

STUDIO CENTRE

A Studio centre of Doordarshan has the following objectives:

1) To originate programmes from studios either for live telecast or for recording on a video tape.

2) To knit various other sources of programs available at the production desk i.e., camera output from studios, feed from other kendras, outdoor, playback from pre recorded tape, film based programs slides, video graphics and characters generator etc. This knitting or live editing includes generation of special effects and desired transitions between various sources.

3) Processing/distribution of different sources to various destinations in technical areas.

4) Routing of mixed programme for recording/transmission via master switching room and Micro Wave to the transmitter or any other desired destinations.

Activities in a television studio can be divided into three major areas such as :

1) Action area,2) Production control room, and3) Central apparatus room,

Action area

This place requires large space and ceiling as compared to any other technical area. Action in this area includes staging, lighting, performance by artists, and arrangement to pick up picture and sound. Hardware required for these activities in a studio (typical size 20 x20x8.5 cubic meters) are:

1. Very efficient air conditioning because of lot of heat dissipation by studio light and presence of large number of persons including invited audience performing artists and operational crew.

2. Uniform and even flooring for smooth operation of camera dollies and boom microphone etc.

3. Acoustic treatment Keeping in mind that a television studio is a multi purpose studio with lot of moving person and equipment during a production.

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4. Supporting facilities like properties, wardrobe, and makeup etc.5. Effective communication facilities for the floor crew with the production control

area.6. Studio cameras (three to four) with one of the cameras fitted with teleprompter

system and pressure dolly.7. Luminaires and suspension system having grids or battens (hand/motorised

operation).8. Pick up wall sockets for audio operations.9. Tie lines box for video and audio lines from control room10. Cyclorama and curtain tracks for blue and black curtain for chroma keying and

limbo lighting respectively.11. Audio and video monitoring facilities.12. Studio warning light and safety devices like fire alarm system and fire fighting

equipments etc.13. Digital clock display.Operational requirement from the technical crew may vary from programme to programme. These requirements for lighting, audio pick up and special effects etc. depends upon the programme requirement such as establishing a period, time, formal or informal situation.

Production control area

Activities in this area are:-1. Direction to the production crew by the producer of the programme.2. Timing a production/telecast.3. Editing of different sources available at the production desk.4. Monitoring of output/off air signal.

Hardware provided in this area include:1. Monitoring facilities for all the input and output sources(audio/video).2. Remote control for video mixer, telecine and library store and special effect

(ADO) etc.3. Communication facilities with technical areas and studio floor.

Vision mixing and switching

Unlike films, television media allows switching between different sources simultaneously at the video switcher in Production control room operated by the Vision Mixer on the direction of the program producer. The producer directs the cameramen for proper shots on various cameras through intercom and the vision mixer (also called VM engineer) switches shots from the selected camera/cameras with split second accuracy, in close cooperation with the producer. The shots can be switched from one video source to another video source, superimposed, cross faded, faded in or faded out electronically with actual switching being done during the vertical intervals between the picture frames. Electronics special effects are also used now days as a transition between the two sources.

Vision Mixer (or Video Switcher)

Though the video switching is done by the VM at the remote panel, the electronics is located in CAR. The vision mixer is typically a 10 x 6 or 20 x 10 cross bar switcher

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selecting anyone of the 10 or 20 input sources to 6 or 10 different output lines. The input sources include: Camera 1, camera 2, camera 3, VTR1, VTR2, Telecine 1, Telecine 2, Test signal etc. The vision mixer provides for the following operational facilities for editing of TV programs:-(i) Take: Selection of any input source

orCut: switching clearly from one source to another.

(ii) DISSOLVE: Fading out of one source of video and fading in another source of video.

(iii) SUPERPOSITION OF TWO SOURCES: Keyed caption when selected inlay is superimposed on the background picture.

(iv) SPECIAL EFFECTS: A choice of a number of wipe patterns for split screen or wipe effects.

The selected output can be monitored in the corresponding pre-view monitor. All the picture sources are available on the monitors. The preview monitors can be used for previewing the telecine, VTR; test signals etc. with any desired special effect, prior to its actual switching.

The switcher also provides cue facilities to switch camera tally lights as an indication to the cameraman whether his camera is on output of the switcher.

Present day PCR’s have:

24 input video special effects switchers (CD 680 or CD 682-SP) Character generators Telecine/DLS remote controls Adequate monitoring equipment

Character Generator(CG)

Character Generator provides titles and credit captions during production in Roman script. It provides high resolution characters, different colours for colorizing characters, background, edges etc. At present bilingual and trilingual C.G are also being used by Doordarshan.

Character Generator is a microcomputer with Texts along instructions when typed in at the keyboard is stored on a floppy or a Hard disk. Many pages of scripts can be stored on the disk and recalled when needed, by typing the addresses for the stored pages, to appear as one of the video sources.

Sync Pulse-Generator(SPG)It is essential that all the video sources as input to the switcher are in synchronism i.e.,

start and end of each line or all the frames of video sources is concurrent. This requirement is ensured by the sync pulse generator (SPG). SPG consists of highly stable crystal oscillator. Various pulses of standard width and frequency are derived from this crystal electronically which form clock for the generation of video signal. These pulses are fed to all the video generating equipment to achieve this objective of synchronism. Because of its importance, SPG is normally duplicated for change over in case of failure.

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It provide the following outputs:

Line drive Field drive Mixed blanking Mixed sync colour subcarrier A burst insertion pulse PAL phase Indent pulses

Camera Control Unit (CCU)The television cameras which include camera head with its optical focusing lens,

pan and tilt head, video signal pre-amplifier view finder and other associated electronic circuitry are mounted on cameras trolleys and operate inside the studios. The output of cameras is pre-amplified in the head and then connected to the camera control unit (CCU) through long multi-core cable (35 to 40 cores), or triax cable.

All the camera control voltages are fed from the CCU to the camera head over the multi-core camera cable. The view-finder signal is also sent over the camera cable to the camera head view-finder for helping the cameraman in proper focusing, adjusting and composing the shots.

The video signal so obtained is amplified, H.F. corrected, equalized for cable delays, D.C. clamped, horizontal, and vertical blanking pulses are added to it. The peak white level is also clipped to avoid overloading of the following stages and avoiding over modulation in the transmitter. The composite sync signals are then added and these video signals are fed to a distribution amplifier, which normally gives multiple outputs for monitoring etc.

Light ControlThe scene to be televised must be well illuminated to produce a clear and noise free picture. The lighting should also give the depth, the correct contrast and artistic display of various shades without multiple shadows.

The lighting arrangements in a TV studio have to be very elaborate. A large number of lights are used to meet the needs of ‘key’, ‘fill’, and ‘back’ lights etc. Lights are classified as spot and soft lights. These are suspended from motorized hoists and telescopes. The up and down movement is remotely controlled. The switching on and off the lights at the required time and their dimming is controlled from the light control panel inside a lighting control room using SCR dimmer controls. These remotely control various lights are inside the studios.

Sound mixing and controlAs a rule, in television, sound accompanies the picture. Several microphones are generally required for production of complex television programs besides other audio sources also called marred sound from telecine, VTR, and audio tape/disc replays. All these audio sources are connected to the sound control console.

The sounds from different sources are controlled and mixed in accordance with the requirement of the program. Split second accuracy is required for providing the correct audio source in synchronisation with the picture thus requiring lot of skill from the engineer. Even the level of sound sometimes is varied in accordance with the shot composition called prospective.

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Audio facilitiesAn audio mixing console, with a number of inputs, say about 32 inputs is provided in major studio. This includes special facilities such as equalisation, PFL, phase reversal, echo send/receive and digital reverberation units at some places Meltron console tape recorders and EMI 938 disc reproducers are provided for playing back/creating audio effects as independent sources (Unmarried) to the switcher.

Video Tape recordersVTR room is provided at each studio center. It houses a few Broadcast standard Videocassette recorders (VCRs). In these recorders, sound and video signals are recorded simultaneously on the same tape.

Most of the TV centers have professional quality B-Format BCN-51 One inch VTRs. For broadcast quality playback it is equipped with correction electronics i.e. a processor which comprises velocity error compensation, drop-out compensation and time base correction. It also comprises a digital variable motion unit enabling still reproduction, slow motion and visible search operation.

New centers are being supplied with Sony U-matic high band VCRs along with ½” Sony Betacam SP VCRs, DVC Pro. High bands VCRs are to be provided with digital time base correctors where as Betacam has got built in DTBC with studio machines.Post Production Suites

Modern videotape editing has revolutionised the production of television programs over the years. The latest trend all over the world is to have more of fully equipped post production suites than number of studios. Most of the present day shootings are done on locations using single camera. The actual production is done in these suites. The job for a post production suites is:-a. To knit program available on various sources.b. While doing editing with multiple sources, it should be possible to have any kind of

transition.c. Adding/Mixing sound tracks.d. Voice over facilities.e. Creating special effects.The concept of live editing on vision mixer is being replaced by “to do it at leisure” in post production suites.

A well equipped post production suite will have:-1. Five VTRs/VCRs, may be of different format remotely controlled by the editor.2. Vision mixing with special effect and wipes etc. with control from a remote editor panel.3. Ampex Digital Optics (ADO) for special effects.4. Audio mixer with remote control from the editor remote panel.5. Multi-track audio recorder with time code facilities and remote operation.6. Character generator for titles.7. Adequate monitoring facilities.8. Supported by “Offline editing systems” to save time in post production suites.9. One man operation.Coverage of Outside events :

Outside broadcasts(or OBs) provide an important part of the television programs. Major events like sports, important functions and performances are covered with an O.B. van which contains all the essential production facilities.

Video Chain :The block diagram on facing page connects all these sections and it can be observed that the CAR is the nodal area. Now let us follow a CAM-I signal. CAM-I first goes to a Camera electronics in CAR via a multi-core cable, the signal is then matched/adjusted for quality in CCU and then like any other sources it goes to video switcher via PP (Patch Panel) and respective VDAs(Video Distribution Amplifiers) and optional Hum compensator/Cable equilizers.

Output from the switcher goes to stabilizing amplifier via PP and VDAs. Output from the stab. Is further distributed to various destinations.

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3TV LIGHTING

1. GENERAL PRINCIPLESLighting for television is very exciting and needs creative talent. There is always

a tremendous scope for doing experiments to achieve the required effect. Light is a kind of electromagnetic radiation with a visible spectrum from red to violet i.e. wave length from 700 nm to 380 nm respectively. However to effectively use the hardware and software connected with lighting it is important to know more about this energy.

Light Source Any light source has a Luminance intensity (I) which is measured in Candelas.

Candela is equivalent to an intensity released by standard one candle source of light.

Colour TemperatureOne may wonder, how the light is associated with colour. Consider a black body

being heated, you may observe the change in colour radiated by this body as the temperature is increased. The colour radiated by this body changes from red dish to blue and then to white as the temperature is further increased. This is how the concept of relating colour with temperature became popular. Colour temperature is measured in

degree Kelvin i.e. )273C(o . The table below gives idea about the kind of radiation from different kinds of lamps in terms of colour temperature.

Standard candle 1930o KGas filled tungsten lamp2760o KProjection bulb 3200o KFlash-bulb 3800o KHMI lamp 6500o KElectronic flash tube 6000o KAverage day light 6500o KBlue sky 12000 - 18000o K

Basic Three Point LightingKey light : This is the principal light source of illumination. It gives shape and modeling by casting shadows. It is treated like "sun" in the sky and it should cast only one shadow. Normally it is a hard source.Fill Light : Controls the lighting contrast by filling in shadows. It can also provide catch lights in the eyes. Normally it is a soft source. Back light : Separates the body from the background, gives roundness to the subject and reveals texture. Normally it is hard source.Background Light : Separates the person from the background, reveals background interest and shape. Normally it is a hard source.In three point lighting the ratio of 3/2/1 (Back/Key/Fill) for mono and 3/2/2 for colour provides good portrait lighting.

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4TV CAMERA

IntroductionA TV Camera consists of three sections :a) A Camera lens and optics : To form optical image on the face

plate of a pick up device.b) A transducer or pick up device : To convert optical image into an

electrical signal.c) Electronics : To process output of a transducer to

get a CCVS signal.CCD CAMERAS

IntroductionAny camera will need a device to convert optical image into an electrical signal. Now let us consider a picture frame made of small picture element. For more sharpness or better resolution we have to increase these elements. This picture frame can now be focused on to a structure of so many CCD elements. Each CCD element will now convert the light information on it to a charge signal. All we need now is to have an arrangement to collect this charge and convert it to voltage. This is the basic principle on which CCD cameras are based.

Latest CCD CamerasCCD were launched in 1983 for broadcasting with pixel count from a mere 2,50,000 which increased to 20,00,000 in 1994 for HDTV application. Noise and aliasing has been reduced to negligible level. CCD cameras now offers fully modulated video output at light level as low as 6.0 lumens. A typical specification for a studio camera now available in market are some thing like 2/3 inch, FIT, lens on chip CCD with 6,00,000 pixel, 850 lines H resolution, S/N more than 60 dB, sensitivity F-8 (2000 lux) etc.

Fig. 5 Block Diagram of a typical Camera

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5VIDEO TAPE FORMATS

Introduction

Format of Video tape recorder defines the arrangement of magnetic information of the tape. It specifies :

The width of tape Number of tracks for Video, Audio, Control, Time Code and Cue, Width of tracks Their electrical characteristics and orientation

All machines conforming to one format have similar parameters to enable compatibility or interchange i.e. the tape recorded on one machine is faithfully reproduced on the other. There are a number of formats in video tape recording and the number further gets multiplied due to different TV standards prevailing in various countries e.g. PAL, SECAM, NTSC and PAL-M.

Classification

Composite Analog Formats (All reel/Spool type)Quadruplex, 1” B format and 1” C format for professional Broadcast use.

Heterodyne formats (Cassette)U-matic LB, HB, SP; for semi-professional work.

Component analog formats (Cassette)Betacam, Betacam SP, M-II; for professional Broadcast use.

Digital Composite/Component formats (Cassette)D1, D2, D3, D5, Digital Betacam and DCT (Ampex); for professional Broadcast use.

Heterodyne domestic (Cassette)VHS, Betamax, Video 8mm, S-VHS, Hi-8; for domestic and semi professional

use (S-VHS & Hi-8)

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Digital Composite/Component Formats :

These are ultimate in video recording as the information is recorded in digital form and multi-generation dubbing is no longer a problem. The various digital formats in use are as under :

a) D1 : It is the first digital standard and uses component system (i.e. CCIR 601 YUV, 4:2:2 format) using ¾” tape with writing speed of about 30 m/s and helical drum running at 150 rps with segmented tracks. Digital coding is in 8-bit words with a raw picture data rate of 216 M bits/sec. The 601 standard now provides the option for 10-bit coding but is not implemented in D1 machines.

b) D2 : To reduce cost, D2 system was introduced by Ampex. D2 uses D1 cassette of high coercivity ¾” metal tape with two pairs of heads scanning eight tracks per field. D2 uses composite video signal for sampling, having 8 bit 4 times sub carrier frequency sampling, with writing speed of about 28 m/s using helical drum running at 100 rps. Data rate is around 150 Mb/s.

c) D3 : D3 was developed by NHK and Panasonic using composite system, ½” metal particle VHS sized cassette thus saving cost. It records 8 bit digital video at a sampling rate of 4 fsc (17.73 MHz) in 8 tracks per field. Data rate is similar to D2. It too offers 4 digital audio 16-20 bit at 48 kHz and cue track with comprehensive slow motion. Head drum rotates at 100 rps with a writing speed of 21.4 m/s. The signal recorded on ½ inch metal tape is more than twice the recording density of any other existing formats. Because of its compact size it is suitable for camcorders. D3 cassette can record 4 hours of continuous recording. Multi-generation suffers in quality in comparison to component machines.

d) D4 : 4 is perhaps an unlucky number in Japan as there is no D4.

e) D5 : Panasonic now has a new component system called D5 using ½” tapes. It is successor to D3. It is digital component using same cassettes as D3 but running at double speed. D5 can handle 4:3 or 16:9 aspect ratio with full restoration. For 16:9 sampling rate is 18 MHz with 8-bit coding.

f) Digital Betacam : Keeping in view the enormous success of Beta-SP, Sony have announced a new digital version. Digital Betacam machine will record component digital to the revised 10 bit CCIR 601 standard providing 2 hours running time large cassette besides small cassettes of 40 minutes duration. Prototype machine has already been displayed by SONY.

g) DCT Format by Ampex : It is a digital component format. DCT is an 8 bit system, and uses ¾ inch tape drive and 2:1 bit rate reduction. Data rate is around half of D1 component system at much lower cost than D1. It offers a record time of three and half hours.

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6HIGH POWER TV TRANSMITTER

DesignAll the TV transmitters have the same basic design. They consist of an exciter followed by power amplifiers which boost the exciter power to the required level.

ExciterThe exciter stage determines the quality of a transmitter. It contains pre-corrector units both at base band as well as at IF stage, so that after passing through all subsequent transmitter stages, an acceptable signal is available. Since the number and type of amplifier stages, may differ according to the required output power, the characteristics of the pre-correction circuits can be varied over a wide range.

Vision and Sound Signal AmplificationIn HPTs the vision and sound carriers can be generated, modulated and amplified separately and then combined in the diplexer at the transmitter output.

In LPTs, on the other hand, sound and vision are modulated separately but amplified jointly. This is common vision and aural amplification.

A special group delay equalization circuit is needed in the first case because of errors caused by TV diplexer. In the second case the intermodulation products are more prominent and special filters for suppressing them is required.

As it is difficult to meet the intermodulation requirements particularly at higher power ratings, separate amplification is used in HPTs though combined amplification requires fewer amplifier stages.

IF ModulationIt has following advantages Ease of correcting distortions Ease in Vestigial side band shaping IF modulation is available easily and economically

Power Amplifier StagesIn BEL mark I & II transmitters three valve stages (BEL 450 CX, BEL 4500 CX and BEL 15000 CX) are used in vision transmitter chain and two valves (BEL 450 CX and BEL 4500 CX) in aural transmitter chain. In BEL mark III transmitter only two valve stages (BEL 4500 CX and BEL 15000 CX) are used in vision transmitter chain. Aural transmitter chain is fully solid state in Mark III transmitter.

Constant Impedance Notch Diplexer (CIND)Vision and Aural transmitters outputs are combined in CIN diplexer. Combined power is fed to main feeder lines through a T-transformer.

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BEL 10 kW TV TRANSMITTER ( MARK – III )

A block diagram of BEL 10 kW TV Transmitter is shown in Fig. 10. It consists of :

a) 10 kW Transmitter MK-III.b) Input Equipment Rackc) Monitoring Equipment Rackd) Control Consolee) Indoor Co-axial Equipment comprising of :

U-link Rack with U-link panel A and B, T-Transformer and 10 kW Dummy Load.

Aural Harmonic Filter. CIN Diplexer Aural Notch Filter and Band Pass Filter.

f) Antenna system with junction box, feeder cables etc.g) Power distribution equipment.

Fig. 10 Block Diagram of 10kW TV Transmitter (Mark-III)

SOLID STATE POWER AMPLIFIERSDual Driver1) Has got two identical sections. Each capable of delivering 10 W.2) Gets 28 V power supply through relay in 80 W AMP.3) Sample of output is available at front panel for RF monitoring.4) Provides A DC output corresponding to sync peak out put for vision monitoring

unit.5) Thermostat on heat sink is connected in series with thermostat or 80 W AMP and

provides thermal protection. (Operating temp. 70oC.)

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Fig. 11 TX. Block Diagram (Mark-III)Ref. Drg.No:-STI(T)745,(DC497)

Fig. 12 Aural PA Chain (Mark-III)

Fig. 13 Vision Chain of Exciter (Mark-III)

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TRANSMITTER CONTROL SYSTEMThe transmitter control unit performs the task of transmitter interlocking and control. Also it supports operation from control console. The XTR control unit (TCU) has two independent system viz.1. Main control system. (MCS)2. Back-up Control System (BCS)

Functions performed by MCS (Main Control System)- XTR control- Interlocking - RF monitoring- Supporting operation from control console- Three second logic- Thermal protection for 1 kW and 10 kW vision PAs- Thermal protection for 130 Watt vision PA and Aural XTRa- Mimic diagram

Functions performed by BCS (Backup control system)- Transmitting control- Interlocking

System Description of Exciter :

Fig. 2 Block Diagram of TV Exciter (Mark-II)

Video ChainThe input video signal is fed to a video processor. In VHF transmitters LPF, Delay equalizer and receiver pre-corrector precede the video processor.

Low Pass Filter : Limits incoming video signal to 5 MHz.

Delay Equalizer : Group delay introduced by LPF is corrected. It also pre-distorts the video for compensating group delay errors introduced in the subsequent stages and diplexer.

Receiver pre-corrector : Pre-distorts the signal providing partial compensation of GD which occurs in domestic receivers.

Both the delay equaliser and receiver precorrector are combined in the delay equaliser module in Mark III version.

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DP/DG CorrectorThis is also used in the exciter preceding LPF (mark III) for pre-correcting the differential gain and differential phase errors occurring in the transmitter.

Video ProcessorThe block diagram of video processor is given in fig. 3. Functions

Amplification of Video signal Clamping at back porch of video signal.

Clamping gives constant peak power. Zero volt reference line is steady irrespective of video signal pattern when clamping takes place otherwise the base line starts an excursion about the zero reference depending on the video signal.

Fig. 3 Block Diagram of Video Processor (Mark-II)

Vision ModulatorThe block diagram of Vision modulator is given in fig. 4 and schematic diagram is shown in fig. 5Functions

Amplification of Vision IF at 38.9 MHz. Linear amplitude modulation of Vision IF by video from the video processor in a

balanced modulator.

IF AmplifierIF is amplified to provide sufficient level to the modulator. It operates as an amplitude limiter for maintaining constant output.

ModulatorA balanced modulator using two IS-1993 diodes is used in the modulator.

Band pass amplifierModulated signal is amplified to 10 mW in double tuned amplifier which provides a flat response within 0.5 dB in 7 MHz band.

Fig. 4 Block Diagram of Vision Modulator (Mark-II)

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Fig. 5 Schematic Diagram of Vision Modulator (Mark-II)VSBF and Mixer :

The block diagram of VSBF and Mixer is given in fig. 6. It consists of following stages :

VSB filter ALC amplifier Mixer Helical Filter Mixer Amplfier

Fig. 6 Block Diagram of VSBF Mixer (Mark-II)

VSB FilterSurface Acoustic wave (SAW) filter provide a very steep side band response with high attenuation outside designated channel. It has a linear phase characteristic with a low amplitude and group delay ripple. (Fig. 7.)

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Fig. 7 Block Diagram of V.S.B.Filter (Mark-II)ALC AmplifierAutomatic level control is provided to maintain the exciter output level constant.Mixer : ANZAC type MD 108 is used as mixer.Helical Filter : to attenuate harmonics by at least 30 dB (similar to the one in aural section).Mixer Amp. : Provides 34 dB gain. Output is + 15 dBm.

Local Oscillator The block diagram of Local Oscillator is given in fig. 8. It supplies three equal outputs of + 8 dBm each at a frequency of fv + fvif. This unit has 3 sub units. (1) fc/4 oscillator : Generates frequency which is 1/4 of desired channel frequency.

Fine freq. control is done by VC1.(2) LO Mixer/Power divider : Here the above fc/4 frequency is multiplied by four to

obtain channel frequency of fc and then mixed with fvif. Power divider is also incorporated to provide three isolated outputs of equal level.

Visual Transistorised Power Amplifier (VTRPA) VSBF & Mixer output is amplified by VTRPA which is highly linear and also sufficient to drive valve stages.

It is a 5 stage amplifier 2 N 3375 for the first three stages. 2 N 3632 for the four stage and BLW 75 for the final stage. All stages are biased for class A operation.

In Mark II later versions only 3 stages are used.1st Stage (1) CA 2870 B Hybrid amplifier.

IInd Stage (2) CD 3400 2 W driverIIIrd Stage (3) CD 3101 Output 10 WThe amplifier is air cooled by two AC fans fixed to the rear of the unit.

Fig. 8 Block Diagram of Local Oscillator (Mark-II)

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AUDIO CHAIN

Aural ModulatorThe aural modulator unit consists of audio amplifier, VCO, mixer and APC.The block diagram of Aural modulator is given in fig. 9.

Fig. 9 Block Diagram of Aural Modulator (Mark-II)Audio AmplifierA balanced audio signal at + 10 dBm from studio is converted to unbalanced signal by audio transformer T4. The output of this is taken through potentiometer to the input of Hybrid Audio Amp BMC 1003. A 50 micro second pre-emphasis is also provided.

VCOThis is a varactor tuned oscillator. Its frequency can be varied by coil L4. Transistor TR-17 forms the oscillator. VCO output is frequency modulated by the audio signal. Output level is 0 dBm.

TV TRANSMITTER ANTENNA SYSTEMTV Antenna System is that part of the Broadcasting Network which accepts RF Energy from transmitter and launches electromagnetic waves in space. The polarization of the radiation as adopted by Doordarshan is linear horizontal. The system is installed on a supporting tower and consists of antenna panels, power dividers, baluns, branch feeder cable, junction boxes and main feeder cables. Dipole antenna elements, in one or the other form are common at VHF frequencies where as slot antennae are mostly used at UHF frequencies. Omni directional radiation pattern is obtained by arranging the dipoles in the form of turnstile (Fig.15) and exciting the same in quadrature phase. Desired gain is obtained by stacking the dipoles in vertical plane. As a result of stacking, most of the RF energy is directed in the horizontal plane. Radiation in vertical plane is minimized. The installed antenna system should fulfil the following requirements :a) It should have required gain and provide desired field strength at the point of

reception.b) It should have desired horizontal radiation pattern and directivity for serving the

planned area of interest. The radiation pattern should be omni directional if the location of the transmitting station is at the center of the service area and directional one, if the location is otherwise.

c) It should offer proper impedance to the main feeder cable and thereby to the transmitter so that optimum RF energy is transferred into space. Impedance

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mismatch results into reflection of power and formation of standing waves. The standard RF impedance at VHF/UHF is 50 ohms.

Fig. 15 Turnstile Antenna and its Horizontal PatternMixerVIF signal from IF osc. and aural IF from VCO are injected at the base of TR1. The mixer output is 5.5 MHz. This is processed, divided to produce a square pulse at 537 Hz. For phase comparison reference pulses are derived from TCXO oscillating at 1.1 MHz after suitable division. The phase difference develops error voltage if the freq variation is present. This voltage is applied to VCO to correct frequency when PLL is unlocked due to freq. shift.

AURAL MIXER This is similar to vision mixer which translates AIF at 33.4 MHz to aural carrier frequency. This unit consists of ---(i) Mixer(ii) Helical filter(iii) Mixer amplifier

Aural Transistor power Amplifier (ATRPA)ATRPA raises the power of Aural carrier to 20 watts. There are four stages giving a 23 dB gain. The transistors used are 2N3856, 2N3375, 2N3632 and BLW 93. The unit is cooled by two small fans fixed at the rear side of ATRPA.

Radiation Pattern and GainThe horizontal and vertical radiation pattern are shown in fig. 19 and 20. The total gain depends upon the type of the antenna panel and no. of stacks as given in table-1.

Fig. 19 Typical Horizontal radiation pattern

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VESTIGIAL SIDE BAND TRANSMISSION

Another feature of present day TV Transmitters is vestigial side band transmission. If normal amplitude modulation technique is used for picture transmission, the minimum transmission channel bandwidth should be around 11 MHz taking into account the space for sound carrier and a small guard band of around 0.25 MHz. Using such large transmission BW will limit the number of channels in the spectrum allotted for TV transmission. To accommodate large number of channels in the allotted spectrum, reduction in transmission BW was considered necessary. The transmission BW could be reduced to around 5.75 MHz by using single side band (SSB) AM technique, because in principle one side band of the double side band (DSB) AM could be suppressed, since the two side bands have the same signal content.

It was not considered feasible to suppress one complete side band in the case of TV signal as most of the energy is contained in lower frequencies and these frequencies contain the most important information of the picture. If these frequencies are removed, it causes objectionable phase distortion at these frequencies which will affect picture quality. Thus as a compromise only a part of lower side band is suppressed while taking full advantage of the fact that:

i) Visual disturbance due to phase errors are severe and unacceptable where large picture areas are concerned (i.e. at LF) but

ii) Phase errors become difficult to see on small details (i.e. in HF region) in the picture. Thus low modulating frequencies must minimize phase distortion where as high frequencies are tolerant of phase distortions as they are very difficult to see.

The radiated signal thus contains full upper side band together with carrier and the vestige (remaining part) of the partially suppressed LSB. The lower side band contains frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is at 1.25 MHz.

StandardsThe characteristics of the TV signal is sections 1 and 2 refer to CCIR B/G standards. Various other standards are given in Table 1.

Table 1Frequency Range Vision/sound carrier spacing channel widthVision sound carrier spacing 5.5 MHzChannel width 7 MHz (B) in VHF OR 8 MHz (G) in UHFSound Modulation FMFM deviation (maximum) + 50 kHz

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