training report on doordarshan kendra

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REPORT ON

INDUSTRIAL TRAINING

DOORDARSHAN KENDRA “PRASAR BHARTI”



The Doordarshan Kendra Gwalior

An Overview

Doordarshan Kendra Gwalior is part of the DD India, the largest

televisionnetwork in the world. Doordarshan with over 5 high power

Terrestrial Transmitters,62 low power,5 very low power transmitter and 3

production centers serve M.P. Inaugurated on 28th may 2000 by the then

broadcast minister mr. ARUN JATELY. Doordarshan Kendra gwalior currently

produces and telecasts 168 hrs of local programmes per week.Now more than

85 per cent of the 60,385,118 populations of M.P., With the introduction of

DTH almost cent percent of the population can now receive DDK Gwalior

programmes without cable connection. Doordarshan studios have been

established at Gwalior, Bhopal and Indore to foster regional diversity. People

all over India are watching Doordarshan’s programmes. It is also received in

64 countries spread over the continents of Asia, Africa, Europe, Australia and

America.

TV Scenario in M.P.

As per the 2001 census there are 60,385,118(5.5 million) house holds in M.P.,

74.9 per cent of them are in the rural sector (44,42550) the remaining 25.1 per

cent (13,52656) are in the urban sector. In 2001, 38.8 per cent of the

households owned TV sets . Of these 62.3 per cent were in rural areas and the

remaining 37.7 per cent in urban areas. Even if we estimate 10 – 15 per cent

growth per annum. Of these estimated 3 million TV households 40 – 45 per

cent is estimated to have cable connection i.e., 1.3 million and the remaining

1.7 million are without cable connection, and totally depend on DDK Bhopal

for their TV viewing. The introduction of DTH, DD Direct Plus has considerably

increased DD viewership in MP. From the available sales estimates of set top

boxes and receivers it is estimated that MP has 3 to 4 lakh DTH households.



TECHNICAL INFORMATION OF TRANSMITTING FACILITIES AT DDK,

GWALIOR:-Doordarshan Kendra, Gwalior is equipped with studio, two

terrestrial transmitters and one digital up-link station. The two terrestrial

transmitters are of 10 KW power each. One is for DD-National and the other is

for DD-News telecasting.

TERRESTRIAL TRANSMITTER PARAMETERS:

DD-NEWS :CH #31 (VHF-Band-III) Pictures IF: 551.25 MHz, Sound IF: 556.75

MHz

DOWNLINK PARAMETERS OF DD-NEWS SATELLITE PROGRAMMES

Latitude Co-ordinates 23 14’25” ( North )

Longitude Co-ordinates 77 23’20” ( East)

Main Sea Level 300 Mtrs.

Antenna Hight 100 Mtrs.

Effective height of the antenna above sea level 400 Mtrs.

Peak power (Both DD-I & DD-II) 10 KW each

Black power 06 KW each

Antenna gainArt Direction 8 Db wide band,

Jampro Antenna.

FREQUENCY OF OPERATION

Band

DD-II

Band

III

Channel

31(.)

Video carrier

551.25 MHz

Audio carrier

556.75 MHz



FUNDAMENTAL 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.

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.

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.

MHz552

10264f

6

highest

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.

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.



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 B

So 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) = 0

Similarly B - Y = 0

As 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.

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.



VIDEO 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, and 3. 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.

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 room

10. 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 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

or

Cut: switching clearly from one source to another.

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

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

2. 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:

1. 24 input video special effects switchers 2. (CD 680 or CD 682-SP) 3. Character generators 4. Telecine/DLS remote controls 5. 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.

It provide the following outputs:

1. Line drive 2. Field drive 3. Mixed blanking 4. Mixed sync 5. colour subcarrier 6. A burst insertion pulse 7. 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 Control

The 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 control

As 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.

Audio facilities

An 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 recorders

VTR 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.

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:-

1. To knit program available on various sources. 2. While doing editing with multiple sources, it should be possible to have

any kind of transition. 3. Adding/Mixing sound tracks. 4. Voice over facilities. 5. 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.



TV LIGHTING:-

GENERAL PRINCIPLES

Lighting 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.

Basic Three Point Lighting

Key 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.



TV CAMERA:-

Introduction

A 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

Introduction

Any 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 Cameras

CCD 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.

Block Diagram of a typical Camera

HIGH POWER TV TRANSMITTER:-

Design:-

All 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.

Exciter:-

The 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 Amplification:-

In 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.

Power Amplifier Stages

In 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.

BEL 10 kW TV TRANSMITTER

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

:

1. Input Equipment Rack



2. Monitoring Equipment Rack 3. Control Console 4. Indoor Co-axial Equipment comprising of :

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

2. Aural Harmonic Filter. 3. CIN Diplexer 4. Aural Notch Filter and Band Pass Filter.

5. Antenna system with junction box, feeder cables etc.

Block Diagram of 10kW TV Transmitter

SOLID STATE POWER AMPLIFIERS

1. 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.)



TX. Block Diagram

Vision Chain of Exciter

TRANSMITTER CONTROL SYSTEM:-

The 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)

System Description of Exciter :



Block Diagram of TV Exciter

Video Chain:-The 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.

DP/DG Corrector

This 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 Processor



The block diagram of video processor is given in fig. 3.

Functions

1. Amplification of Video signal 2. 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.

Block Diagram of Video Processor

Vision Modulator

The block diagram of Vision modulator is given in fig. 4 and schematic diagram

is shown in fig. 5

Functions

1. Amplification of Vision IF at 38.9 MHz. 2. Linear amplitude modulation of Vision IF by video from the video

processor in a balanced modulator.

IF Amplifier

IF is amplified to provide sufficient level to the modulator. It operates as an

amplitude limiter for maintaining constant output.



Modulator

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

Band pass amplifier

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

Block Diagram of Vision Modulator

Schematic Diagram of Vision Modulator

VSBF and Mixer :

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



1. VSB filter 2. ALC amplifier 3. Mixer 4. Helical Filter 5. Mixer Amplfier

Block Diagram of VSBF Mixer

VSB Filter

Surface 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.)

Block Diagram of V.S.B.Filter



.

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.

Block Diagram of Local Oscillator

AUDIO CHAIN:-

Aural Modulator

The aural modulator unit consists of audio amplifier, VCO, mixer and APC.

The block diagram of Aural modulator is given in fig. 9.



Block Diagram of Aural Modulator

Audio Amplifier

A 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.

VCO

This 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 SYSTEM:-

TV 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 :

1. It should have required gain and provide desired field strength at the point of reception.

2. 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.

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

Turnstile Antenna and its Horizontal Pattern

Radiation Pattern and Gain

The 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

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:



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

2. 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.

Standards

The 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 1

Frequency Range Vision/sound carrier spacing channel

width

Vision sound carrier spacing 5.5 MHz

Channel width 7 MHz (B) in VHF OR 8 MHz (G) in UHF

Sound Modulation FM

FM deviation (maximum) + 50 kHz

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