Minor Training

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An

Industrial Training Report

“DOORDARSHAN KENDRA”

Bhopal(11.07.2011-05.08.2011)

Submitted in Partial fulfillment of the Requirement for the award of Bachelor of Engineering in Electronics & Communication

Submitted to:

RAJIV GANDHI PROUDYOGIKI VISHWAVIDYALAYA,BHOPAL (M.P.)

Submitted By:

Shubhankar Chanda0191EC081079

Under the Supervision of

Mr. P.K.PatiSuperintendent Engineer

Doordarshan, Bhopal

Session: 2011-2012


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TECHNOCRATS INSTITUTE OF TECHNOLOGY (EXCELLENCE)

BHOPAL (M.P.)


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TECHNOCRATS INSTITUTE OF TECHNOLOGY (EXCELLENCE)

BHOPAL (M.P.)

CERTIFICATE

This is to certify that the major training done in Doordarshan Kendra Bhopal by

Shubhankar Chanda student of final year is the bonafide of the work done by him as a part of

the partial fulfillment of the requirement for the award of Bac!"#$ #% E&'&!!$&' in

ELECTRONICS COMMUNICATION ENGINEERING of Raji !andhi "roudyogiki

#ishwaidyalaya during the academic year $%&&'&$(

A**c+. P$#%. H!a S&'

HEAD OF DEPARTMENT


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ACKNOWLEDGEMENT

It takes an immense gratitude to thank the member

staff involved in the vocational training done from theDoordarshan Kendra, Bhopal.

I would like to thank Mr.P.K. Pati without whom the

completion of the training would not have been possible. a

humble gratitude towards all of the team members who

gave us an opportunity to explore the world of microwaves

and satellite communication.

Shubhankar Chanda

0191EC081079


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Doordarshan Kendra Bhopal

“An Overview”

Doordarshan Kendra, Bhopal was inaugurated on

20th October 1992 by then President of India Hon’ble lateShri Shankar Dayal Sharma since than Doordarshan Kendra,

Bhopal is serving to the people of Madhya Pradesh.

It has two studios where in house programmes produces and

it covers 76.4% of land area of the State and 77.1% of population

watches the programmes. The total coverage area is about 70 Kms

radius and approximate viewership is 61.76 lacs.

The telecast timings generally are 4.00 PM to 8.00 PM on

Monday to Saturday and 6.30 PM to 8.00 PM on Sunday at anaverage. The total programme telecast duration in a week is 26.5

hours, which includes sponsored programmes.

Doordarshan Kendra consists of distinct units, which can be

described as follows:

1) Programme wing

2) Engineering wing

3) Administrative & Finance wing

4) News wing5) Audience Research unit

6) Rajbhasha (Hindi) Unit

The Director is the head of the Kendra. Being a major

Doordarshan Kendra one chief Producer, two Deputy Director of

Programme, one Executive Producer and four Assistant Station

Director who look after various aspects of programme work.

Similarly, there are Sr. Administrative officer and Administrative

officer to look after Administration and Accounts work.

The Assistant Station Directors are assisted by Programme

Executive, Transmission Executive and other programme staff, and

some clerical staff. The Administrative officer/Senior Administrative

officers are assisted by Accountant/Assistant, UDC, and LDC &

Telephone Attendants.


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The main function of Doordarshan Kendra Bhopal is to plan

and telecast programmes to viewers all over the Madhya Pradesh

State.

TV Scenario in M.P.:

As per the 2001 census there are 60,385,118(5.5 million)

households 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

lakhs DTH households.

Technical Information:

Doordarshan Kendra, Bhopal 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.

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

Sound IF: 556.75 MHz.


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TECHNICAL BRIEF

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

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

Main Sea Level 550 Mtrs.

Antenna Height 110 Mtrs.

Effective height of the antenna above sea level 660 Mtrs.

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

Black power 06 KW each

Antenna gain Art Direction

12.3 Db wide band,

Jampro Antenna.

FREQUENCY OF OPERATION

Band DD-I DD-II

Band III III

Channel 5 7

Video carrier175.2396

MHz189.25 MHz

Audio carrier180.7396

MHz194.75 MHz

Primary coverage 70 KMs.

DD-I HPT 10 KW Commissioned 24th October,1984

Bhopal Studios Commissioned 20th October,1992

DD-II (NEWS) HPT 10 KW Dedicated on 12th May, 2001

Coverage ( Population-wise) 61-76 Lakhs


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Coverage ( Percentage-wise) 77.1%

Coverage ( Area-wise in percentage) 76.4%

EARTH STATION COMMISSIONED

Analogue 19th November , 1993

Digital Earth Station (2+1) 19th January, 2005

Uplink Frequency U/L 6174.5 MHz ( H )

Downlink Frequency D/L 3949.5 MHz ( V )

Satellite INSAT-3A

Transponder C-6

Digital Satellite News gathering Band C-cum-KU

(DSNG)VAN MOBILE UPLINKING November 2002.

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 pickup device convert theoptical 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.


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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.

Therefore the horizontal resolution of the system is 5 MHz. A

similar calculation for 525 lines system limits the highest frequency


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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 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. Additive Colour Mixing


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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

consist 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 makesit 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 TVsignal 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.


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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 bemuch 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 that,

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 fulfil the compatibility andreverse 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.


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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 subcarrier 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:

&( To originate programmes from studios either for live telecast or

for recording on a video tape.

$( To knit various other sources of programs available at the

production desk i.e., camera output from studios, feed from other

Kendra, 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. 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,


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• 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 multipurpose 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. Luminaries andsuspension system having grids or battens (hand/motorised

operation).

7. Pick up wall sockets for audio operations.


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8. Tie lines box for video and audio lines from control room.

9. Cyclorama and curtain tracks for blue and black curtain for

chroma keying and limbo lighting respectively.

10. Audio and video monitoring facilities.

11. Studio warning light and safety devices like fire alarm system

and fire fighting equipments etc. 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 informalsituation.

Production control area: Activities in this area are:-

1. Direction to the production crew.

2. Timing a production/telecast.


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3. Editing of different sources available at the production desk.

4. Monitoring of output/off air signal.

1. Hardware provided in this area include:

2. Monitoring facilities for all the input and output sources

(audio/video).

3. Remote control for video mixer, telecine and library store and

special effect (ADO) etc.

4. 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


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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:-

1. TAKE: Selection of any input source or Cut: switching clearly

from one source to another.

2. DISSOLVE: Fading out of one source of video and fading in

another source of video.

3. SUPERPOSITION OF TWO SOURCES: Keyed caption when

selected inlay is superimposed on the background picture.

4. SPECIAL EFFECTS: A choice of a number of wipe patterns forsplit screen or wipe effects.

The selected output can be monitored in the corresponding

preview 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, priorto 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,


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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 thiscrystal 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 provides 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:


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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.


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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 withthe 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 centre. 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 centres have professional quality B-Format

BCN-51 One inch VTRs. For broadcast quality playback it is


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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 centres

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 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.

5. Audio mixer with remote control from the editor remote panel.


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6. Multi-track audio recorder with time code facilities and remote

operation.

7. Character generator for titles.

8. Adequate monitoring facilities.

9. Supported by “Offline editing systems” to save time in post

production suites.

10. One man operation.


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• 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 equalizers. Output from theswitcher 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 toachieve 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.


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• Basic Three Point Lighting:

Key light: This is the principal light source of illumination. It

gives shape and modelling 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 hardsource.

Background Light: Separates the person from the background,

reveals background interest and shape. Normally it is a hard

source.

In three point’s lighting the ratio of 3/2/1 (Back/Key/Fill) for

mono and 3/2/2 for colour provides good portrait lighting.

TV CAMERA : - A TV Camera consists of three sections:

i. A Camera lens and optics: To form optical image on the face

plate of a pickup device.

ii. A transducer or pick up device: To convert optical image into

an electrical signal.

iii. Electronics: To process output of a transducer to get a CCVS

signal.

CCD CAMERAS:

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.


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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.


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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 offer 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 something 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


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HIGH POWER TV TRANSMITTER:-

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.

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. 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 auralamplification. 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.

In BEL mark I & II transmitters three valve stages (BEL 450CX, 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.


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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.

A BEL 10 kW TV Transmitter consists of Input Equipment

Rack monitoring Equipment, Rack Control Console, Indoor Co-axialEquipment comprising of U-link Rack with U-link panel A and B, T-

Transformer and10 kW Dummy Load, Aural Harmonic Filter,

CIN Diplexer, Aural Notch Filter and Band Pass Filter Antenna

system with junction box, feeder cables etc.


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Block Diagram of 10kW TV Transmitter


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Solid State Power Amplifiers:

Ithas got two identical sections. Each capable of delivering 10 W. It gets 28 V power supply through relay in 80 W AMP. Sample of

output is available at front panel for RF monitoring. 4) Provides ADC output corresponding to sync peak output for vision monitoring

unit. 5) Thermostat on heat sink is connected in series with

thermostat or 80 W AMP and provides thermal protection.

TX. Block Diagram


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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:


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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

precorrecting the differential gain and differential phase errors

occurring in the transmitter.

Video Processor:

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 anexcursion about the zero reference depending on the video signal.

Vision Modulator:


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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 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. 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


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Schematic Diagram of Vision Modulator


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VSBF and Mixer :

It consists of following stages:

1. VSB filter

2. ALC amplifier

3. Mixer

4. Helical Filter

5. Mixer Amplifier

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.


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Block Diagram of V.S.B.Filter

• Local Oscillator:

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

Power divider is also incorporated to provide three isolated outputs

of equal level.


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Block Diagram of Local Oscillator

Audio Chain:

The aural modulator unit consists of audio amplifier, VCO,

mixer and APC.

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 MC

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,


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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 and exciting the same in

quadrature phase.


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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 centre of the service area and directional one, if the

location is otherwise.

c) It should offer proper impedance to the main feeder cable andthereby 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.


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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 isused 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.


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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.

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