documentation on dd07

8/8/2019 Documentation on Dd07

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CERTIFICATE

DEPARTMENT OF ELECTRONICS AND COMMUNICATIONS

Industrial training in doordharshan

Low power T.V transmitter


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ACKNOWLEDGEMENT

We are deeply indebted to DOORDARSHAN for giving us permission to accomplish our

project in their esteemed guidance.

We express our deep sense of gratitude towards the management and administration for

having shown keen interest at every stage of development of our project work and guiding us inevery aspect.

We are grateful to our revered guide Mr.P.Satyanarayana (A.E) for his excellent support

throughout the course of the project. We would like to thank him for energetic guidance because

of which we could complete our project successfully with a lot of practical knowledge.

Finally we are extremely thankful to all the DOORDARSHAN staff.

With regards

B.SATYA SAGAR

07A91A0476


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ABSTRACT

Studying low power TV transmitter in doordarshan relay station. In this

transmission of signals are possible from cable TV and DTH service. Low power transmitter inKakinada is very high frequency and ultra high frequency.

VHF transmitter is used within channel 11 for DD1.

UHF transmitter is used within channel 33 for DDnews.

And here we are studying about three sections respectively

Receiving section

Transmitter section

Antenna section

And some specifications


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INTRODUCTION

Doordarshan is the public television broadcaster of india and a division of prasara

bharati a public service broadcaster nominated by the government of india. It is one of the largest broadcasting organization in the world in terms of the infrastructure of studios anf transmitters.

Recently it has also started digital terrestrial transmitters. On September 15 2009, doordarshan

celebrated its 50 th anniversary.

BEGINNING:

Doordarshan had the modest beginning with the experimental telecast starting in delhi

On 15 september 1959 with a small transmitter and a makeshift studio. The regular daily

transmission started as a part of all india radio. The television service was extended to Bombay

and Amritsar in 1972. Till 1975 seven Indian cities had television service and doordarshanremained the only television channel in india. Television services were separated from radio in

1976. Each office of all india radio and doordarshan were placed under the management of two

separate director generals in new delhi. Finally, doordarshan as a national broadcaster came into

existence.

CHANNELS:

Presently, doordarshan operates 19 channels-two all india channels-DD national and DD

news, 11 regional languages satellite channels(RLSC), four state networds(SN), an international

channel, a sports channel and two channels (DD-RS& DD-LS) for live broadcast of

parliamentary proceedings.


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On DD national (DD-1), regional programmes and local programmes are carried on time-

sharing basis. DD-news channel, launched on 3 November 2003, which replaced the DD-metro

entertainment channel, provides 24-hour news service. The regional languages satellite channels

have two components- The regional service for the particular state relayed by all terrestrial

transmitters in the state and additional programmes in the regional language in prime time and

non-prime time available only through cable operators. DD-sports channel is exclusively devoted

to the broadcasting of sporting events of national and international importance. This is the only

sports channels which telecasts rural sports like Kho-Kho, Kabaddi etc. some thing which

private broadcasters will not attempt to telecast as it will not attract any revenues.


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CONTENTS

CHAPTER-1 Basic television systems

a) Cable transmission

b) Direct to home (DTH)

c) Transmitter service

CHAPTER-2 Block schematic of LPT

CHAPTER-3 VHF Transmitter

CHAPTER-4 UHF Transmitter

CHAPTER-5 Transmitter and Receiving section

CHAPTER-6 Antenna section

a) Self supporting mast

b) Guided wire mast

CHAPTER-7 INSAT

CHAPTER-8 Conclusion


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BASIC TELEVISION SYSTEMS

1. Cable transmission

2. Direct to home3. Transmitter service

CABLE TRANSMISSION:

In addition to wireless transmission by broadcast stations, the cable TV system provides a

distribution system with co-axial cable. It is similar to a wired telephone system but it is used for

TV programs. The RF carrier signals ate supplied so that a tuner can be used to select the desired

channel cable tv has become very popular because more channels are provided and strong signals

can be supplied for areas on which the antemma signal is not good enough cable television

started as a means by providing signals to communities that could not receive broadcast stations,

either because of distance or shadow areas in which the signal was too weak.Today cable TV has

developed far beyond that into huge systems that cover huge areas; even for locations having

food reception the reason is that cable TV does not have the restriction of channel allocations for

broadcasting. It offers up to 36 channels so many programs that not available on broadcast

television reach the cable operator via satellite transmission.

Cable channels :

Each cable channel is 6MHz wide for the AM picture signal and the FM sound signal.

However the cable signals are not radiated therefore , the frequencies in betqeen channels 6 and

7 can be used without interfacing with other services. These mid band cable channels range from

88 to 176 MHz also all the low band VHF channels (7 to 13) are used for cable TV. Those VHF

channels not assigned in a given area.


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Cable distribution:

The head end provides the program signal for all channels. Local and distant broadcasts are

picked up by an antenna which is mounted on a very high tower, in order to extend the line-of-sight distance.

The RF losses in co-axial cable are high especially in the 36 channel system that operates

in the cable TV super band in the distribution system the main line the trunk. From the trunk

branch lines extend out for groups of subscribes the line for each subscriber is called a drop.

Power Supply

1.Maximum demand/capacity: 30KW

2.Monthly average consumption : 6000 units

3.Monthly average expenditures 40000/month

DIRECT TO HOME (D.T.H)

Satellite TV, a direct to home (DTH) from the satellite through set-top box that means thereis no middle man (cable operator). So DTH puts an end to all the problems like unreasonable

charges, cable operators strike, power outages, not getting your favourite channels and channels

shifting their channel number positions.

WORKING OF DTH :

In DTH you receive the signal from satellite to a small dish antenna installed at the roof top

of your house. This signal is decoded by a set-top box which is provided by the broadcaster

and connects to the dish antenna directly with a cable. The set-top box in turn connects to your

TV. So you become the master of your entertainment and watch the channel you wish and pay

for only those channels which you wish to watch.


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Advantages of DTH TV:

1. Digital picture:The picture quality in DTH is much better. The quality of the

picture is uniform across all channels.

2. Digital audio:

We get the stereo phonic sound. So if we have got a home theatre, connect it to

your set-top box we will get better sound effects.

3. Electronic program Guide (EGP):

Its an on-screen guide that shows the program schedule or listing of all

channels. So we can find out whats playing on any channel. We can also set remainders

for programs we wish to watch and get synopses of the program.


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TRANSMITTER SERVICE

1. High power transmitter (HPT):

Transmitter power 10KW Distance covered by above transmitter is 60km-100km

Eg: located in Rajamundry

2. Low power transmitter (LPT) Transmitted power 100w-500w Local area transmitter covers distance around 21kms

Eg: located in Kakinada

3. Very low power transmitter (VLPT) Transmitted power 10w Distance covered is around 5-10Km

Eg: located in peddhapuram


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BLOCK SCHEMATIC OF LPT

Work shop

Generator power supply

Receiver dish area

P.D.A

Monitoring transmitter input

rack rack rack

Mast antenna


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DG ROOM:

The genetator generates 35KVA power supply.

Receiver Dish Area:

In receiver dish area parabolic dipolic antennas (P.D.A) are used. The shape of the

dish mist be parabola because the parabola has specific focal point. When the information from

satellites through space is incident on parabolic dishes it reflects back and for parabolic surfaces

by the principle of foci, the rays incident on parabolic surfaces reflects back by the cross the

focal point. So that at focal point the receiver information by the dish is the exact replica of

transmitted information by the satellite.

P.D.A:

Passive receiver It receives signal from satellite If the size of the dish increases gain is also increases. So that receiving capability

increases.

MONITORING RACK INPUT RACK TRANSMITTER RACK

T.V

WAVE FROMMONITORING

DEMODULATION

RECEIVER-1

RECEIVER-2

RECEIVER-3

V.C.R

P.G

SWITCHER

EXCITER

DRIVER AMPLIFIER

DIVIDER

COMBINER


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For case of understanding we can divide the functioning of input rack in to three blocks

1.Receiving section

2.Transmitting section

3.Mast and antenna

RECEIVING SECTION:

P.D.A receives information from satellites which are located in geostationary orbits.

The following are the point lobe considered while placing P.D.As

Look angle Azimuthal angle Elevation angle Latitudes and longitudes

PARKING ANGLE:

The angle at which the satellite placed in geostationary satellite is called parking angle.

LOOK ANGLE:

The angle at which the P.D.A is placed on earth with respect to latitudes and longitudes is

called look angle

To fix the look angle, azimuthal angle and elevation angle should be fixed.

Azimuthal angle determines the look angle in horizontal direction.

Elevation angle determine the look angle in vertical direction.Latitudes and longitudes steels

about the situation of P.D.A in geometrical plan

TRANSMITTER RACK:


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1. Audio- Video switcher:

This unit performs the function of selecting one of the four sets audio and video

inputs. The video input levels to the unit are 0.5 1.5 Vp-p and +10dBm respectively.This unit as an associated power supply to derive +15v, +5v and -15v required for its sub

units from 230V AC. One of the programme sources (video or audio) can be selected

using PUSH button switches available on the front panel.

2. Exciter:

The audio and video outputs from audio-video switcher unit are fed to exciter unit. The

audio input is fed directly to the aural modulator while the video signal is passed through

a low pass filter before being fed to its respective modulator. The audio is frequency

modulated using 33.4MHz IF. While video signal is amplitude modulated using 38.9

MHz IF. The modulated signals are combined and then up converted to the desired

transmitted channel frequency. The video output power level after vestigial sideband

filter and mixer is 10MW synchronous peak while audio is 1mW ALC (automatic level

control) input is available on VSBF mixer unit which can be fed from P.A stages to keep

the overall transmitter power output constant. The power supply need +16V and +28V

for the unit is supplied by P.S.U.

AUDIOVIDEO

SWITCHER

EXCITER DRIVER AMPLIFIER

DIVIDER COMBINER


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3. Driver Unit: The up- convertor signal from the exciter is fed to an attenuator which is

placed at the front panel and adjusting the input levels suitably. The signal is amplified

using class A driver stages. The overall gain of the amplifier can be adjusted by the front

panel attenuator control to be about 33db.

The output of the amplifier is fed to the directional coupler where in samples of

transmitted and reflected power is obtained and fed to metering unit which defects the

signal and feds suitable voltage to a DC meter placed at the front panel.The three position

switch on the front panel selects the parameters to be monitored viz. vision, power, aural

power and reflected power. Readings are to be read with black picture aural power

indication is valid for black picture only.

A separate exhaust fan operating at 230V AC is provided for blowing off air inthe driver unit to control the temperature raise for operation of driver amplifier.

A portion of output power is taken to the back panel of the driver unit for

monitoring purposes. The front panel output constant called Ale can be fed to the

exciter ALC in to the driver output constant at the set level. The availability of the input

power 28V to the unit is indicated through a green L.E.D on the front panel DC

Check facility is provided to monitor currents of 4 stages of power amplifiers by

patching a chord meter on combiner /divider unit.

4. Power Amplifier Unit:

The power amplifier unit comprises of two similar 60W power amplifier

modules.The R.F power output from the driver unit is divided in to two parts using the

divider in the divider/combiner unit and fed to each 5.0W power amplifiers. Each power

amplifier is fed with power input which is amplified to SOW(Sync peak) by four class A paralleled power amplifier stages with a gain of approx 6 & 10dB for channel 9.10 & 11 ;

12 respectively.- this output is fed to a directional coupler for obtaining samples of

forward & reflected power (30 db coupling) for monitoring purposes for the control unit.

The control unit also obtains the temperature of heavy sink assembly through a

thermistor.


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Separate power supply is made available for each power amplifier (28V,20A). the

power supplies are placed at the bottom portion of chassis assembly. A DC voltage

proportional to current drawn by each of the transistor in power amplifier is available

from bias unit on DC check connector placed over the front panel. This can be

monitored on the current meter provided on divider combiner unit through suitable patch

cord provided separately.

There are two types of transmitters:

1. V.H.F transmitter

2. U.H.F transmitter


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VHF TRANSMITTER

In this transmitter the frequency ranges from 224 MHz-231MHz.

BLOCK DIAGRAM OF EXCITER UNIT

Video signal DRIVER

LPF VIDEOPROCESS

OR

VISIONMODULA

TOR

POWERCONTROLL

ER

VESTIGIAL SIDEBAND

FILTER

CONTROL OSCILLATOR

IFOSCILLATOR

AURAL MODULATOR

AUDIO SIGNAL


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EXCITER :

Exciter provides amplitude modulated visual drive of 10MW. Sync peak and a frequency

modulated all drive of 1mW required for the power amplifier stages of 100W TV transmitter at

the designated channel frequencies. It consists of the following individual units:

1. Video signal

2. Low pass filter

3. Video processor

4. Vision modulator

5. IF oscillator

6.

Control oscillator 7. Aural modulator

8. Audio signal

9. Power combiner

10. Vestigial side band dilter

11. Driver

12. +12V regulated power supply

Video signal :

The video signal is limited to 5 MHz by the low pass filter and group delay by its corrected

group delay introduced by it is corrected by the active group delay equalizer.

Low pass filter:

The LPF is used to limit the video frequency to 5MHz only, and it attenuates the video

signal more than 20dB above 5.5MHz the group delay introduced by steep falling characteristic

at 5.5 MHz is corrected using 5-6 active group delay equalizer LPF unit consists of single PCBconsisting of a video amplifier section and clamp pulse generator section.

Video Amplifier:


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It amplifies the video signal to level sufficient to modulate the vision carrier in the visual

modulator unit. The video input to this unit is at level of 1Vp-p clamp pulse.

UHF TRANSMITTER

In this transmitter the frequency range is from 564-574MHz. it requires 500W power. DD news is

broadcasted in channel 33. This transmitter is manufactured by Bharat Electronics (BEL)

BLOCK DIAGRAM OF UHF TRANSMITTER

Video signal splitter & PA

Linearity corrector :

Linearity corrector operates in the UHF TV band of 470-600MHz and its function

is to correct the non-linearitys that occur in power amplifiers operated in this band. Non

linearity in TV amplifiers are measured in terms of 3-tone IMD and differential gain . Thelinearity corrector is a pre-distorter circuit that is placed ahead of the power amplifier and pre-

corrects the above mentioned distortion so as to reduce them at the power amplifier output.

Linearitycorrector

Base bandcorrector

exciter Up convertor


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Up- convertor:

The up-convertor unit combines modulated vision IF an aural IF signals and translates to

respective channels frequency suitable for transmission. The unit has in-built power supply. Thestatus and fault information are displayed on front panel of the unit.

Splitter:

The linearity corrector output is divide into four equal amplitude and phase outputs to fed four

PA to get the required output power. To achieve this connection, a four way splitter by

terminating unused parts. The four ways splitter doesnt have any avhieve vomponents for

isolation resistor. It is a micro strip circuit desigh based on Wilkinsons power divided

principles.

Combiner:

The two way power combiner is a sub unit in the 500W transmitter there are such units. Two

way combiner is used to combine the outputs of four amplifiers. For the first level combining

pairs of amplifiers are combined output or pairs of amplifiers is combined in a second kevel of

combining resulting in 600W peak sync output power. All units are identical electrically and

mechanically and are interchangeable. It is based on the Wilkinsons power combiner principle.

The combiner is realized as a micro strip line on a PCB substrate with a isolation resistor for

isolating all the ports.


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RECEIVING SECTION

BLOCK DIAGRAM OF RECEIVING SECTION

The parabolic dish antenna is metal structure with a shape of half circle, and apart from

that at a distance a feed arm is held with support in air to which a low noise amplifier in addition

to the low noise block convertor and the internal relay station there is a digital broadcast receiver

in for monitoring and later on re-transmission of the signal is done in the transmitting section.

TRANSMITTER SECTION

BLOCK DIAGRAM OF TRANSMITTER SECTION

Parabolicdish antenna

Low noiseamplifier

Low noise block convertor

Digital video broadcastreceiver

antenna

V1 AUDIO &

V2 VIDEO

V3 SWITCH

EXCITER

DRIVERAMPLIFIER

POWERAMPLIFIER

A1 A2 A3


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For transmission, a signal -- sound or light -- is first converted to an electrical signal using a

microphone or a video camera. These signals are then superimposed on to a carrier wave

before being sent to the transmitter. Different channels of a radio or television use carrier

waves of different frequencies. Radio/TV receivers are so designed that they can tune in to a

particular frequency of electromagnetic waves at a time -- they ignore other signals. The images

and sounds corresponding to one particular signal can then be selectively processed and

reproduced by the receivers. It is important to remember that radio waves are only part of a

extensive spectrum of electromagnetic waves. This spectrum includes such familiar radiations

as visible light; ultraviolet and infrared radiations, as well as X-rays and gamma rays. As you may

be aware they all exhibit phenomenon of reflection, refraction, diffraction, interference and

absorption. Since sound or the image signals both travel as an electromagnetic wave, their

velocity is the speed of light and they reach any destination on Earth almost instantaneously.

The process of mixing electrical signals from a microphone or a TV/video camera with an

electromagnetic carrier wave is known as modulation. At present two kinds of signal

modulation are commonly used -- Amplitude Modulation (AM) and Frequency Modulation(FM).

In amplitude modulation the amplitude of the carrier electromagnetic wave at any instant of

time is changed corresponding to the amplitude of the signal electric current and in frequencymodulation, the frequency of the carrier wave at a particular instant of time is changed

according to the amplitude of the signal wave. Normally amplitude modulation is used for

carrier frequencies corresponding to the short, medium and long wavelength bands of radio

frequencies. The television signal is made up of two parts, both related to each other by the

frequency of the carrier signal. The image signal is amplitude modulated and occupies about

three fourth of the total bandwidth (which is usually 6 megahertz) the audio component of the

TV signal is frequency modulated and has a frequency in the range of the upper quarter of theband frequency range. Television and radio receivers are generally designed to process signals

in a certain frequency range which are globally allotted for the respective signals. Normal radio

receivers therefore cannot process sound signals meant for televisions. But such radio

receivers can be designed.


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A Radio/TV transmitter performs essentially three functions: generation of the carrier currents

for the sound and/or light signals, modulation and amplification of the resulting signal (so that

it has enough energy to dissipate over a large area). The carrier currents have frequencies

accurate to roughly one part in 2 00,000. The signals are then sent to the transmission antenna,

which in turn, sends the signal out into air as electromagnetic waves. The receiver receives the

electromagnetic waves through its own antenna, demodulates the received signal (by mixing

with an electromagnetic wave corresponding to the carrier wave frequency generated within

itself) and then recreates the original sounds and/or images.

The broadcasting range of a transmitter depends both on its power (measured in watts) as well

as the frequency of the transmitted signals. For example, A typical strong AM radio station --which broadcasts signals in the range of 1000 kilohertz ( in medium wave band)-- has a power

of 5 0,000 watts and its signals can be received far away. For example, programs broadcast on

Delhi-B by a 5 0,000 watts transmitter in New Delhi can be heard at night as far as Calcutta

(about 1 5 00 km away). The least powerful AM stations operate at 25 0 watts and usually serve

only neighboring areas. AIR Chandigarh programmes can be received in Delhi but not beyond.

The power of FM stations which broadcast signals in the range of 100 megahertz, ranges from

100 watts (which can broadcast up to 3 0 kilometers) to 100,000 watts, (which can broadcast upto about 100 kilometers). The dependence of the range of a radio signal on its frequency is

essentially due to absorption of these waves by matter. As the frequency of the carrier wave

increases it is absorbed more readily by the structures in its way. Since the television signals are

generally transmitted on carrier waves popularly known as VHF (very high frequency waves)

they can only be transmitted and received more like light i.e. in straight lines. VHF signals,

unlike short-wave radio signals transmitted by radio stations, transmitting in short-wave

frequency band, cannot be reflected by the upper layers of atmosphere. Television or FMsignals therefore cannot be transmitted directly to receivers located at very long distances. But

now with the advent of satellites it is possible to beam signals both in VHF and UhF (ultra high

frequency) range to geostationary satellites, which can then transmit these signals back to

another far off location on Earth.


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Television, The Magic Lantern :

y How does the screen of a television set acquire static electricity?y What is `High Definition TV'?y What is a cable TV channel?y While switching on or off the TV our hair tends to stick to the screen, why?y What is the nature of the substance coated on the inside surface of a co lour TV screen?y Why does the image on a TV screen get distorted if we bring a magnet near it?y Why is it that we can see colour pictures on a colour TV and not on a black and white TV?

y How does a colour TV work?y Why dont we get clear photographs of images on the television screen?y Why doesn't a colour picture come on a black and white television ?y In the dark when we focus a torch on TV screen and turn it off immediately, why is the

image of the focused light is seen on the screen?

y What is S-Band in modern television sets?y If a colour picture tube is fitted in a black and white TV set will it function as a colour TV?

y How are we able to watch pictures through video on TV?y Why does an airplane flying overhead distort a TV picture.

In order to answer the above queries, we need to understand the working of a television

receiver. The radio signals received by the TV antenna are normally weak, they have to be

therefore first amplified using a rf (radio frequency) amplifier. After amplification the signals

are demodulated, this is done through a tuner. A tuner can produce electric currents

(electromagnetic waves) having same frequencies as the carrier waves used to broadcast

various channels. Therefore when we select a particular band on the TV set, an electric signal

having a particular frequency is mixed with the amplified signal received through the antenna

to produce a demodulated signal. From the tuner, the television signal goes through

complicated electronic circuits in the set. These circuits further process the signal to separate


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the audio and video portions of it. The audio signals are changed into sound waves by the

speaker, the video signals go to the picture tube where they recreate the picture.

The picture tube transforms the video signal into patterns of light that duplicate the scene infront of the camera at the time of the broadcast or when the programme was recorded. One

end of the picture tube is rectangular and nearly flat which makes up the screen of the TV set.

At the other end the picture tube tapers off to a narrow neck. The neck of the picture tube

holds three electron guns -- one each for red, blue, and green signals. The tube of a black and

white TV set has only one gun. Each electron gun in a colour picture tube shoots a separate

beam of electrons at the screen. The screen of most colour tubes is coated with more than

3 00,000 tiny phosphor dots -- made-up of coating of phosphorescent materials which emit lightwhen exposed to some radiation such as light or electrons. They continue to emit light for some

time even after the source of radiation is turned off. This explains why the TV screen glows

after a torch light is switched off. The dots on a colour TV picture tube are grouped in triangular

arrangements of three dots each -- one red, one blue, and one green. These dots glow with

their respective colours when struck by an electron beam. A metal plate perforated with

thousands of tiny holes lies about 1 3 millimeter behind the screen of a colour picture tube. This

plate, called the shadow mask, keeps the beams from hitting any other colour dots but theirown. When the television set shows a colour program, the neuron signals produced by light

emitted from the three coloured dots blend together in the viewers brain to produce the

perception of all the colours of the original scene (see Mystery of Colours , Curiosity Corner ,

SR, Oct. 98 ) In case of a black and white program the dots appear to produce differing amounts

of white light.

The picture on the TV screen is produced by the process of scanning the electron beam

horizontally on the screen at a very fast rate. The beam is guided by the magnetic field

produced by the signal electric current fed into coils which are located around the neck of the

picture tube. The electron beam scans the screen much as a person reads -- from left to right,

top to bottom. The scanning pattern for the normal TV sets used in India is made up of 625

lines. In a high definition TV it is made up of 11 25 lines


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As electrons constantly strike the screen, it acquires static electric charge, which can induce

electric charge on our hairs on our hands/arms and make hair stick to the screen. A magnet

distorts the path of the electron beams used to create the picture and hence the picture

formed on the screen gets distorted when a magnet is brought close to it. Transmitting or

receiving video signals without modulation/demodulation is indeed possible for short distances

it is used in a close circuit television setup. A video tape recorder records and replays the video

signal and the audio signals on a magnetic tape very much like a audio tape recorder records

audio signals on audio cassettes.

A normal TV receiver can receive transmitted TV on fourteen different channels. The frequency

of these ranges from 54 ,000,000 hertz or 54 megahertz to 2 16 ,000,000 hertz, or 2 16 megahertz. The bandwidth of each of these channels is 6 megahertz. Signals transmitted on

such frequencies are known as VHF, or very high frequency signals. TV signals can also be

broadcast on frequencies between 47 0 megahertz and 89 0 megahertz known as the UHF

range. Doordarshan in India broadcasts TV programs on the national channel (DD1) in VHF

range. Both VHF and UHF signals act much like light, not bending much around the curvature of

Earth and pass through the atmosphere. They are also blocked by structures and hills. An

airplane coming in the way of the signal and the receiver therefore disturbs TV reception.

Television broadcasting antennas are usually placed on tall towers standing on high ground, so

that the radio signal which carries the television programme may travel as far as possible. But

still the maximum range of a TV broadcast signal is in between 100 to 25 0 km. Television signals

are therefore sometimes broadcast via satellites to reach an audience farther off. The signal

sent to the satellite are in the UHF range and are sent back towards the ground by the

satellites. These signals are received by cable operators using large dish antenna. Often there is

a limitation posed by the design of older television sets, which allots 6 MHz. bandwidth to each

channel. In order to overcome this limitation cable operators/recent television sets use single

sideband (S Band) technology to modulate/demodulate the signals. Using this technology a

larger number of TV signals can be distributed through a cable network. Such signals have

bandwidth less than 6 MHz.


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ANTENNA SECTION

y Why is it possible to increase the sound level of a radio set by just changing the direction?y Why are the shapes of different TV antennas different?y If an antenna wire can be used for connecting a bulb to the mains, then why can an ordinary

wire not be used to connect a television set to the antenna?

y How can a TV with no connection with any cable receive some satellite channels,?y Why does the reception of a TV programme depend on the orientation of the antenna?y Why do we need a dish antenna to receive television programmes telecast through satellites?y Why is it that inside a train compartment, a transistor set is unable to receive radio

programmes?

y If a TV antenna replaces a radio antenna, will a radio receive a TV program?y Why do we sometimes see ghost images on the TV screen?y Can there be some place where radio waves cant reach?

An antenna is a piece of a conducting material which facilitates the resonance between

the receiver and transmitted electromagnetic waves. It can be in the form of a length of a wire, a

number of metallic rods, a coil or a dish. The dimensions and the design of an appropriateantenna for a reciever located at a particular location depends both on the strength of the signal

in that area as well as on the electronic design of the receiver instrument. While in the

neighborhood of a radio or TV transmitter a piece of wire may be often sufficient to produce

good reception, in far off places one needs an antenna designed for better reception of the signal.

To be highly efficient, an antenna must have dimensions that are comparable with the

wavelength of the radiation of interest --the wavelength of a 66-72 MHz. wave (the frequency of

channel 4 on which DD1 is transmitted) is about 4 meters, A conductor having this length is

often sufficient to receive strong TV signals. The folded rod in the middle of a common

television antenna is also about this length -- the other rods essentially serve as reflectors to boost

up the signal. Radio signals are usually strong. We therefore can pick up signals from a local

radio station even without an antenna. Sometimes a long metallic rod which has length equal to a

near fraction of the wavelength (one tenth or a quarter of the wavelength of the transmitted


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radiation) or a coil of wire wound on a ferrite core can be sufficient provided the receiver has

been adequately designed to receive and process weak signals.

A radio frequency signal is often much stronger in a certain direction ( the direction in

which the transmitter is located) than others. Also some antennas are directional - they are more

effective in a particular geometry e.g. when the rods of a TV antenna are aligned perpendicular

to the direction in which transmitter is located the antenna is more effective. Similarly a radio

antenna of a AM radio receiver ( made up of a wire coil wound on a ferrite rod) if aligned toward

the transmitter yields a higher output signal. The changing intensity of the volume of a radio

program broadcast on an a medium wave band is due to such directional characteristic of its

antenna. The antenna of a FM radio set is often in the form of a stretchable metallic rod, which

normally points towards the sky, but one does sometimes experience changes in the sound

intensity in an FM set. This is due to the fact that the wavelength of the electromagnetic waves

used to carry FM signals is in the range of a few meters--the dimensions of our usual rooms.

These waves are reflected from the walls of the room and can be absorbed by objects coming in

between. Hence sometimes signals become very weak or very strong when the receiver is placed

pointing to a certain direction or someone comes in between..

The metro channel of Doordarshan is transmitted on a frequency band ranging from 174-

180 megahertz, hence an antenna having a dipole about 1.3 meter length is sufficient provided

the signal does not get attenuated by the time it reaches the location. You may recall, we have

said earlier that as the frequency of the signal increases, it is more susceptible to be absorbed by

objects in the way. The metro channel therefore can be easily received only in the cities where it

is broadcast, or through the satellite. The signals transmitted from a satellite are also in high

frequency range. Ordinary rod antennas cannot pick up such signals, one needs specially

designed dish antennas linked to appropriate tuners to pick up and process signals from them.

The energy in an electromagnetic wave is easily dissipated by inducing electric current

(motion of electrons) in a closed loop of metallic conductors. A receiver enclosed in a cage like

structure made up of a metal ( popularly known as the Faradays cage) is therefore unable to

receive electromagnetic signals, because there is no electromagnetic energy left. One can

therefore understand why our transistor radio cannot receive radio programs inside a train or a


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bus. Such a cage also acts a no entry zone for electromagnetic waves. This also explains the use

of shielded wires used for transmission of TV programs by cable networks. A metallic shield

around the metallic wire carrying the signal prevents outside interferences as well as attenuation

of the signal by leaking of waves to neighboring locations. But then cable operators often have to

compromise with low cost joins between two cables, which are often sufficient for neighboring

TV sets to pick up which explains the reception of cable programs even when you may not have

a cable connection. TV signals are easily reflected by huge buildings in the neighborhood of a

transmitter, the reflected signal has a slightly different phase than the original signal and gives

rise to ghost images.

For the propagation the electrical energy is converted into electro-magnetic wave.

This is done by antenna section and the different types of propagation are explained below as

1.Sky wave or Ionospheric wave propagation [between 2 to 30MHz]

The sky wanes are of practical importance for every long radio communications at medium and

high frequencies i.e medium waves and short waves.

In this mode the EM waves transmitted by the transmitting antenna reach the receiving antenna

at very long distance away from transmitting antenna after the reflection from the ioinized region

in the upper part of the atmosphere of the earth.

This part is called ionosphere and it is located above earths surface at about 70km to 400km

height. The ionosphere acts as the reflecting surface and reflects the EM wave back to the earth if

the frequency is between 2 t0 30 MHz.

As the sky wave propagation is useful for the frequencies between 2MHz to 30MHz only this

mode of propagation is also called short wave propagation.

As the waves propagate due to the reflection by the ionosphere the mode of propagation is also

called ionospheric propagation using the skywave propagation is also called ionospheric

propagation. Using the skywave propagation a long distance point to point communication is

possible and hence it is also called point to point propagation or point to point communication.


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space wave and the surface wave. Where the VHF and UHF transmissions are different.Here the

antennas are of two types where the propagation of the signal is done. The word mast means that

a supporting structure.

1.Self supporting mast:

It is a general broadcasting purpose antenna here the antenna is held at height so that the

transmission of the signals would be without any obstacles. It is generally almost used in all

media using sectors.

2.Guided wire mast:

The mast here is suspended from the ground and it is supported by some wires so that it would

with stand to the climatic conditions.


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INSAT

INSAT or the Indian National Satellite System is a series of multipurpose Geo-stationary

satellites launched by ISRO to satisfy the telecommunications, broadcasting,meteorology,

and search and rescue needs of India. Commissioned in 1983, INSAT is the largest domestic

communication system in the Asia Pacific Region. It is a joint venture of the Department of

Space, Department of Telecommunications, India Meteorological Department, All India

Radio and Doordarshan. The overall coordination and management of INSAT system rests with

the Secretary-level INSAT Coordination Committee.

INSAT satellites provide 199 transponders in various bands (C, S, Extended C and Ku) to serve

the television and communication needs of India. Some of the satellites also have the Very High

Resolution Radiometer (VHRR), CCD cameras for metrological imaging. The satellites also

incorporate transponder(s) for receiving distress alert signals for search and rescue missions in

the South Asian and Indian Ocean Region, as ISRO is a member of the Cospas-

Sarsat programme.

The Indian National Satellite (INSAT) system was commissioned with the launch of INSAT-1Bin August 1983 (INSAT-1A, the first satellite was launched in April 1982 but could not fulfill

the mission). INSAT system ushered in a revolution in

Indias television and radio broadcasting, telecommunications and meteorological sectors. It

enabled the rapid expansion of TV and modern telecommunication facilities to even the remote

areas and off-shore islands. Today, INSAT has become the largest domestic communication

satellite system in the Asia-Pacific region with ten satellites in serviceINSAT-2E, INSAT-3A,

INSAT-3B, INSAT-3C, INSAT-3E, KALPANA-1, GSAT-2, EDUSAT, INSAT-4A and INSAT-

4B. Together, the system provides 199 transponders in C, Extended C and Ku bands for a variety

of communication services. Some of the INSATs also carry instruments for meteorological

observation and data relay for providing meteorological services. KALPANA-1 is an exclusive

meteorological satellite. The satellites are monitored and controlled by Master Control Facilities

that exist in Hassan and Bhopal.


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CONCLUSION

We would like to conclude this training as a very great and enriching the experience to

learn about the low power TV transmitter.The transmitter service involves great equipment that

deals with monitoring section exciting system and we learn about the above equipment of the

doordarshan relay centre and its working.

We also learned about the procedure of transmission, reception. And strengthening of the signal

and retransmitting the signal into space for the broadcast around the range of propagation.


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