ORIGIN OF DOORDARSHAN:

Doordarshan is the national service of India and is also one of the

largest broadcasting organizations in the world. A network of three nationals, two special

interest channels; 10 regional language channels, 4 state network and an international

channels. Through a network of 868 terrestrial transmitters of varying powers it makes

available television signals for over 87% of population. 300 million viewers in their

homes watch Doordarshan programmes. Television sets established under various

schemes in community centres in villages for a total number of 450 million viewers

(India, 1998) . The countrywide class room on national network is aimed to reach quality

education of students in small villages.

Television in India has been in existence for decades now. India

did not begin till September 15, 1959 with a small studio. The service was called

“Doordarshan” for the first 17 years, it spread haltingly and transmission was mainly in

black and white. Doordarshan was established as a part of AIR, until 1976, it consisted of

one national network and seven regional networks. In 1992 there were sixty three high

power television transmitters, 369 medium power transmitters, 76 low power station and

23 transposers. Regular satellite transmission began in 1982.

Television has come to the forefront only in the past 21 years and

more so in past 13. There were initially two ignition points, the first in the 80’s when

color television was introduced by state owned broad caster. Doordarshan (DD) timed

with 1982 Asian games which India hosted. It then proceeded to install transmitter nation

wide rapidly for terrestrial broadcasting. In this period, no private enterprise was allowed

to set up television signals. The second spark came in early nineties with the broadcast of

satellite television by foreign programmers like CNN followed by STAR T.V and a little

later by domestic channels such as ZEE T.V and SUN T.V into Indian homes.

The number of Televisions sets in India increased from around 500,000 in

1976 to 9 million in early 1987 and to around 47 million in 1994; increases are expected

to continue at around 6 million sets per year.

If all the doordarshan centres, Mumbai has the most acute language

problem, having to cater to a cosmopolitan and varied audience in Hindi, English, Urdu,

Marathi and Guajarati. In 1984, doordarshan introduced a second channel for the big

cities and permitted cable operators to transmit locally made programs to fill the gaps in

the schedule when doordarshan was not in air. These cable operators grew from a few

100’s in the eighties to more than 20,000 in the nineties.

Presently Doordarshan operates 19 channels, two All India Channels, 11

regional languages satellite channels (RLSC), four State Networks (SN) an international

channel and a sports channel. Regular satellite transmission began in 1982. Now more

than 87% of population of the country can receive Doordarshan programmes through a

network of nearly 1044 Terrestrial Television Transmitters. About 46 Doordarshan

studios are producing television software.

DD - NATIONAL (CHANNEL-7) DD-NEWS (CHANNEL-9)

INTEGRATED ON 02-08-1986 01-04-2003

TRANSMITTER TYPE BEL NEC

CHANNEL VHF(band-III) 07- 09

VISION POWER 10KW 10KW

AURAL POWER 1KW 1KW

VISION FREQUENCY 189.2396 MHZ 203.26MHZ

AURAL FREQUENCY 194.7396 MHZ 208.76 MHZ

SATELLITE INSAT 3A INSAT 3C, INSAT 4B

LOOKING ANGLE 93.50740, 93.50

AZIMUTH 14902200 ,1490

ELEVATION 67063.20,670

DOWNLINK FREQUENCYREGIONAL : 3820MHZ, NATIONAL : 3725 MHZ NEWS 3725MHZ

MEAN SEA LEVEL350 METRES

350 METRES

FIG: FEATURES OF DOORDARSHAN KENDRA HPT

PROCESS GOING ON IN DOORDARSHAN

SATELLITE COMMUNICATION

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

Satellite

Uplink

Antenna

Downlink

Antenna

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

SatelliteSatellite

Uplink

Antenna

Uplink

Antenna

Downlink

Antenna

Downlink

Antenna

TV Signals from studio are processed and up-linked to the satellite where these

signals are further processed and then down linked to the Terrestrial T.V Transmitters

with the help of transponders of the satellite. At the Doordarshan stations these signals

are received with the help of the Parabolic Dish Antenna (PDA), amplified with the help

LNA and converted to Intermediate Frequency with the help of LNBC (Low Noise Block

Converter). The IF Signals are fed to the Transmitter input stage for further processing

and amplification and finally connected to terrestrial antenna for radiation in to the space.

The reason for using parabolic dish antenna is, as it uses physical features as well

as multiple element antennas to achieve extremely high gain and sharp directivity. These

antennas use a reflective dish in shape of a parabola to focus all the received

electromagnetic waves on antenna to a single point called as feed point. The parabolic

dish antenna also works to catch all the radiated energy from antenna and focus it in a

narrow beam when used for transmitting. By harnessing all of the antenna’s power and

sending it in same direction, this type of antenna is capable of providing high gain. The

parabolic dish antenna is prepared by aluminum as it is a good reflector. Feed point is

also called as centre point . Centre point should be perfectly feed. From the centre point

signal are reflected back to the satellites.

There are also some other type of antenna’s like dipole antenna, flat panel antenna,

slotted antenna, micro strip antenna. Dipole antenna offers best flexibility but it provides

no gain. Flat panel antenna offers greater directionality and would be suited for a fixed

installation. Slotted antenna and micro strip antenna are well suited for moderate

performance applications that need to integrate the antenna and radio into OEM

application. But the parabolic dish antenna provides sharp directionality which provide

point to point link over a long distance. A 2- foot dish at 10 GHz can provide more than

30 db of gain. The gain is only limited by the size of parabolic reflector. These high gains

are achievable if the antennas are properly implemented and dishes have more critical

dimensions than horns and lenses.

The signal received by the parabolic dish antenna is sent to the TVRO of input

output chain with the help of coaxial cable. The signal is divided into visual signal and

aural signal. Demodulator separates audio and video signal. The video Signal is further

amplified to 1V peak to peak by Video Distribution Amplifier ( VDA) and Stab

Amplifier in Video Signal Chain where as the Aural Signal is amplified to 10 dbm by

Audio Distribution Amplifier.

First let us consider VISUAL SIGNAL CHAIN :

This visual signal first undergoes the input monitoring section. Input monitoring

section consists of mainly three section.

Video distributor

Colour Stab Amplifier

Video Equilizer

VIDEO DISTRIBUTOR:

The video distribution amplifiers are employed in Transmitters to distribute

composite video signals to a number of units. This contains two identical distribution

amplifiers each providing five outputs. Here the input signal coming from TVRO is

amplified to 1V peak to peak.

COLOUR STAB AMPLIFIER:

So when the visual signal goes to this amplifier block, the signal is amplified, and

we get perfect 1v peak to peak signal. This is used with sync. processor to process the

composite color video signal. The composite video signal is processed to remove hum

and noise from the timing components in the sync. Processor which produces regenerated

noise free sync and blanking signals. These noise free signals are added to the composite

color video signals in the color stabilizing amplifier.

VIDEO EQUALIZER:

The video equalizer is used to compensate for the video signal attenuation in

cables (OD lengths up to 300 mts ). The equalizing can be adjusted in 21 steps.

AUDIO DISTRIBUTION AMPLIFIER

In the similar way audio signal undergoes many process to get the signal in the

range of 0 to10 dbm. First the signal go through audio distribution block where audio

level correction is carried out. Then the signal undergoes pre-emphasis technique to get

perfect audio signal.

PRE-EMPHASIS - DE-EMPHASIS

In processing audio signals, pre-emphasis refers to a system

process designed to increase, within a band of frequencies, the magnitude of some

(usually higher) frequencies with respect to the magnitude of other (usually lower)

frequencies in order to improve the overall signal-to-noise ratio by minimizing the

adverse effects of such phenomena as attenuation distortion or saturation of recording

media in subsequent parts of the system.

De-emphasis is a process designed to decrease, within a band of frequencies, the

magnitude of some frequencies ( usually earlier pre-emphasised ) with respect to the

magnitude of other frequencies in order to improve the overall signal-to-noise ratio by

minimizing the adverse effects of such phenomena as attenuation differences or

saturation of recording media in subsequent parts of the system. It is the mirror of pre-

emphasis, and the whole system is called emphasis. The frequency curve (response) is

decided by special time constants, from which one can calculate the cutoff frequency.

It may be recalled that 7 MHz bandwidth is provided in band 3in VHF range. At

these frequencies, propagation takes place by space waves limited by maximum line of

sight distance between transmitting and receiving aerials. The signal strength at any place

in the service area must be large enough to overcome noise at that place and provide

satisfactory picture. The radiated power of transmitter is usually expressed as effective

isotropic radiated power (EIRP). In a TV transmitter, amplitude modulation of picture

carrier by video signal can be carried out at high level or a low level modulation.

In early transmitter designs, direct modulation was used. The picture was directly

modulated by video signal. This can be done at a high level modulation in final power

amplifier or at low level RF driving amplifier. At present, I.F modulation at low level is

used.

High level modulation has many disadvantages like,

Video modulator has to supply high video voltages of up to several 100v into

highly capacitive loads, taking amperes of current at video frequency and

therefore employs transmitting type of tubes.

The grid modulated stage presents a varying load to its driver.

The V.S.B characteristics are obtained here by a filter that must handle high

power at operating channel. In a high level modulation, linearity and high

video driving power presents design problems.

The V.S.B filter, to filter out the lower side band partially, must handle total

transmitter output power.

Low level modulation has many advantages like,

In this one or more stage of linear amplifiers follows modulated stage. These

stages must be designed for linear and wideband operation.

The V.S.B filter can be located immediately after modulated amplifier and

hence can be operated at lower power.

Linear amplifier follows modulated stage has to be tuned with care and skill

with help of special apparatus.

Earlier power tubes used for linear amplification suffered from non linearity;

low gain and bandwidth.

Improvements in vacuum tubes have made available a new family of high

power ceramic tetrodes with high gain and bandwidth at V.H.F frequency

with excellent linearity.

High power RF transmitter can now be designed with one or more R.F linear

amplifier following low level modulated stage.

I.F modulation is done at low level modulation in which video signal is carried

out at intermediate frequency, 38.9 MHz video I.F used in TV receiver. The modulated

I.F is then converted to channel frequency by heterodyning it with a suitable oscillator

frequency.

IF modulation has many advantages like;

Since modulation takes place at low power level , modulator section and

visual exciter can be built with solid state devices, giving greater efficiency

and reliability.

It is possible to introduce V.S.B filter at a low power level. Just after I.F

modulator and can be designed with lumped components, not only to shape

lower side band but also upper side band response.

Subsequent stages following V.S.B filter can be tuned permanently for wide

band linear operation.

Visual exciters for all channels in V.H.F or U.H.F bands are identical in

nature in I.F modulated systems. This results in considerable economy.

VESTIGIAL SIDE BAND TRANSMISSION

In the 625 line TV system, frequency components present in the video signal

extend from 0 Hz to 5 MHZ. A double side band AM transmission would occupy a total

bandwidth of 10 MHz. To reduce the channel bandwidth and power, Vestigial sideband

Transmission is in practice. In the video signal very low frequency modulating

components exist along with rest of signal. These components give rise to sidebands very

close to carrier frequency which are difficult to remove by physically realizable filters.

Suppressing one complete sideband also not possible. The low video frequency contains

the most important information of picture and any effort to completely suppress the lower

sideband results in objectionable phase distortion at these frequencies; it will look in the

picture as smear. Therefore only a part of lower side band is suppressed and radiates

signal with full Upper Side Band together with carrier and vestige of the partially

suppressed Lower Side Band. This is called V.S.B or A5C transmission. In the 625 line

system, frequencies up to 0.75 MHz in the lower sideband are fully radiated. So it is a

double sideband transmission for lower video frequency.

Because of filter design difficulties it is not possible to terminate the bandwidth of

signal abruptly at the edges of sideband therefore attenuation slope covering 0.5 MHz is

allowed at either end.

Now these visual and aural signals are given to the exciter for further processing.

In the exciter stage, blocks like video processing unit , diode bridge modulator , delay

equalizer , V.S.B filter , video up converter , linear amplifier , power amplifier and

diplexer and frequency multiplier process the video and audio signals. The combined

visual and aural signal after arriving the diplexer block is transmitted to mast antenna.

BEL TRANSMITTER (CHANNEL-7)

Let us see how the video input gets processed to visual output?

EXCITER UNIT:

1Vp-p Vif

38.9MHZ

Exciter unit consists of four blocks that is :

Video processor

Video modulator

IF synthesizer

Vision output

VIDEO PROCESSOR:

VIDEO INPUT

1V(P-P) 75Ω

VIDEO

PROCESSOR

VESTIGAL SIDE BAND

BAND

IF SYNTHESIZER

BALANCED MODULATOR

1VP-P

It consists of five sections

input buffer

video gain amplifier

peak white limiter

delay equalizer

receiver pre-corrector

First the signal enters into the buffer amplifier for good input return loss and then

passes through a variable video gain amplifier. The gain of this amplifier can be adjusted

to 0.5V to 1.5V (p-p).The output of this is adjusted to 1V (p-p). Then the video signal

enters into the peak white limiter section where it is split into two paths, main signal and

limiting signal generator path. The limiting signal generator path takes the video through

a gain stage where the signal is amplified to 2.5Vp-p and then through a clamping stage

where the video pedestal is clamed to 0V. The video signal which swings above peak

white will generate an error signal in turn is fed to non-inverting input of Video amplifier

with video fed to inverting input. The resulting video available at video amp is clipped

approximately at 1V.

After the peak white limiter section the video passes through 5 stages of phase

equalizers which are used for correcting the group delay that is introduced to the video

INPUT BUFFER

VIDEO GAIN AMPLIFIER

PEAK WHITE LIMITER

DELAY EQUALISER

RX PRECORRECTOR

signal in the transmitter line up. The output of the transmitter delay equaliser is clamped

to remove sag and then buffered through a video amplifier to get 1 Vp-p. Since TV

broadcasting employs vestigial side band transmission, TV receivers have nyquist

characteristics for reception. These nyquist filters introduce group delay errors in the low

frequency region of the detected signal. Notch filters are used as aural traps in video IF

stage and video amplifier in receiver and introduce group delay errors in the high

frequency region of the detected video signal. The receiver pre-corrector is used to pre-

correct these group delays introduced in the receiver.

VSB MODULATOR:

The VSB modulator generates the amplitude modulated vision IF signal at

38.9MHZ. The video signal is first fed to the boot strap buffer circuit, here the signal is

inverted and then fed to the Darlington pair. The Darlington pair feeds the video signal to

clamping stage where the video signal is clamped. The IF signal (+13dbm) at 38.9MHZ

is fed from IF synthesizer and followed by buffer amplifier.

The balanced modulator has the clamped video signal and the buffered IF carrier

signal as its inputs. The diodes modulate the IF carrier. The video signal is applied in

parallel to the modulator, while the IF signal is applied in push-pull, thus the diode

outputs are IF signals modulated to a small percentage and hence a very linear

modulation results. The modulator output is filtered and amplified by the IF amplifier.

The amplifier is set operate between 33 to 41 MHz. The vision IF signal is then passed

through a saw filter to achieve accurate vestigial side band response of the IF carrier.

IF SYNTHESIZER:

It consists of the 38.9MHZ IF synthesizer and 5.5MHZ sound inter carrier

synthesizer and FM Modulator. The 38.9MHZ aural IF is also fed to a mixer to produce

33MHZ.

DIFFERENTIAL GAIN CORRECTOR:

DG corrector corrects luminance and chrominance non-linearities introduced by

power amplifier stages. And it also corrects the differential gains of the video signal. The

out put of the DG Corrector is adjusted to 0 dBm

VISION UPCONVERTER:

VIF I/P

VRF O/P

The vision up converter unit consists of :

DG CONTROL

CIRCUIT

DOUBLE BALANCED MIXER

HELICAL

FILTER

AGC/MGC CIRCUIT

BPFFINAL

AMPLIFIER

LOCAL OSCILLATOR

SIGNAL

DG control circuit

Double balanced mixer

Helical filter

AGC/MGC circuit

Final amplifier

Band pass filter

Local oscillator signal

The composite IF signal is up-converted to channel dependent RF signal using

synthesized local oscillator signal from LO module. Variable attenuator has been

provided to adjust the module gain for +10 dBm RF output.

The vision up converter unit will accept the vision modulated IF signal at 38.9

MHz from DG corrector at 0 dbm. First IF signal is passed through differential gain

corrector stage which precorrects DG distortions at black level introduce by power stage.

The DG corrector IF signal is fed to double balanced mixer where it is mixed with local

oscillator signal which is at +5 dbm level at frequency of fv+fif.

At the output of mixer we have the vision modulated signal at desired channel

frequency (189.25 MHz). Because of mixing unwanted products like multiples of sum

and difference frequencies components harmonic are generated at the output of mixer.

The 5 section cavity tuned helical filter is used for filtering out unwanted components out

of the required channel. The output of helical filter is amplified by hybrid amplifier with

a gain of 27 dB and fed to voltage variable PIN attenuator.

The PIN attenuator is a part of AGC/MGC circuit transmitter. There is a

AGC/MGC switch on up converter. In AGC mode attenuator of pin attenuator is

controlled by the dc voltage fed to its control pin. In AGC mode sample of final vision

RF,(the output of power amplifier ) is fed to vision up converter with a normal level of

+7dBm. Thus any variation in input level is corrected by AGC loop. The output of AGC

amplifier stage is fed to band pass filter before being fed to a output amplifier. The

output amplifier is a broad band amplifier with high power output and low distortion.

.

POWER AMPLIFIER:

The solid state power amplifier module operates over the TV Broadcast band of

174- 230 MHz and can deliver 125 watts (vision) or 370 watts (aural) into 50 ohm load.

The solid state power amplifier consists of two heat sinks, driver amplifier and power

amplifier with various controls and monitoring circuit.

The driver amplifier consists of three gain stage, hybrid amplifier followed by two

power transistor stages operating in class-A mode.

Driver Amplifier output is split in a 90 degree hybrid coupler and fed to two

Power Amplifiers. The Power amplifier consists of two push-pull devices operated in

class AB mode . The outputs of these devices are combined in a high power 90 degree

hybrid coupler to get 125 watt sink peak vision power or 390 watt (aural). A directional

coupler is used at the power amplifier for monitoring and control purpose. The power

amplifier is protected from bad VSWR at the output. A control board monitors the

VSWR through Directional coupler and in case VSWR exceeds limit it immediately

releases a command to switch off the drive to the driver amplifier through pin switch

located in the hybrid amplifier mode. In the same way, abnormal rise in the Driver and

Power Amplifier heat sink temperature are also sensed resulting in drive removal and

switching off the power supplies. The P A is also protected against overdrive conditions

due to high RF input.

At the output of the power amplifier we get 100watts of power. Power is then

given to the circulator for impedance matching and to absorb the reflected power. This

power is then given to 7F VPA (valve), which converts 100watts of power to 1kw of

power. This 7F VPA is one type of high capacity and efficient ceramic Tetrodes. This

1kw power is then given to the circulator for impedance matching and to absorb the

reflected power. Now this power is fed to final stage amplifier, 8F VLPA a ceramic

tetrode, which converts 1kw of power to 10kw. The 10 KW visual power is then fed to

CIN DIPLEXER.

Let us see how the audio input gets processed to aural output?

AURAL EXCITER UNIT:

AIF OUTPUT

The IF synthesizer operates at 5.5 MHz working as a aural

modulator. The incoming audio is amplified and 50 µsec, pre-emphasis is given before

feeding into a varactor diode in the 5.5 MHz synthesizer. It achieves a very low level of

audio distortion and FM noise over a whole audio range. The output of aural exciter goes

to aural up converter.

AURAL UPCONVERTER:

AUDIO INPUT

BALANCED +10dbm 600Ω

IF SYNTHESIZER

AUDIO IF INPUT

MIXER BPF HYBRID FILTER

Aural RF O/P

The Aural IF signal from exciter and local oscillator signal from

local oscillator synthesizer are fed to mixer unit. At the output of mixer we have aural

modulated signal at Fa (channel frequency 194.75 MHz) because of mixing operation

unwanted products like multiple of sum and difference frequency components and

harmonics are generated at the output of mixer. A three section active band pass filter is

used to for filtering off the unwanted components of the required channel. The output of

band pass filter is then amplified and fed to PIN attenuator for automatic gain control.

The pin attenuator output is further amplified by two stage amplifier and fed as aural RF

OUT. This is given to a 4-way divider.

This 4-way divider unit consists of a single 4-way 00 divider. The 4-way divider is

used to divide the exciter aural drive into four parts to drive four aural power amplifiers.

The output of RF patch panel at a level of +3dbm is fed to 4-way divider. Here it gets

divided into four, thus obtaining four final output at a level of -4.4dBm in same phase.

Each of these outputs will drive an aural power amplifier. The output of the power

amplifiers is fed to a 4 way combiner.

4 WAY COMBINER

The 4 way combiner is made up of following assemblies:

LOCAL OSCILLATOR

1. 4 way combiner block assembly

2. Heat sink assembly

The 4way combiner block assembly consists of the combiner PCB mounted on a

block for mounting the connectors. The input has ‘N’ connectors and the output is

through a 1-5/8 adaptor. The combiner PCB has micro strip lines etched on it and

forms a modified Wilkinson type of combiner. Isolation between different inputs is

provided by 50Ω resistors located on a heat sink. The design of the PCB is such that

it can meet the specifications in the complete televisions band-iii. It is important to

note that the optimizing of the band width of each channel is done by proper selection

of cable lengths connecting the terminating isolation resistors.

The combiner PCB by itself does not provide isolation and hence to provide

isolation, resistors are placed on a heat sink and cables of proper design lengths

connect the two. As the unbalance power and the rejected power of the combiner

dissipates in these 50Ω/800w resistors, these are mounted on a heat sink and force

cooled by a blower. The selection of the cable length plays a key role in getting the

bandwidth and isolation of the combiner. In case the combiner unit for one channel is

required to be used for a different channel ensures that the cable assemblies to the

isolation resistors are changed to proper length. Moving the resistors away from the

combiner PCB and placing them separately on a heat sink provides easy cooling and

maintenance but puts more emphasis on design and testing.

Hence at the output of the 4-way combiner we get 1kw power which is then given

to circulator for impedance matching purpose. Thus we obtained the aural output.

Hence, we finally obtain two outputs that are visual and aural outputs in the ratio of

10:1 which is then fed to CIN DIPLEXER.

CIN DIPLEXER:

CIN diplexer (Constant Impedance Notch) is equipment used to combine visual and

aural output of a TV transmitter into a single composite output, to feed a common

antenna while maintaining a sufficient isolation between visual and aural input. It

provides constant input impedance at that input ports.

ANTENNA

S1

DL

S2

ANC aural notch cavity

DC1, DC2 3db couplers

S1, S2 Stubs

DL Dummy Loads

It consists of :-

ANC1

ANC2

VISUAL

TRANSMITTER

DC1 DC2

AURAL

TRANSMITTER

3 db coupler

Aural notch cavities

Dummy load

Rigid coaxial line

Blower (band -3)

AURAL NOTCH CAVITY:

Aural notch cavity is a coaxial resonant cavity tuned precisely to

present a short circuit at its input port at aural carrier frequency. It consists of inner

conductor of quarter wave long, an outer conductor, a coupling loop and tuning plunger

for fine frequency adjustments. The cavity has a very high quality. The inner conductor is

made of “invar” which has very low thermal coefficient of expansion so that fluctuations

in ambient temperature have negligible effect on cavity tuning.

FINE FREQ ADJUSTMENT

COUPLING LOOP . INNER CONDUCTOR

OUTER CONDUCTOR

A coaxial stub is connected across notch cavity. The cavity is

adjusted for series resonance at aural frequency fn. the length of short circuited stub is

chosen such that it forms an anti-resonant circuit (parallel resonance) at vision frequency

fv . Thus the cavity presents a very high impedance in visual band and does not effect

visual signals on the line, while the aural band of frequencies are presented are short

circuit at the series resonance.

3dB COUPLERS:

Any signal given at port1 is divided equally to port2 and port4 with phase

difference of 900 and no output appears at port3. If two signals at same frequency are

applied at port2 and port4 with phase relation, then combined output is obtained at port1

with no power output appears at port3.When port3 is terminated with characteristics

impedance Zo and port2 and port4 by another impedance, the impedance at port1 equals

Zo.

When both port2 and port4 are terminated with Z0, impedance at port1 equals Zo,

even if port3 is terminated with any impedance.

It consists of two 3db couplers dc1 &dc2 which are connected together using co-

axial Transmission lines of equal lengths. Two notch cavities ANC1 & ANC2 are

connected in shunt to the two Transmission lines at the same distance from coupler dc2.

The dummy load DL is connected to port3 of dc1. The visual signal is applied at port1 of

dc1 is split into port2 &4. The signal out of port2 is in phase with input 4 at port4 being

lagging signal input by 90.

These two signals arrive at port1 and port4 of dc2 & are combined at port3 feeding

antenna. No signal appears at port1 of dc2 over visual band. The aural notch cavities

present the high impedance across the lines as they are in parallel resonance with stubs s1

& s2 hence presence of ANC does not effect visual signals. The aural signal input at

port1 of dc2 is split into two parts at port2 & port4 of dc2.

Again with phase relations as mentioned before these two signals get reflected at

ANC1 & ANC2 because of short circuits presented by ANC1 & ANC2.The reflected

signals arrive at port2 and port4 of dc2 & combined signal output from port3 is fed to

antenna. Any aural signal that leaks past the cavities or any visual signals reflected at the

dc2 goes into port3 of dc1 where dummy load DL absorbs signal. Thus isolation between

visual and aural signal are maintained while same antenna is used to radiate two signals

simultaneously.

A meter is provided on CIN diplexer which monitors power flowing into dummy

load which is a measure of performance of CIN diplexer. Normally this absorbs power

less than 10% of aural carrier power so that monitoring meter reading can indicate any

abnormalities in the operation of diplexer. A blower is provided in band-3 to cool

cavities.

Positioning of various parts in diplexer is very important as the parts are made of

copper tube. sufficient care should be taken as use of improper tools are over tightening

may result in the permanent damage .The adjustment process is time consuming so it is

not attempted unless it is necessary.

Hence the visual and aural outputs are given to kathrein 2-way directional filter

combiner through channel-7. This block consists two 3db couplers and two band pass

circuits.

DD-NEWS : NEC TRANSMITTER

(CHANNEL-9)

EXCITER UNIT:

The exciter section consists of various parts like

AD-DA converter

Visual modulator

IF corrector

Digital video compression

Synthesizer

Visual mixer

Aural modulator

IM corrector

Aural mixer

A/D-D/A CONVERTOR

VISUAL MODULATOR

IF CORRECTOR VISUAL

MIXER

VIDEO I/P

VISUAL

REF I/P

VIF O/P

10MHz O/P

600Ω AURAL O/P

75Ω

NICAM IN

A MOD MONITOR A AGC IN A O/P MONITOR

AURAL MODULATOR:

This unit generates a frequency modulated aural IF signal by modulating a voltage

controlled oscillator with an audio input. Two sets of audio inputs are provided one for

600Ω balanced line and other 75Ω unbalanced line used for sound multiplexer

broadcasting. To fix the average frequency of modulator oscillator at the reference input

the automatic phase control (APC) circuit is provided. This unit is applied to a VHF or

UHF mixer circuit.

FEEDBACK IN

DIGITAL VIDEO COMPENSATOR

SYNTHESIZER

AURAL MODULATOR

IM CORRECTOR AURAL MIXER

IM CORRECTOR:

This unit is used for dual sound having two carriers in which some correction of

IM due to the non-linearity of the power amplifier stage can be carried out. This unit

contains a low level and a high level correction circuit each having correction circuits for

amplitude and phase. In the low level circuit, correction can be performed by combining

a non-linear signal generated by class B amplifier with the linear signal, as a result the

phase combining of the linear signal with the non-linear signal produces amplitude

correction. Difference phase combining in the rated phase difference produces phase

correction. High level correction of amplitude is performed by the use of a saturated class

A transistor amplifier high level correction of phase is performed by the use of a class C

transistor amplifier.

AD-DA UNIT:

This unit has functions that converts video input signal supplied to the exciter into

PCM signal and sends the PCM signal to a unit for digital correction and which converts

the video PCM signal after the digital correction into analog video signal and supplies the

analog video signal to a visual modulator unit.

The functions exercised by the AD-DA unit are as follows:

1. Allows switching between input video signal from the main line and that from a

feedback line

2. Converts analog video signal fed to exciter into a PCM signal and supplies the

video data to DVC unit at a next stage.

3. Converts the 12 bit video data from DVC unit in to an analog video signal, and

supplies the signal to V Mod unit.

4. Clamps the pedestal potential of the video signal.

5. Carries out synchronous separation of the main input video signal, the feedback

input video signal and that of the input synchronous signal when scrambling is

used.

DIGITAL VIDEO COMPENSATOR:

This unit compensates by the use of digital signal processing technology

distortion of input video signals and different types of distortion like linear and

nonlinear distortions produced in transmitter and receiver. It is composed of a non-

linear distortion compensating circuit, linear compensating circuit, control circuit,

etc. It will receive the demodulated output signals of the transmitter and

automatically compensates for the di The non-linear distortion compensating

circuit (for differential gain, differential phase and luminance linearity)

compensates as an auxiliary circuit to the IF corrector. The non-linear distortion is

caused by the power amplifier of the transmitter. Furthermore, the circuit is an

APL follow-up type that can compensate for variations in the characteristics of the

power amplifier by APL.

The linear distortion compensating circuit (pre-compensation circuit

for compensating, or the frequency and group delay characteristics of the

transmitter, and the group delay characteristics of receiver) performs high

accuracy compensation by using a 128 tap digital filter (for input signals with the

filter co-efficient of 12 bits) The non-linear and linear distortion compensating

circuits respectively can be bypassed.

The control circuit is provided with a DSP (digital signal processor)

that analyze demodulated output of the transmitter and automatically computes the

compensation values.

VISUAL MODULATOR:

This unit is intended to convert a base band video signal into a modulated IF

signal with ring modulator in which IF carrier is also phase modulated by a processed

video signal to pre-correct the incidental carrier phase modulation (ICPM). The video

signal for the IF phase modulation is arbitrarily sliced into three regions of sync, black

and white in which each signal is individually expanded or compressed then summed into

the processed video signal by which the carrier for the ring modulator is phase

modulated.

IF CORRECTOR UNIT:

This unit is generally used for correction of non-linear distortion generated in the

power amplifier stage enables correction of DG and DP characteristics of visual signal.

This also contains a means to combine two modulated IF carriers of visual and aural

allowing multiplex operation of the transmitter.

VHF MIXER UNIT:

In this unit the IF signal applied at input is converted to an RF signal and the RF

signal is then passed through filters to separate out only the specified band and amplified

to obtain an RF signal of +20dbm. By applying AGC to IF signal, the output of the

transmitter is maintained at a constant level.

SYNTHESIZER UNIT:

This unit generates signals at three frequencies visual IF and local frequency.

The exciter section is to receive two input of both video and audio incoming to the

TV transmitter. It contains same signal processing plug in board by which signals are

converted into IF modulator visual and aural TV signals. The visual and aural outputs are

then given to exciter switch. The modulated visual IF signal passes through a VHF mixer

unit and IF corrector unit and modulated aural IF signal passes directly through a separate

VHF mixer unit of same type, thereby producing VHF TV signal on desired channel. The

visual output power that is 100 mw is given to 2-way divider block, which is used for

dividing the signal to two transistor power amplifier blocks and two transistor power

amplifier in parallel. This two 50 mw power is then given to 5-way divider block, where

the power gets divided into five parts producing 1kw each which gets combined in a 5-

way combiner resulting in a total power of 5kw. The two 5kw power (10kw) gets

combined into 2-way combiner and results in 10kw of power which is then given to V/A

combiner.

V/A COMBINER UNIT:

V/A combiner work in the similar fashion as CIN –diplexer in BEL transmitter,

and are used to combine the visual and aural outputs.

VISUAL I/P

AURAL I/P DL

O/P ANTENNA

V/A

COMBINER

HARMONIC FILTER

OUTPUT FILTER

CIBD UNIT:

CIBD stands for Constant Impedance Band Diplexer which is similar to CIN

Diplexer in BEL transmitter. The output filter and harmonic filter are used for combining

visual and aural signals of a VHF TV transmitter and attenuating spurious and harmonics.

A simple explanation of the CIB Diplexer is as follows:

The signal of the aural transmitter applied at terminal (1) of the 3db coupler H1

appears at terminals (2) and terminal (3) with the same amplitude but with a phase

difference of 900 (terminal (2) is 900 ahead in phase of terminal (3) , because of nature of

3db coupler, no output appears at terminal (4).

The signals appearing at terminal (2) and terminal (3) then passes through aural

band pass filters respectively, and terminal (21) and terminal (31) of other 3db coupler H2

still with 900 phase difference.

OPF

Vin

CIBD

1 2 A 21 11

Ain BPF1

H1 H2

4 3 BPF2 B 31 41 O/P

The signals appearing at 3db coupler H2 terminal (21) and terminal (31) with 900

phase difference are combined in the H2 into terminal (41) because signal at terminal (21)

has a 900 lead phase from that at terminal (31) but no output appears at terminal (11). The

signal appearing at terminal (41).

Mean while, the visual transmitter output is connected to terminal (11) of 3db

coupler H2. The visual signal entering this terminals does not appear at terminal (41), but

at terminal (21) and terminal (31) with same phase difference of 900 (terminal (21) is 900

ahead in phase of terminal (31)).

HF

The visual signals are reflected at points A and point B, then reaching terminal

(21) and terminal (31) of 3db coupler H2. Since the length of point A to terminal (21) of

H2 is equal electrically to that of point B to terminal (31) of H2, the visual signal entering

to terminal (21) of H2 is combined with that to terminal (31) because of nature of 3db

coupler. Then the combined visual signal appears at terminal (41).

The CIB Diplexer has constant input impedance as viewed from visual and aural

input and allows sufficient isolation between visual and aural signals. Accordingly, it can

supply visual and aural signals to antenna without mutual interference.

The absorbing resistor absorbs aural signal components reflected by

filters and the visual signal components passed through filters.

ADVANTAGES OF NEC TRANSMITTER OVER BEL TRANSMITTER

High reliability and low maintenance are two reasons NEC’s transmitters are

known for.

1. HIGH PERFORMANCE EXCITER:

Nonlinear distortions are corrected at the videos and IF stages. Among the nonlinear

parameters are luminance distortion, differential gain (DG) , differential phase(DP) , and

incidental carrier phase modulation(ICPM). All of these items are individually corrected.

Higher reliability, performance and maintainability can be obtained. The exciter also

employs a pedestal AGC circuit and a surface acoustic wave vestigial side band filter.

2. Highly efficient transistor power amplifier unit

3. Compact components for ease of installation newly developed, greatly maintained

component parts are mounted on each cabinet in such a manner as to facilitate ease

of maintenance service.

4. EASE OF OPERATION:

The transmitter is equipped with a display panel for visually displaying all

operating and faults status for the operator in one convenience location. Supplementing

the EL display, the fault displaying functions distributed among several units. in order to

improve the operational flexibility of the television transmitter and the expectancy of the

visual and aural transistor power amplifier unit, a wide variety of circuit design

improvements have been incorporated.

FUNCTIONING OF CONTROL SYSTEM OF NEC TRANSMITTER:

CONTROL SYSTEM:

The transmitter can be controlled in three modes

Remote

Local

Maintenance

In either mode all controlling signals are processed in the transmitter control

which is also capable of controlling signals are processed in the transmitter control which

is also capable of controlling several equipments such as V/A combiner and main

blowers.

TRANSMITTER CONTROL SYSTEM:

This control system applies to TV transmitters. This system is composed of

different types of equipment that are described in the following section and which

respectively have special functions. The details of monitor and control by this system are

all displayed on a display unit, the EL display, where operators can read the necessary

information. Furthermore, an RS-232C connector for serial data communication fitted to

this system allows for connection of this system to PC, thus enabling monitoring of

information the same as is on EL display.

EL CONTROL:

This will control the display on the EL DISPLAY unit that receives information

signals from transmitter control and which makes display for monitoring the TV

transmitter. This EL control is mounted with a CPU and performs control operation by

means of control software. Furthermore, the EL CONTROL is provided with an RS-

232C serial interface connector to send out the data displayed on the EL DISPLAY. This

performs control of the display but has no relation to control of TV transmitter.

TRANSMITTER CONTROL:

The transmitter control is a controller system for television transmitter series. This

controls and monitors the television transmitter and in the event of a serious alarm in the

transmitting system shuts off the transmitter in order to protect it from the serious

damage. All of the information monitored by the transmitter control is displayed on the

EL control. The information is also sent to the relay card and transmitted to external

equipment. To enable analogue values to be displayed on the EL display, the output

power of the television transmitter, reflective power V/A combiner , absorbed power,

information from transistor power amplifiers and the output level and deviation of the

mixer in the exciter are converted to digital signals.

TX-PA MONITOR:

This TX-PA monitor is a part of monitors and control system of television

transmitters. This distributes the signals coming from transmitter control to transistor

power amplifiers (TR-PA’S) and power supplies for transistor amplifiers. Furthermore

this monitor samples output power , reflection power, temperature, DC voltage, drain

current, alarm etc in sequence and time division multiplexes and sends this data through a

data bus line to transmitter control.

TR-MONITOR:

This unit indicates the transmitter output power and detects any abnormal

condition

Hence the DD-NATIONAL and DD-NEWS channels are combined in kathrein

block through channel-7 and channel-9 producing the total output power of 20kw which

is again further divided into two parts each 10kw and is given to mast antenna and finally

from mast antenna consumer receives the channels.

LATEST ADVANCEMENT IN TRANSMISSION:

Direct-to-Home (DTH) satellite television is becoming a buzzword in the satellite

broadcast industry due to the fact that DTH offers immense opportunities to both

broadcasters and viewers. Thanks to the rapid development of digital technology, DTH

broadcast operators worldwide have been able to introduce a large number of new

interactive applications in the television market besides a large number of entertainment

programmes over a single delivery platform. In addition, since digital technology permits

a highly efficient exploitation of the frequency spectrum, the number of TV channels that

can be broadcast using digital technology is significantly higher than with analogue

technology. The increased number of television channels allows the operator to satisfy

the demand of a number of niche markets with dedicated transmissions.

In general, DTH service is the one in which a large number of channels are

digitally compressed, encrypted and beamed from very high power satellites. The

programmes can be directly received at homes. This mode of reception facilitates the use

of small receiving dish antennas of 60 to 90 cm diameter installed at convenient location

in individual buildings without needing elaborate foundation /space etc. Also, DTH

transmission eliminates local cable operator completely, since an individual user is

directly connected to the service providers. However, a digital receiver is needed to

receive the multiplexed signals and view them on a TV. DTH, in sharp contrast to Cable

TV, lends itself to easy monitoring and control.

DTH is digital in nature hence more number of channels are available and

bandwidth is reduced. This works in Ku-band for up linking and down linking process.

DEFINITION:

DTH is defined as the reception of satellite programmes with a personal dish in an

individual home. And an individual Set Top Box empowering you to pick & choose you

bundles of choice and pay for what you watch.

In DTH, TV channels will be transmitted from the satellite to a

small dish antenna mounted on the window or rooftop of the subscriber's home. So the

broadcaster directly connects to the user.

DTH can also reach the remotest of areas since it does away with the intermediate

step of cable and wires (cables) that come to your house. DTH offer digital superior

quality picture against cable TV today which is most analog.

DTH is an encrypted transmission that travels to the consumer directly through a

satellite. DTH transmission is received directly by the consumer at his end through a

small dish antenna. The encrypted transmission is decoded by an individual STB at your

home.

The other advantage of DTH is the availability of satellite broadcast in rural and

semi-urban areas laying of cable is difficult. Due to digital compression techniques, many

more niche channels are available than cannot be provided by cable operators. DTH also

offers digital quality signals which do not degrade the picture or sound quality. It also

offers interactive channels and program guides with customers having the choice to block

out programming which they consider undesirable.