Download - Study of Nec Transmitter

1. INTRODUCTION

1.1 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

programs. Television sets established under various schemes in community centers 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 nationwide 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.

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1.2 FEATURES OF HPT’S:

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 93.50 740, 93.50

AZIMUTH 1490 2200 ,1490

ELEVATION 670 63.20,670

DOWNLINK FREQUENCYREGIONAL : 3820MHz, NATIONAL :

3725 MHz NEWS 3725MHz

MEAN SEA LEVEL 350 METRES

350 METRES

Table 1.1 features of doordarshan Kendra HPT

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2. PROCESS GOING ON IN DOORDARSHAN

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

FIGURE 2.1 processes going on doordarshan

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.

The signal received by the parabolic dish antenna is sent to the TVRO (television receiver

only) 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 dB by Audio Distribution

Amplifier.

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2.1 VISUAL SIGNAL CHAIN:

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

consists of mainly three sections.

Video distributor

Color Stab Amplifier

Video Equalizer

2.1.1 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 (television receiver only) is

amplified to 1V peak to peak.

2.1.2 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.

2.1.3 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.

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2.2 AUDIO DISTRIBUTION AMPLIFIER:

In the similar way audio signal undergoes many processes to get the signal in the range of

0 to10 dB. First the signal goes through audio distribution block where audio level correction is

carried out. Then the signal undergoes pre-emphasis technique to get perfect audio signal.

2.3 PRE-EMPHASIS and 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-emphasized ) 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.

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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.

2.4 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.

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3. DD-NEWS (NEC TRANSMITTER)

(CHANNEL-9)The block diagram of the NEC transmitter is as shown in figure bellow.

NEC 10 KW Transmitter

Figure 3.1 Complete Block Diagram of a 10 kW TV Transmitter (NEC)

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3.1 EXCITER UNIT:

The exciter section consists of various parts like

A/D-D/A converter( analog to digital-digital to analog converter)

Visual modulator

IF corrector( intermediate frequencey corrector)

Digital video compression

Synthesizer

Visual mixer

Aural modulator

IM corrector( image corrector )

Aural mixer

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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

Figure 3.2 block diagram of exciter unit

3.1.1 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.

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A/D-D/A CONVERTOR

VISUAL MODULATOR

IF CORRECTOR VISUAL

MIXER

VIDEO I/P

FEEDBACK IN

DIGITAL VIDEO COMPENSATOR

SYNTHESIZER

AURAL MODULATOR IM CORRECTOR AURAL MIXER

3.1.2 IM CORRECTOR (image 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.

3.1.3 A/D-D/A UNIT(A/D Convertor D/A convertor):

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 A/D-D/A 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 (digital video compensator) 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.

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3.1.4 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

digital video. 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.

3.1.5 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.

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3.1.6 IF CORRECTOR UNIT:

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

amplifier stage enables correction of DG (differential gain) and DP (differential phase)

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.

3.1.7 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 (automatic gain control) to IF signal, the output of the

transmitter is maintained at a constant level.

3.1.8 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.

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3.2 POWER DIVIDERS

3.2.1 2-WAY POWER DIVIDER

1) GENERAL:

This 2-way power divider for dividing the signal to two transistor power

amplifier blocks and two transistor power amplifiers is parallel. 2-way power divider

using a 2-way zero degree type 3db hybrid as shown in figure.

Figure 3.3 2 way power divider

2) FEATURES:

This 2 way power divider has the following features

Wide frequency range

Low transition loss

Low input return loss(VSWR)

3) SPECIFICATION:

The specification of this 2 way power divider is shown in table 3.1.

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Table 3.1 specifications of two way power divider

4) DESCRIPTION:

The outside view of 2-way power divider as shown in figure 3.4.

Figure 3.4Figure 3.4 outside view of 2 way power divider outside view of 2 way power divider

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ITEM SPECIFICATION

1.frequancey range 170-230MHz

2.VSWR ≤1.2

3.return loss ≥20dB

4.insertion loss ≤0.6dB

5.maximum input power rating 10mw

6.input/output impedance 50Ω

7.input/output terminal BNC-j

8.ambient temperature -10 to +45°C

9.dimensions W×H×D 200×55×22mm

3.2.2 5-WAY POWER DIVIDER

1) GENERAL:

This 5-way power divider for dividing the signal to two transistor power

amplifier blocks and five transistor power amplifiers is parallel.5-way power divider

using a 2-way zero degree type 3dB hybrids as shown in figure.

Figure 3.5 5 way power divider

2) FEATURES:

This 5 way power divider has the following features

Wide frequency range

Low transition loss

Low input return loss(VSWR)

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3) SPECIFICATION:

The specification of this 5 way power divider is shown in table

ITEM SPECIFICATION

1.frequancey range 170-230MHz

2.VSWR ≤1.2



5.maximum input power rating 2mw


7.input/output terminal BNC-j

8.ambient temperature -10 to +45°C


Table 3.2 specifications of 5 way power dividers

3.3 POWER AMPLIFIERS:

3.3.1 OPERATING PRINCIPLES:

The drive PA1 comprises two stage of power amplifier circuits

containing PIN attenuator, a phase shifter, RF limiter, hybrid IC MC5388 and FET 2SK1543

A signal from J1 (BNC-J) on the front panel passes through input and the

PIN attenuator, Where the gain of the unit is adjusted. The phase shifter adjusts the phases

between two units when this unit runs in parallel with another. The phase shifter can adjust the

phase in a range within approximately 40 degrees. The RF amplifier protects the unit from

overdrive input signal and has been adjusted so as to operate 1dB above the input rating.

The MC5388(IC 301) at the first stage, which is a class A hybrid IC, has a

gain approximately 18DB. The next stage amplifier circuit having an FET 2SK1543 (TR301)

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which is a single class AB has a gain of approximately 18dB. The output from the FET 2SK1543

(TR301) passes through an isolator W1 and then is to the drive PA2.

3.3.2 DRIVE PA2 CIRCUIT:

The drive PA2 comprises one stage of power amplifier having a

2SK1543M. The drive PA2 is class AB operation with a gain of approximately 16dB. The output

from the drive PA2 circuit passes through an isolator W2 and is distributed by a Wilkinson 3-

way divider and then fed to the final PA.

3.3.3 FINAL PA CIRCUIT:

The final PA circuit is the final stage amplifier circuit supplied with six

2SK1543MPS which uses a pair of characteristic matched 2SK1543M in parallel and it

comprises two stages of 2 way distribution circuits of distributed type and one stage of power

amplifier using 2SK1543S as well as a circulator combiner. The signal from the drive PA is

divided into six sub signals and distributed by the distributor and each final stage amplifier

circuit. the final stage amplifier circuit of optimum class AB push pull design using

2SK1543MPs recurs a gain of approximately 16dB, maximum out of 250W or more and a drain

efficiency of 70% or higher at the maximum output. The 2SK 1543s used in this units are

developed MOSFETs of a push pull constructions which are of a high output, a high gain, a high

reliability for use in particularly VHF band TV transmitter.

Six outputs of the signals amplified at the final stage amplifier circuit

lead to a 6-way combiner through circulator.

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3.4 POWER COMBINERS:

3.4.1 5-WAY POWER COMBINER

1) GENERAL:

This 5-way power combiner is used for combining five transistor power amplifiers in

parallel to obtain the required power. The general diagram for 5 way power combiner is given

below.

Figure 3.6 5 way power combiner

2) FEATURES:

This 5-way power combiner has the following features

1) wide frequency range

2) low insertion loss

3) low input return loss(VSWR)

3) SPECIFICATION:

The specification of this 5-way power combiner is as shown in table

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Table 3.3 specifications of 5 way power combiner

4) DESCRIPTION:

The type of this 5-way power combiner uses quarter wave impedance

transforms into combiner

More ever this 5-way power combiner is made of copper plate strip line

with in aluminum case.

The copper plate strip line is supported from the aluminum case by Teflon

insulators.

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ITEM SPECIFICATION

1.frequencey range 170-230 MHz

2.VSWR ≤1.2



5.maximum input power

rating

1000W


7.input/output terminal W×-20D/W×-39D

8.ambient temperature -10 TO 45°C


3.4.2 2-WAY POWER COMBINER

1) GENERAL:

This 2-way power combiner is used for combining two transistor power

amplifier blocks and two transistor power amplifiers in parallel to obtain the required

power. The general diagram for two way power combiner given below.

Figure 3.7 two way power combiner

2) FEATURES:

This 2-way power combiner has the following features

1) wide frequency range

2) low insertion loss

3) low input return loss(VSWR)

3) SPECIFICATION:

The specification of this 2-way power combiner is as shown in table.

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Table 3.4 specifications of 2 way power combiner

4) DESCRIPTION:

The type of this 2-way power combiner uses quarter wave impedance

transforms into combiner

More ever this 2-way power combiner is made of copper plate strip line

with in aluminum case.

The copper plate strip line is supported from the aluminum case by Teflon

insulators.

3.5 V/A COMBINER UNIT:

V/A combiner work in the similar fashion as CIN(constant impedence notch) –diplexer in

BEL transmitter, and are used to combine the visual and aural outputs. V/A combiner 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.

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ITEM SPECIFICATION

1.frequencey range 170-230 MHz

2.VSWR ≤1.2



5.maximum input power

rating

100W


7.input/output terminal W×-20D/W×-39D

8.ambient temperature -10 TO 45°C


VISUAL I/P

AURAL I/P DL

O/P ANTENNA

Figure 3.8 V/A combiner unit

3.6 CIBD UNIT:

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

impedance notch) 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).

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V/A

COMBINER

HARMONIC FILTER

OUTPUT FILTER

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

Figure 3.9 block diagram of CIBD

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).

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HF

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)).

The visual signals are reflected at points A and point B, then reaching terminal (2 1) 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.

3.7 FUNCTIONING OF CONTROL SYSTEM OF NEC TRANSMITTER:

3.7.1 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.

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3.7.2 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(electroluminicent) 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.

3.7.3 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.

3.7.4 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.

3.7.5 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

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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.

3.7.6 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 Kathrin 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.

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4. 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.

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5. 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 programs 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 nice 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 programs 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.

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5.1 DEFINITION:

DTH is defined as the reception of satellite programs 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.

Figure 5.1 DTH transmission

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.

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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 (set top box) 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.

CONCLUSION & OBSERVATIONS

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We have done the project at doordarshan Kendra Simhachalam in Visakhapatnam. There we

have gone through all the different varieties of transmitters and antennas. There we have studied

the digital transmitter NEC (DD-NEWS) and DTH in detail. We came to conclusion that the

digital transmitters are much more effective than the solid state transmitters in terms of audio and

video quality and also maintenance cost. We have seen the improved version of DTH receiver

which will give good clarity of TV pictures and quality with the expense of high bit rate

transmission and it is beneficial to the remote areas where the cost of installation of cable

transmission is high.

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