Study of Nec Transmitter
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1. INTRODUCTION1.1 ORIGIN OF DOORDARSHANDoordarshan 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 80s 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.
1.2 FEATURES OF HPTS:
DD - NATIONAL (CHANNEL-7)
INTEGRATED ON TRANSMITTER TYPE CHANNEL VHF(band-III) VISION POWER AURAL POWER VISION FREQUENCY AURAL FREQUENCY SATELLITE LOOKING ANGLE AZIMUTH ELEVATION DOWNLINK FREQUENCY MEAN SEA LEVEL
02-08-1986 BEL 0710KW 1KW 189.2396 MHz 194.7396 MHz INSAT 3A 93.50 1490 670 REGIONAL : 3820MHz, NATIONAL : 3725 MHz 350 METRES
01-04-2003 NEC 09 10KW 1KW 203.26MHz 208.76 MHz INSAT 3C, INSAT 4B 740, 93.50 2200 ,1490 63.20,670 NEWS 3725MHz 350 METRES
Table 1.1 features of doordarshan Kendra HPT
2. PROCESS GOING ON IN DOORDARSHANSatellite
U lin p k A ntenna
Dow link n A ntenna
E arth station Terrestrial system User
E arth station Terrestrial system U ser
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.
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.
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. 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.
3. DD-NEWS (NEC TRANSMITTER) (CHANNEL-9)The block diagram of the NEC transmitter is as shown in figure bellow.
NEC 10 KW Transmitter6
Figure 3.1 Complete Block Diagram of a 10 kW TV Transmitter (NEC)
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 )
VISUALDIGITAL VIDEO COMPENSATOR
REF I/P VIF O/P 10MHz O/P 600 75AURAL MODULATOR
AURAL O/PIM CORRECTOR AURAL MIXER
A MOD MONITOR
A AGC IN A O/P MONITOR
Figure 3.2 block diagram of exciter unit
3.1.1 AURAL MODULATOR: