19-12-2012, 04:21 PM
A INDUSTRIAL TRAINING REPORT ON TELEVISION TRANSMISSION STATION
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INTRODUCTION
The Television Relay Kendra is divided into 2 major areas :
1. Transmitter
2. Receiving Equipments
The report describes each of these 2 sections along with suitable block diagrams and along with a high level implementation perspective of each.
THE TRANSMITTER
KW VHF TRANSMITTER ::
The analog transmitter uses AM for video and FM for audio.
Exciter
The first block of the transmitter is the Exciter . This unit is used to produce the required IF signal of 38.9 Mhz for video signal obtained from the receiver and the audio IF of 33.4 Mhz . In this exciter the RF frequency ( channel frequency ) is allocated for each channel and the necessary frequency conversion is made . There are 2 exciters of which the second one is for stand by purpose. The exciter has a AGC unit and the audio and video signals are given to the exciter from the receiver. This signal consists of the video signal with the sync and blanking pulses along with the audio signal. The AGC circuit is used to maintain the signal amplitude levels constant with separate AGC for audio and video signals. It is to be noted that the amplitude of the voltage of the video signal is always maintained at 1V p-p and it is the power of the signal alone that is boosted in the transmitter. No voltage amplifiers are used and only voltage driver amplifier are used which in spite of input fluctuations in amplitude maintain a constant output amplitude .
Power Amplification
The output from the exciter is fed to an exciter switch. Here the video and audio signals are separately selected. The video signal is fed to a 2-way divider . Each of the 2 output of the divider is fed separately to a 5-way divider. The 5-way divider has a 1KW power amplifier at each of it's 5outputs. The 10 video signals from each of the power amplifiers has a total power of 10KW ,which is the required output power to drive the transmitter . The outputs of the PA are fed to a 5-way combiner to give a single output . The outputs of each of the 5-way combiner is fed to a 2-waycombiner to get a single 10KW video signal .The exciter switch also selects the audio signal and feeds it to a 2-way divider to get 2audio signals. Each of the divider output is fed to a PA with a power rating of 1KW but operating at 500W.So the 2 outputs of the PA give a 1KW audio signal . This is the required audio signal power for transmission , as the video power to audio power must have a ratio of 10:1 . this 10 : 1power ratio was used in older transmitters , while modern transmitters have a power ratio of 20 : 1. The PA output is fed to a 2-way combiner to give a single 1KW audio signal. Thus the audio signal goes through a chain of 2 amplifier stages and the video signal goes through a chain of 3amplifier stages .
Combining Video & Audio For Transmission
The audio and video power outputs are combined in the CIN ( Constant Impedance Notch )diplexer and tuned notch filter section and the fed to the antenna through a tee – transformer for transmission. The signal to be transmitted can also be fed to a dummy load for measurement. This is used to test the transmitter in the absence of the antenna, such as the case of troubleshooting the transmitter, when the dummy load accepts the transmitted power. This enables to isolate the cause of the fault, either in antenna section or in the transmitter section. This load must be able to absorb the total10KW + 1 KW power and so is rated for 20KW. The combined audio and video signal from the tee – transformer is fitted into 2 feeder cables. Each of the feeder cable is connected to a junction box. Each junction box has 12 Bunk Feeder Cables (BFC). Each BFC goes to a dipole antenna. So there are a total of 24 BFCs connected to the dipoles. The dipoles are arranged such that each side has 8 dipoles leaving the side facing the sea. These dipoles are mounted on a 150m high mast which makes up the TV transmitting tower. Signal in the transmitter is carried in hollow metallic copper pipes. These pipes are called coaxial pipes. The outer copper is the shield and is grounded. The video and audio signals propagate through the inner hollow pipe in the form of electrical signals. The audio signal pipes are noted to have a smaller diameter than those carrying video signals. The power amplifiers used are solid state amplifiers using MOSFET technology though valve amplifiers are still in use. The advantage of using MOSFET amplifiers lies in the easy isolation of faults in case of amplifiers failure. Also solid state amplifiers modules of 1KW rating are readily available and any wattage rating can be easily obtained by just pulling back a particular module i.e. by just varying the position of the module in the package the configuration can be changed to give a different rating.
Modulation & Bandwidth
The television transmitter uses negative modulation technique. The video signal in an analog transmitter is VSB modulated. The need for using VSB is that it provides improved low frequency response and can also have DC response. This helps in ease in the filter design at the receiver. In the VSB modulation the portion of the lower side band is transmitted. This is because of the sensitivity of the human eye to low frequency signals. Hence any attenuation of the low frequencies due to filtering will produce a visible distortion to the viewer. To avoid hid the lower portion of the sideband is transmitted. The VSB signal has a bandwidth of 7 Mhz. The carrier frequency for each channel is specified in terms of the video carrier frequency . The audio carrier is always placed at 5.5 Mhz. away from the video carrier in the positive direction . The audio signal has a bandwidth of 0.5 Mhz centered around the carrier . The VSB signal has a lower vestige of 1.25 Mhz to the left of the video carrier in the negative direction
Low Power Television Transmitters
The transmitter used at the TV broadcast station is a High Power Transmitter (HPT). As it's range is limited to a 70km radius , regions beyond use LPTs. These Low Power Transmitters have video powers of 100W(18-20 km ) and with powers of 300W, 500W for UHF ranges ( this increase in power is because of the increase in attenuation with increase infrequency ) in the older equipment systems. Modern LPTs have 1KW power and they consist of a small area. Here the signal from the satellite is received and it is then broadcast through terrestrial transmitters. In India there are above 1200 LPTs in operation with a coverage of 90% for DD. AIR has nearly reached 100% coverage. Baseband signal for audio information is 20Hz – 20KHz Baseband signal for video information is 0MHz - 5MHz.
DIGITAL TRANSMISSION - DVB – T
Digital transmission is provided by one transmitter which can handle 5 channels at a time. It uses Quadrature Amplitude Modulation(QAM) for signal transmission.
Encoding
The video signal and the audio is digitized at the MSR and they are routed through a Input Router Video / Audio to the encoders. Here the Serial Digital Input ( SDI ) signals from the satellite receiver is also routed to the encoders . The data signals from the RS – 232 and RS – 485 interface panels is also routed through Input Router Data. These signals are fed to the Encoder units. There are totally 5 encoders used. There is one standby encoder and two unfitted encoders. The digital transmission uses MPEG – 2 (Motion Pictures Expert Group) encoding. The outputs from the encoders go to a multiplexer unit. The multiplexer also receives input from the computer for control signals. The multiplexer output labeled as ASI is fed to an output router which is controlled by a terminal server. The output router is terminated by a 75 ohm load and the other output feeds an IF splitter. The IF splitter output goes to the ASI interface panel .This panel provides two outputs.
Modulation
Each output is fed to a DVB-T modulator. Here Quadrature Amplitude Modulation (QAM) is used to produce the required IF signal. The output of each modulator is fed to an exciter. The exciters have a AGC control signal, a reference monitor output and the RF output. The exciter converts the IF input to the required high frequency signal for transmission.
MULTIPLEXER AND MODULATOR
The compressed signals are fed to a multiplexer unit which combines the three different streams into a single transport stream. The multiplexer has external controls that enable selection of e and the clock rate for multiplexing. The output of the multiplexer is modulated using Quadrature Phase Shift Keying ( QPSK ) technique. This gives a signal with a symbol rate of 8.6Mbps. This is the intermediate frequency of 70MHz. The IF signal is fed to the Up Converter that increases the signal frequency to the Giga Hz range to enable satellite communication.
TRANSMISSION
The C - Band transponder used with the satellite INSAT 3A has a up link frequency of 6056 Mhz. and a down link frequency of 3831 Mhz. The transponder has a bandwidth of 36 Mhz., with total BW of 40 Mhz. and a guard band of 4 Mhz. The up link frequency is then fed to a Klystron High Power Amplifier which provides the necessary power to the signal for up link to the satellite. The up link signal has a power of 3KW. The klystron must provide the minimum power required to be maintained which is specified as the signal Effective Isotropic Radiated Power (EIRP) and is 30W / 63dBW. The HPA feeds the signal to a parabolic dish through a waveguide. The waveguide feeds the signal at the Feed Point of the dish. The dish then focuses the EM waves as a fine pencil beam and directs it to the satellite. [ Now one may consider the Feed Point of the dish and the Transponder in the satellite as the two focus of an ellipse. ] The up link uses horizontal polarization and the down link uses vertical polarization. This up linked signal is received by the transponder and is down converted to a slightly lower frequency. This is done in order to reduce the loss of the signal and is achieved by beating the uplinked signal with a 2Ghz. local oscillator.
DOWN LINK SEGMENT
The satellite transmitting antenna also serves as the receiver. In the receiver the conversion from the C – band to the L – band takes place. The antenna has a Low Noise Block Converter (LNBC ) unit in the antenna unit itself. This is where the frequency conversion takes place. Also the received signal is of very low strength due to the high attenuation suffered during propagation. Hence the LNBC amplifies the signal without causing any distortion to the received signal. The down link power is fixed at 10W. So the receiving antenna size must be varied to receive the signal at different regions of the world. Hence the areas that lie at the satellite foot print receive the signal of good strength. However the areas at the corners tend to receive a very distorted signal from the satellite. Hence the fringe areas will use receiver dish antennas of larger diameter. This is so because the received power is directly related to the dish radius. The areas under direct footprint need only smaller radius dishes. The signal from the antenna unit is then converted to the common IF frequency of 70 Mhz. at the tuner section of the set top box. The signals are de-multiplexed and then the desired signals viewed in the monitor.
TVRO ( T V Receive Only ) EQUIPMENT
This equipment is used to receive the signal from a different TV station telecast by satellite. For example the DD-News channel signals are up linked to the satellite from the New Delhi kendra and they are received at the various kendra using the Television Receive Only apparatus. This signal is then processed, wherein, it is converted to the standard IF of 70MHz from the higher down link frequency. By using the super heterodyne principle the audio and video are recovered from the 70Mhz IF frequency. The recovered signal is then modulated with RF carrier signals by standard analog modulation schemes. Using the VHF transmitter the signal is then transmitted through surface wave transmission to be received by the regional audience.
DIRECT TO HOME (DTH ) SERVICE
This service was introduced by Doordrashan to provide programmes to the fringe areas of the nation. This concentrated mainly on the North – Eastern states ( sister states ) where the terrain makes the any terrestrial transmission and reception difficult and remote hilly area of India. The DTH uses digital transmission technology to provide the digital content to it's subscribers. The DTH service provider receives the signals from all the TV content providers ( i.e. From all the channels broadcasting ) using custom receiving equipments.