Introduction: Dated on 12th March, 2011, we had the tour to the Khulna television substation located at Khalishpur. This substation has been operational since 1977. The job of a substation is to retransmit the audio-video signal received from the mother station, so that the receivers in that locality can get the low video feed amplified from that substation.
Objective: Our objective is to observe and understand the system of operation of that substation. To see how the signal is received, amplified, combined and retransmitted.
Basic structure and operating principle: Things we found out in that substation are as follow.
Signal reception: For signal reception BTV uses ASIA SAT 3S, location 105.50, east. It is a geostationary satellite about 36000 km above the sea level. This satellite has three levels, leo, meo and geo. The geo level contains the C band ranging from 3.7GHz to 4.2GHz which is used in this signal reception. Signals are divided in some bands. 41 to 68 MHz is the first band used for navigation. 88 to 108 MHz is the 2nd band used for FM. 3rd band is 174 to 230 MHz. For Khulna channel no. II there is 216 to 223 MHz.
Composite video signal: The composite video signal
as seen from the substation is given here. We were
not allowed to use any camera, otherwise genuine
photos could be attached to this report. As seen from the
figure, there is a pair of signals for two frames taken
per steel picture.
Modulation: Frequency modulation for audio signals and amplitude modulation for video signals are applied.
Mixing: There is a combiner named diplexer which combines the audio and video signals and sends them to the antenna (tower).
Amplifiers: There is a space for ten amplifiers in a machine made by Digital Axcera in the substations air conditioned room. Amplifier no. 1 and 10 are for audio amplification and amplifiers 2 to 9 are for video amplification. Although there were nine amplifiers in that space and amplifier no. 4 was absent. There are two balancing networks that balance the signal and make sure that absence of any amplifier can not affect the signal power to be transmitted.
Transmitter: Transmitters used in that substation are air cooled. The first transmitter we saw was made by NEC, Japan. One thing that all of them have in common is that they produce a lot of heat while operational and this heat must be removed instantly. Although as a safety precaution, the transmitter shuts down itself if the body temperature tends to go beyond 450 Celsius. The air conditioning system that keeps the room cool, always keeps the room temperature below 220 Celsius. The hot air is removed continuously filtered air is entered into the transmitter. The filter is cleaned after every 5-6 days.
As they said, the transmitting power 10 kW is maximum for video and 1 kW is maximum for audio. Right now two channels are being transmitted through the same devices (transmitters, antenna etc.). They are BTV and BTV World. Another third channel is also possible to be transmitted. And so the channel Songshodio will soon be on air.
Antenna: The amplified video signal is passed through a band pass filter giving a bandwidth of 7MHz (216MHz to 223MHz). Then it is combined and sent to the tower. Then the tower transmits the signals. The tower is Omni directional, has a height of 500ft and was set up in 1964.
Power system: Although the substation has own power system, it uses the line supply. In case of power failure, backup power is provided shortly.
Conclusion: The way our Khulna substation gets its signals is as follows.
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