Channel Limitation

The initial TV channel spacing of 6 MHz was adopted based on the 525 lines US system and the 405 early British system. Thus the channels were spaced (still so with the US NTSC system) 6 MHz apart. With the later systems 625 lines and 819 lines coming into use the actual bandwidth required for transmission of TV programmes on these standards is more than the 6 MHz. However the terrestrial TV stations are so distributed and channel frequencies carefully spread over these stations, so that at any location a TV set will receive only one or two stations (metropolis), on frequencies quite apart and thus there is no interference between the signals from the different IV stations. The 625 line CCIR standard adopted by India needs about 7 MHz bandwidth, but since there is no adjacent channel trans¬ mission in the same area (primary service area), there is no difficulty.

Satellite transmissions when received in a ground station, have to be converted from their FM modulation into the normal VSB TV signal and on the assigned channel frequencies used in a TV set. Each of the Sat TV desired transmission will be converted to one of the TV channel. If MTV is put on CH 2, say, if you put the next signal say BBC news on channel 3, PRIME Sports on CH 4 and so on, when they are combined on a single cable and fed into the TV set, because each signal has a bandwidth of 7 MHz and sidebands of one could will spillover into adjacent channel's sideband resulting in mutual interference between the channel signals. You have to therefore skip a channel between two adjacent signals to provide a guard band. As a matter of fact even if the transmission bandwidth were to be 6 MHz only as also the channel spacing, you would still need to skip channel between two TV signals when combining the signals on to a single cable, to avoid mutual interference, as an appreciable guard band is in any case required to provide about 40 dB attennuation needed for interfering signals in case of vision signals, which are amplitude modulated. (This is not so for the Satellite TV transmissions which are Frequency Modulated and hence, because of the capture effect interfering signals need be about merely 10 dB down as a conservative measure).



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