Choosing a channel for a VCR/STB
If you need to use an RF link to connect a VCR or set top box or video game to your television, choosing the correct output channel is not as simple as it used to be. In the old days, Channel 36 was often used as this was not used by the boradcasters. Since Channel 5 and the new digital services now do use this channel another channel sometimes has to be selected.
If you tune the video or set top box to a random clear channel you may find wavy lines or worse across another analogue channel, or lose reception from a digital channel. The reason for this is complicated, depending on the design of the television tuner. There are some combinations of channels that should be avoided when selecting a channel for the VCR/set top box output.
These channels are easily calculated. If channel N is in use, channels N, N+5, N-5, N+9 and N-9 should not be used. Adjacent channels N+1 and N-1 should also be avoided.
The channels that should be checked are all those used by transmitters in the area, including digital channels even if you do not have a digital receiver. As in many areas, there are now 10 or more channels used, it may be hard or impossible to find a good channel to use. In these cases, a compromise will have to be reached.
An example
Around Oxford the following channels are used:
N 34 47 48 49 52 53 56 57 60 63 67 68
The channels that can also not be used are therefore:
N-9 25 38 39 40 43 44 45 46 51 54 58 59
N-5 29 42 43 44 47 58 51 52 55 58 62 63
N+5 39 52 53 54 57 58 61 62 65 68
N+9 43 56 57 58 61 62 65 66
Leaving:
21 22 23 24 26 27 28 30 31 32 33 35 36 37 41 50 64
Avoiding adjacent channels leaves:
21 22 23 24 26 27 28 30 31 32 36 37 41 50
Quite a good choice - it looks carefully planned, and indeed it was. If channel 36 is chosen for the VCR, channels 37 and 41 are unavailable, leaving plenty of choice. However, many of the seemingly clear channels are used by nearby transmitters narrowing the choice.
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