There could be 5 or 6 transmitters on a high power frequency in Britain for the network radio 5 Live. So, how do they position those transmitters so that if I'm listening in a certain place, a wave coming from one transmitter isnít going to mix with the wave coming from another and cancel out?
John - That is exactly what happens, in fact. If you get midway between two transmitters of similar power, even though we synchronise the transmitters so that they're absolutely in phase and audio is delayed such that they get to their transmitters at the same time, you will still get a situation where youíll get horrible phasing distortion.
A good example of how we dealt with this was radio 4 long wave which has 3 transmitters, one in Droitwich, near Birmingham, one at Westerglen between Glasgow and Edinburgh, and one at Burghead near Inverness in Scotland. All of these operate on 198 kilohertz. What they do is they steer the audio delay such that they can create a mush area in centres where there's hardly any population.
So, if you were to take the difference between Westerglen and Droitwich, the mush area tends to be around Cambria. But how to deal with that is to put a medium wave filler in that area so that although you got horrible radio 4 long wave signal, you get a medium wave alternative. Likewise, the delay between Westerglen and Burghead is steered, such that the mush area is in the highlands of Scotland where the population is very low.
Chris - What about with FM networked radio? They're using different frequencies in different parts of the country to avoid that problem arenít they? I've noticed that when I drive from one part of the country to another, Iíll stay tuned to the same network radio programme, but my fancy radio would retune itself to a slightly different frequency between the two places on my journey.
John - That's right. Each national network occupies around about 2.2 megahertz worth of space on the FM dial. So, 5 national networks Radio 1, 2, 3, 4, and Classic FM. They all get around just over 2 megahertz each to plan the national networks. So that comprises quite a number of transmitters at high power that cover the bulk of the population and in lots of low power fillers in between, but they have to be on different frequencies because as I said earlier, itís an analogue system. If you get two transmitters on the same frequency that donít arrive at the same time at the receiver, you get distortion and all sorts of nasty noises. So that's how they deal with that on FM.