Exactly -- I saw this one coming awhile back.
Radio may soon be nobody's bomb.
Building a MW / LW radio receiver is not hard — years ago, nearly every schoolboy did it. Building one that works well under all circumstances is tricky, but amplitude modulation — which is used on the medium and long wave bands — is basically easy. You have a high-frequency carrier signal, which you make get weaker and stronger in time with the audio signal getting weaker and stronger. You feed this into an aerial system, and invisible electromagnetic waves travel away from it in all directions. The person listening has a tuned circuit, which lets through just the frequency of your carrier signal; a rectifier, which converts the high-frequency alternating current into direct current (which is still rising and falling in time with the audio signal); and an amplifier, which boosts the signal enough to move a loudspeaker cone. That’s a bit of an oversimplification, but I don’t want to lose my audience.
Frequency modulation is a little bit harder. This time, instead of varying the strength of the carrier signal, you vary the frequency Where your oscilloscope trace is above the zero line, you move the peaks closer together; where it goes below the line, further apart. So the frequency of your carrier signal is changing. This isn’t as big a problem as it sounds, because most tuned circuits aren’t perfect; so the one in the receiver will let through frequencies that are within a certain tolerance anyway. You also need a different kind of detector, which responds to changes in frequency as opposed to amplitude; but once you have built that, you can make use of the same power supply, amplifier and loudspeaker as before.
Doing it with pictures involves another layer of complication since there is no single, universally-accepted way of representing a picture as an electrical signal. And that’s just in mono — don’t get me started on the various ways people have actually transmitted colour pictures. In practice, though, agreements were thrashed out between governments, broadcasters and setmakers on a country-by-country basis, so all the TV sets in any given country worked on the same broadcast standard. And home video equipment works to national broadcast standards precisely so that an ordinary television set can be used to view home recordings.
Still, the fact remains that anyone with the right knowledge can build a radio transmitter. (Actually using it is another matter; if your signal travels far enough to interfere with other users, you can expect to end up in court. Not much is likely to happen if you build a small, low power transmitter and nobody finds out about it. And if the Rule of Law has already broken down …..)
But digital broadcasting is a whole other kettle of fish. Even building a digital receiver requires access to proprietary technologies (and this includes mathematical operations over which some people claim to hold patents!), although they may be available under what appear to be generous licencing terms. This is only because the big corporations are aware that in order to sell transmitters, receivers need to be almost given away. Building a transmitter is what requires access to the seriously expensive stuff, and that’s what they aren’t going to let Our Sort near.
Call it paranoia on my part if you like, but there’s no denying that wholesale adoption of digital broadcasting will end up making it nigh-on impossible to start an underground radio station — and in so doing, will deprive The Population At Large of a potentially extremely useful weapon against a corrupt government.