release_the_bats wrote:
Yeah, all sorts of weird and wonderfully interesting things can happen with radio signals.
For example, tube amplifiers and older analog stereo systems can pick them up. What's interesting is that the particular piece of equipment always picks up the same station.
I had an old receiver + speakers in upstate New York that picked up the signal for a strange evangelical fundamentalist Christian station. Between playing records, I'd get to hear some preacher's voice reading about "The Rapture" or something like that. It was creepy, but I found it entertaining because I understood the phenomenon.
It was annoying on my ex's 1969 Marshall tube amp, though. When turned on, its station of choice was one that played bad "classic rock", which is kind of a distraction when you're trying to use the thing to play good music.
I don't fully understand the mechanics of this, but it does interest me.
All older tube equipment had a very basic sound system. All radio signals would be normalized to a single internal intermediate frequency, or IF, before being fed to the speaker. Tube radio circuitry only works with a single frequency, so the incoming signal has to be normed with an IF system to be usable by the radio. In the early days of radio, the IF didn't exist, so all stations had to broadcast on a single freq. This led to the Coolidge Administration in the US trying to enforce all sorts of weird timesharing plans with stations in rural areas that tended to hate the Federal Radio Commission. (For shortwave enthusiasts, imagine trying to get WWCR to obey the dictates of the Obama Administration. SW is basically left alone by the FCC because of the crazy white guys with machine guns angle.)
Anyway, all tube equipment used 455 khz as the IF for AM (MW), and 10.7 Mhz for FM. Because of the properties of hollow state electronics, stations on a certain harmonic, or multiple, of the IF can be picked up in the circuitry. The engineering of the system will determine what it gets, also if you have or don't have the magic freq being used in your area.
In the early days of radio, this was no big deal, what with only a thousand or so Class A "clear channel" stations. (Today, there are 13,000 or so, all playing right wing talk.) As the bands got crowded and high fidelity equipment became the norm, it became more and more of a problem. People then and now were stupid and thought that their stuff was haunted etc. Finally the feds forced tube gear off the market because the problem was too severe. It's MUCH easier to control IF issues in a transistorized or microchip set. PLL technology has practically eliminated the problem. If you ever try and use a radio that has its PLL chip go on strike, you will get harmonic problems all over the place.
16 Jan 2010, 3:15 pm
