I was testing a recording setup just now and needed an input source, so I hooked up the audio out of my TV. When I looked at a frequency analysis of the recording, I found a strong hum in one of the higher frequencies.
For the less scientifically-minded out there, this is basically a visualization of sound. It graphs frequency against time. Time progresses from left to right. Frequencies go from low (0 Hz) at the bottom to high (22050 Hz) at the top. Each pixel's color represents the power, or volume of that frequency at that time. White is silence, and dark blue is maximum volume. Low rumbles will make dark blue blotches very close to the bottom. A blood-curdling scream will make a line around a quarter of the way up from the bottom, plus other blotches around it (harmonics). The limit of normal human ears can be anywhere from half way up to the top. (This frequency scale is linear, while we hear changes in frequency logarithmically, so most of the normal sounds we here are squished to the bottom.)
Well it's clear that there is a constant hum present whether the TV audio is on or off, but it disappears when the TV is turned off. I determined the line to be at roughly 15730 Hz. I didn't hear it when playing back the recording, but then, its power isn't very high. Using a test waveform, I determined that my hearing drops off right around 18000 Hz, so I should be able to hear it. So I simply cranked up the volume. Guess what...
Suddenly it sounded like my TV was on. :-P
Just to make sure, I Googled for "television 15730 Hz" and found this:
... In accordance with the U.S. NTSC commercial television standard, a clock frequency of 3.579545 megahertz (Mhz) is used to establish both a reference for color information demodulation and image scanning. The horizontal synchronizing pulse frequency which establishes the start of each scanning line is 2.times.3579545/455 or 15730 Hz. ... (full page)
So yup, it's the squeal. And it shows up in the audio line out for some reason. It might be a well-known fact about TVs to the digital video/audio field, because I noticed that in another recording setup where I used my Belkin USB VideoBus II, instead of seeing the line where the squeal was, I saw an absence of data where the line should be, as if it was filtered out with a digital equalizer. Either the Belkin circuitry is designed to attenuate that frequency specifically, knowing that TVs always produce it, or it's designed to attenuate any frequency at which a hum is detected.
Hmm. Now, why am I thinking about stuff like this instead of sleeping?