Log in

No account? Create an account

Previous Entry Share Flag Next Entry
TV squeal...
Not everyone can hear the high-pitched squeal a television makes. One day in my high school biology class, a student pointed out to the teacher that the TV was on. It wasn't displaying an image or playing any sound, and there weren't any indicator lights on it. The teacher went over and hit the button, and it made a brief flash on the screen as it normally would when turned off. The teacher was puzzled as to how the student knew it was on without any indication, and the student explained that he could hear it. Many of the other students concurred, including myself. The teacher simply couldn't hear it.

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?

  • 1
I can always hear the TV when it's on but not showing anything- I can hear that whine it makes. The weird thing is, our big screen TV doesn't make that sound. It's not projection, so I don't know what the difference is, but it doesn't make it.

And speaking of TVs, I'd really like one of those flat screen TVs that they advertise during football that you can hang on your wall like a picture. Could you look into that for me? Thanks! (By look into, I mean purchase, of course)

Yeah, those flat plasma screens wouldn't make that whine either, I don't think... And the picture quality.... *drool*

Why? Because you are a true GEEK. Not one of those half-hearted geeks that just walks around with a pocket protector or knows how to use a slide-rule. You are an honest to goodness GEEK and I hope you realize how cool that is.....

Now if we could only harness this GEEK power and use it for Good.....

You're talking about a project the scale of splitting the atom here... hehe

That was mind expanding, thanks. I've always been able to hear the tv also, and other various high pitched sounds that drive me to distraction if I don't hunt them down and kill them. I must be part dog.

  • 1