Sometimes my tv goes crazy with audio static when certain graphics or text appears on screen, even if it is part of the program. Why is this?
I have no good answer, but I feel like posting to say that you are NOT crazy, I know exactly what you mean. I always thought it was happening more or less depending on the amount of white or black on the screen… too much white, and the buzz starts? Something like that.
The last time it happened I was watching a movie and a man was standing by his office door with lettering on the glass. It seemed as if every time they cut to him the static would start, and then the end credits did the same thing, depending on how much lettering was present.
I’m sure someone will be along who can give a more precise answer, but here’s my vague understanding of what’s happening:
The audio signal is just modulated at a slightly higher frequency than the video signal. So you’ve got to “slice” the audio waveform out of the video waveform – they’re parallel: AAVAAVAAVAAVAAVAAVAAVAAV (This is simplified a bit, since there are three video “slices” per frame, Red, Green, and Blue. The audio is stuck in the high end of each slice.)
When the video signal contains fields of solid white, its amplitude goes up to the maximum in portions of all three slices, making it bleed into audio slices. What should be a slice with a much lower amplitude becomes one with a higher amplitude – so it’s noisy. You’re hearing the video signal interpreted as audio.
This works backwards sometimes, too. Audio information will leak into the video area. When this happens, it looks like a wavy pattern of white dots.
The audio signal is above the video, but it’s not really “sliced”. The video is amplitude modulated and the audio is frequency modulated. There are two video signals, called chromanance (or just chroma) and lumanance (or luma). Luminance is the “brightness” and is all that you get on a black and white TV. Chromanance is the color portion of the signal. This site seems to be accurate if you want more details: http://www.maxim-ic.com/appnotes.cfm/appnote_number/734
I’ve never really studied it, but I suspect that what happens is that the high video signal level (“white” corresponds to the video signal being near the top end of its range) causes some distortion in the amplifier circuits inside the TV which results in higher frequency signal artifacts, which get coupled into the audio signal as noise.
When a high-contrast pattern is shown on the screen, the CRT is rapidly switching the electron guns on and off as the beam scans over the pattern; I suppose this rapid switching could cause electromagnetic noise that the audio circuits could pick up
The graphics you see are probably being made–or at least broadcast–incorrectly. The brightest white should actually be a little gray (about 7%). Pure white is “illegal” in NTSC video, and can cause the interference you hear. Usually, some piece of equipment in the process will correct illegal levels, but maybe not in this case. Also, if your TV is getting old or malfunctioning, it may do this even if the signal is legal.