On some of the crappier TV sets that I’ve watched, I’ve noticed that sometimes the speaker will make a buzzing sound whenever there was a high-contrast image on the screen, like large dark black text on a bright white background.
What’s the connection between the type of image on screen and the buzz coming out of the speaker?
Well, when there is a certain type of image on the screen, the picture tube is sending out a stream of electrons in a certain patern. All electrons create a magnetic field, and since speakers function magnetically, the two may disrupt each other.
This works out well, as it seems my picture tube sends out the right frequency to disrupt the sound whenever Kathy Lee Gifford comes on 
If you can’t dazzle them with brilliance, baffle them with bullshit.
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One explanation that I’ve heard is:
Let’s say you’re watching your favorite commercial. The commercial dissolves to the final shot of the new Neon/Camry/Accord/Lexus with all the tiny type about leasing details. The tiny type has actually been superimposed on the regular picture of the car. That’s two distinct video signals, which slightly overloads the bandwidth, and causes interference in the audio part of the signal.
Thanks for the explanations guys.
“The world ends when I die. And as far as I’m concerned, the rest of the universe might as well call it a day too.” – Matt Groening
kunilou, i would presume that those two images are mixed prior to being sent to the transmitter. Are you serious that they broadcast two distinct images which my set knows how to merge?
Video level is measured in IREs (don’t ask me what it stands for). 0 IRE is black (actually, you try not to get lower than 7.5). 100 IRE is the highest signal you’re allowed to use. Anything higher than 100 IRE tends to cause problems with other parts of the signal, i.e., tha audio.
The problem today is with computer character generators and other graphic devices. Anyone familier with RGB color values knows that white is measured as 255 on a scale of 0 - 255. Unfortunately a 255 white is going to be much brighter than 100 IRE. A white value of 240 is much better for NTSC video.
What you’re hearing (and seeing) on your television is an overdriven video level spilling over into the audio. Some television sets are better able to handle this error than others, so don’t expect the same signal to look/sound the same on every TV. Also, some broadcasters clip the signal before it goes on air, while others may not, so the same spot may buzz on some channels but not on others.
Kunilou: All switching and superimposing is done at the studio. You’re getting the final signal. The only superimposing your television is capable of is the closed captioning.
My apologies for not explaining it clearly. What I should have said was that two separate video signals were mixed and then transmitted.