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.