Technical Addendum | Updated: 07/16/2004 |
|
The TV Broadcast Signal From the discussion of the waveform monitor you've probably gotten the impression that white in a TV signal is transmitted at some sort of maximum power and black is just the opposite—a minimum signal. Not so; in fact, it's just the opposite. In the transmission process the brighter something is in a scene, the lower the amplitude of the modulated video signal becomes. This explains an audio problem that sometimes occurs. Recall that the color information in a TV signal is added to the luminance (black and white) signal information. Abrupt transitions from one brightness/color level to another can cause "signal overshoot." Since TV sets base audio demodulation on the video signal, this "overshoot" results (since things are reversed in the transmission process) in signal "undershoot" or, in extreme conditions, the video signal momentarily disappearing. Since this happens at the field rate of 60 times a second, when the audio loses its reference in this situation, you end up with a very annoying audio buzz. This problem typically occurs with TV graphics that contain contrasty lettering—for example, gold lettering on a dark background. |
To Home Page © 2005, All Rights Reserved
|
