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 luma (black and white) signal information. Abrupt transitions from one brightness/color level to another can cause "signal overshoot."

Because TV sets base audio demodulation on the video signal, this "overshoot" results in signal "undershoot" -- a badly damaged video signal.

Since this happens at the field rate of 60 times a second, when the audio loses its video reference, 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. This is just one more reason that all subject matter should be kept within the system's optimum contrast ratio.

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