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.

With most analog broadcast systems 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" -- a badly damaged video signal.

Because TV sets base audio demodulation on the video signal, this can affect audio. Since video is based on field rate of 60 Hz, 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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