Essentially all consumer video tape recorders use frequency modulation for the luminance component of the video signal and demodulation is performed by zero-crossing detection. Distortion-free video requires that the zero crossings of the frequency modulation waveforms be preserved exactly. In this paper, the complete set of mathematical conditions for zero-crossing preservation is derived from first principles. While some of these conditions have been discussed previously, the complete set derived here is believed to be novel.
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