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NTSC Legalacy Formats



Chris,

You are correct, the subcarrier frequency of NTSC--3.579545 MHz--was
specifically chosen to be 227.5 times the line rate 15,734 Hz to
interleave.  Interleave (e.g. the reversal of chroma phase on the same line
numbers in alternate frames) cancels out the picture interference that
would otherwise result because the chroma subcarrier frequency is inside
the video frequency range. It has to be that way for NTSC transmission. But
for home recording like VHS,  VCR's record the color under the video by
hetrodyning the subcarrier down to a low frequency, 688 KHz I beleive. When
played back the crystal oscillator hetrodynes in the VCR back up to 3.58
MHz. Unfortunatly there is jitter in the tape playback of the color signal
so the chroma is not locked to the horizontal therefore it doesn't
interleave well. But it is still within the range of the color detector
circuits the receiver has. This is not tolerated on profesional machines
and they must have a reference, be it internal or external, to play back in
the interleaved mode. You can verify this by putting an osciliscope on the
video output of your home VCR (with no video source and no antenna) and
note that the picture on playback is locked to the line and the burst is
free running around 3,579545 Hz.

Sincerly,

Jim Mendrala
Real Image Technology, Inc.

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