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Re: NTSC Legalacy Formats



In a message dated 98-02-17 13:46:39 EST, Jim Mendrala writes:

<< 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. >>

Hi Jim,

In NTSC and PAL, color information is carried by quadrature-modulating the
phase of subcarrier.  Regardless of image content, the frequency is always
3,579,545 +/- 10 Hz, give or take some phase difference (in broadcast).  All
colors of the rainbow (or at least of the TV system) are represented in one
360-degree phase swing of I and Q.

In color-under recording, chroma phase errors introduced by the scanners and
tape are reduced by the same factor as the frequency conversion, which is 5
for 688-KHz machines.  This reduces the phase error on playback to about one
line, so a color picture good enough for consumer and industrial/eduactional
use can be obtained without a TBC despite the fact that the phase jitters all
over the place.   As you rightly point out, with all the jitter, interleave is
pretty well ruined.  But it gets ruined in such a way as to add what appears
to be random noise to the picture, rather than the fixed pattern of fine,
faint vertical bars that indicate a breakdown of interleave.

<< 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. >>

In an analog videotape recorder, the frequency of the off-tape chroma
subcarrier--along with the line and field rates--is controlled by the tape
speed, which in turn is controlled by the control track.  A feedback loop
between the two is what regulates the capstan servo in playback.  (The servo
is locked to incoming video in record).  

The practice in broadcast is to turn off the subcarrier burst when a
monochrome picture such as an old movie is being transmitted.  This causes
color TV sets to revert to monochrome mode so you don't see false colors
running through the image.  Of course, people expect their VCRs to work in
monochrome as well as color, so some alternate means of referencing the
capstan servo is necessary in case there's no off-tape subcarrier.  In some
VCRs, this is line lock; in others (portables, camcorders, etc.), it is a very
stable servo reference oscillator.  

Best regards,
Christopher Bacon 



---
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