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From: Jan Panteltje <alien@comet.invalid>
Newsgroups: sci.electronics.design
Subject: Re: S-VHS cassette recorders
Date: Wed, 08 Jan 2025 07:42:36 GMT
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On a sunny day (Mon, 6 Jan 2025 17:27:21 -0500) it happened bitrex
<user@example.net> wrote in <677c5899$0$1787$882e4bbb@reader.netnews.com>:

PS
from a TECHNICAL POV the system used in for example VHS is very interesting!

Color in he analog days, be it NTSC or PAL, needs phase accuracy for the color sub-carrier of a few nanoseconds
(part of period of 4.3 MHz for PAL)

So HOW can you do that with all those tape speed variations? rotating head, tape transport, what not?
The trick WAS SIMPLE

You probably remember analog audio tapes, those used RF bias for recording.

In video recording to tape an FM modulated carrier is used for brightness, few MHz ..

So the trick:
Use the FM modulated brightness info as RF bias and record the lower frequency color info on it.

But the timing>
They use an about 4.9 MHz something oscillator locked to an harmonic of the horizontal sync
and mix the 4.3 MHz color down with it to about 560 kHz or so,
and super-impose that on that BW info FM carrier, using the FM as RF bias (for the audio guys)
But what about tape speed variations?
Genius, on playback the 560 kHz color info is mixed with the same horizontal from tape locked oscillator!
So now if the tape runs faster and the color carrier from tape with it, the 4.9 MHz also runs faster,
the mixed up 560 kHz, now 4.4 MHz  stays exactly stable to a few nano seconds.

Genius!
I did read a description of the system in 'Radio Electronica' one day  and in a few days had it working on my BW video tape recorder....

There are lessons to be learned from that!!