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From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.graphics
Subject: Re: Mixing computer and video signals
Message-ID: <15342@onfcanim.UUCP>
Date: Tue, 23-Jun-87 21:41:09 EDT
Article-I.D.: onfcanim.15342
Posted: Tue Jun 23 21:41:09 1987
Date-Received: Sat, 27-Jun-87 04:48:41 EDT
References: <173@mv06.ecf.toronto.edu> <3119@eagle.ukc.ac.uk> <804@puff.WISC.EDU>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 32

In article <804@puff.WISC.EDU> upl@puff.WISC.EDU (Future Unix Gurus) writes:
>
>By the way, a good professional studio should be able to genlock itself to
>the NTSC out from yor computer, thus you could use the studio's chroma
>key to build the images you want. Using decent studio equiptment has some
>real advantages in terms of control and flexability, if you can afford it.
>
>Jeff Kesselman

This is *not* true in general (in fact, I don't know of any frame buffer/
workstation for which it is true).

The problem is that, though your frame buffer may generate video that
matches the NTSC timing parameters, it is RGB video, not NTSC.
A studio's sync generator will indeed lock its own sync to the computer's
sync, but there is no colour subcarrier coming out of the computer for
the studio's sync generator to lock to.  Thus colour subcarrier phase
will wander randomly with respect to sync.  (I know of no sync generator
that will lock its *subcarrier* output to a video input that contains
only sync.)

This produces a signal which is good enough to record on some recorders,
but it does not meet broadcast standards, and can't be used for single-frame
recording on 1 inch machines.

The only way to get broadcast-quality video is to have the subcarrier
and sync generated within the same piece of equipment, generally divided
down from the same oscillator.  The normal way of doing this is to use
a studio sync generator as the master timing source, and slave the
computer's video to sync from the sync generator (this is what the genlock
option of the computer does).  It could also be done by having the
computer generate a subcarrier signal, but I've never seen it done that way.