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From: dsi@unccvax.UUCP (Dataspan Inc)
Newsgroups: net.video
Subject: Re: Help w/VHS slow dubbing
Message-ID: <451@unccvax.unccvax.UUCP>
Date: Thu, 27-Feb-86 13:41:28 EST
Article-I.D.: unccvax.451
Posted: Thu Feb 27 13:41:28 1986
Date-Received: Sat, 1-Mar-86 17:08:17 EST
References: <5031@alice.uUCp>
Organization: UNC-Charlotte
Lines: 35


     The problem is that during slow advance on your PV8000, the relationship
of the equalisation pulses to the vertical sync pulse is destroyed. I doubt
that your sync generator will genlock to the vertical sync pulse which is 
provided during slow motion and stop frame reproduction. The reason that this
is done is to essentially force the synchronisation at the monitor to 
"noninterlaced" (only one field is being displayed at a given time) and because
there is no control track to provide reconstitution of the vertical sync
during normal playback.  (Note also that the "dead zone" occurs during the
vertical blanking interval).

     The solution is to get a time base corrector. Most of them which are
designed for use with the heterodyne VTR send an advanced V-sync tach signal
to the VTR.  The advanced signal is used to precorrect the headwheel phase
and the control track is simply ignored.  Remaining timebase errors are 
removed by digital techniques.  Now, when you go into slow/stop motion,
the sync which will be on the TBC out will have the correct synchronizing
waveforms impressed on it.  However, still frame reproduction will actually
"jitter" now because the TBC out signal is always interlaced.

     In short, when you go into slow/stop motion, the signal isn't NTSC 
anymore, and your 1/2 inch machines (utilizing digital techniques to find
sync pulses, etc) don't know what to do with non NTSC sync pulses. Try 
taping a cheap "Pong" game or something to test the effect.

     What this world needs is 1) a GOOD but CHEAP time base corrector with,
say, a 32 line window and 2) VHS and Beta machines with provisions to 
disconnect the control track servo and provide advanced Vsync tach signal
to the headwheel servo comparator.  Everyone seems to be using video these
days, but no one seems to have TBC's.  You don't need a $15,000 Tektronix
110 TBC (which is extremely nice, by the way) to do what you want to do,
but the manufacturers haven't gotten the message yet.

David Anthony
DataSpan, Inc