Date   : Fri, 05 Apr 1985 19:14:38 GMT
From   : Dataspan Inc <dsi%unccvax.uucp@BRL-TGR.ARPA>
Subject: Re: async camera switching.

     You neglected to state whether or not the VTR was consumer or professional.
The motor slows down during the switch because (evidently) this is its free
running speed in absence of a tach pulse (vertical sync). You can adjust this
parameter in some VHS recorders but it screws up freeze frame, etc.

     It might be possible to genlock the two cameras; if they use a sync 
generator such as the MM5321 (National) or the "other" popular one made by
Fairchild. What has to happen here is to drive both cameras from a single
timebase (probably originating in a 14.318 mc crystal) as well as sending the
frame 1 line 1 signal to reset the "other" camera(s). This will have other
benefits as well, not the least of which is a consistent colour burst phase
from camera to camera WITH RESPECT TO THE 4 FIELD SEQUENCE. 

     The vertical sync (and horizontal sync) is then hopefully genlocked, which
would allow hot switching during the vertical interval.

     There is simply no hope otherwise, as consumer recorders (indeed, all
recorders) cannot cope with hot asynchronous switching without a time base
corrector. The time constant in the head servo of a VTR is extremely long,
and cannot be made to recover in a frame time. Besides, how do you expect to
handle wrong field edits asynchronously?

      I could see a way if you could send "advanced VTR sync" such as a line
store type time base corrector uses, but this is strictly a hackage for the
not-so-weak-of-heart.

     Sorry, but genlocking is a fact of life for those "professional" switchings,
and there just isn't any other economical way to do it. The time base correctors
required cost $10,000 or so each minimum, you'll need 1 and want 2.

     If you are serious, get some reasonable JVC E.N.G. type cameras that can
be genlocked.....

David Anthony
Sr. Analog Nut
DataSpan, Inc.