[BC] Fun with Quad Videotape

[Xen Scott]

At 08:19 PM 12/03/2011 -0500, Tom Spencer wrote:
>Quads were always easier to do insert edits rather than assemble edits.

That's because with an insert edit, the capstan motor and thus the forward
speed of the tape is controlled by the control track signal from the tape
during the edit interval.  An assemble edit causes new control track to
be recorded, beginning at the point of the in-going edit and the reference
for the capstan motor switches to a divided-down signal from the video
head wheel.  Sometimes this causes the capstan to change speed slightly
if the control track reference is not an exact match to the divided down
head wheel reference.

I once had an extreme situation where assemble edits made on one particular
Ampex VR-2000 would not playback reliably on other VTRs.  Also, tapes made on
this machine would not counter-time correctly although those tapes would 
always
playback to the correct time duration.  A one hour recording would counter-
time 18 seconds short.  After some investigation, it was discovered that this
VTR had left Ampex as a black & white machine and later field upgraded to 
color.
Unfortunately, the capstan motor was never changed, so when it was fed a 
59.94hz
color reference during recordings instead of the previous 60hz black & white
reference, it ran slower.  It would playback tapes just fine as the capstan 
servo
and thus the forward speed would be determined by the control track signal 
from
the tape.  However, when doing an assemble edit on a tape recorded on a 
different
machine, the edit would breakup because of the abrupt capstan speed change 
at the
edit point.  Changing the capstan motor fixed the problem.

How many of us have had to trouble shoot a problem and discovered it was 
caused by
incomplete upgrade modifications?

Xen Scott