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Mike Waldie on Capstan Wrap
Message for Rob Lingelbach's "Capstan wrap" discussion from Mike Waldie at
Hi Rob! - Long Time no speak.
Firstly, we have seen all of the problems mentioned at one time or another.
We have even seen an operator(?) lace negative onto the mag follower because
it was brown! But these occasions aside, here are things as we understand
1. Originally the Wrap was conceived to achieve the correct sound
spacing on comopt 35 utilising a common sound head position alongside 16mm
within the capstan. This removed the necessity to have a sound flywheel as
was the norm before the MkIII.
2. The original reason for not using this loop is because of
interference with the vertical stability of the image especially over
splices. The bearing and machining accuracy in terms of runout for the
intermediate rollers has to be very precise hence the different material
used for these rollers. As a very approximate example, discounting any
effects of the servo loop filters, a runout of plus or minus 0.0005 inch
will result in a vertical stability error of more than 2 vertical lines.
Multiply this by 2 rollers in phase and you can easily end up up with a 4
3. As vertical stability on occasions appears to be on the limits of
acceptability, most colorists I know opt for the direct route.
4. Scratching of the negative with excessively curved film by the gate
exit roller has been noted and seen on rare occasions. The two fixes for
this have been to machine the centre well of the exit roller deeper or to
remove the roller altogether and use the delays in the audio desk to
compensate for the sound spacing.
5. The reason for the direct path with Pin-Up has nothing to do with
stability but all to do with controlling the uncontrolled mass of these very
heavy rollers with fast stepping speeds. The capstan itself simply does not
have sufficient low torque resolution for accurate positioning of the film.
With these roller masses between the capstan and its positional sensor (gate
6. The sprocket jumping phenomenon is usually associated with deck and
capstan alignment. Many machines around will exhibit deck alignment
problems and this will frequently be seen in the form of the film creeping
back and forth across the capstan when running forward and then reverse.
Sorting this out, sorts the problem.
7. Capstan slippage due to the decreased wrap is not normally a problem
unless the capstan is dirty or severely hardened or the silicon sleeve on
the sound cell wires within the capstan groove actually touch the film and
act as a very effective brake (very common). When slippage does occur,
usually 16mm because of its decreased contact area, it is nearly always the
active stop back and forth oscillation that causes the slippage/scratching.
Run up, run down and shuttling speeds do not cause the capstan to slip
unless there is something else retarding the film. Lowering the active stop
torque or modifying the active stop by way of servo mod kits will usually
sort this problem.
Video Engineering Ltd, London, Eng.