Re: syncing 2 16mm projectors

From: Jeff Kreines (email suppressed)
Date: Wed Sep 26 2007 - 09:39:28 PDT

On Sep 26, 2007, at 5:09 AM, Joost Rekveld wrote:

> I did, for two projections during a dance-piece, and two dvd-
> players of the same brand, same type, will typically give you about
> 5 to 10% difference, say about two minutes out of sync after half
> an hour. I've never met somebody who could explain to me why this
> happens, but it is reality.

That's a lot of drift. Most digital consumer devices will hold sync
pretty well for 15-20 minutes -- which is why cheap MD recorders can
work nicely for double system sound with a DV or HDV camcorder.
Drift is maybe a frame every 15-20 minutes, easy to deal with.

It's hard to common-start DVD players perfectly. But two minutes!
Wow! I guess there's still a place for time code in this modern world.

As for sync motors and selsyns, the were cool at the time and worked,
but Magnatech went to stepper motors in the 70s, and MTM went to
servos, and both were simpler and more elegant and easier to keep in
sync, with simpler wiring. You can slave a Magnatech et al to any
projector using just a simple shaft encoder, which transmits pulses
based on the frames, and will follow the projector in forward and
reverse faithfully.

If you are only needing to start once and go forward, synchronous
motors "common started" (the same switch is ideal) will hold, but you
need to test to see if one machine lags on start time. You can do
this with loops of punched leader, with non-even spacing, so drift is
apparent. Then just start the machines threaded up to compensate for
the offset. A pain, but it's how mixes were done in the old pre-
selsyn days. (Yes, I go back that far...)

Jeff "old fart" Kreines

For info on FrameWorks, contact Pip Chodorov at <email suppressed>.