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It is possible that the new motor replaced a nonsync motor and that the gearing of the sound head or projector did not change to reflect the difference in speed between a non-sync motor and a sync motor. The difference in speed is only about one perecent and it would be faster than than the other machines. If you have 5-stars the main pulley being driven by the motor should have 90 teeth on it for sync and 89 for non-sync.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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He wanted to interlock a Westrex 2001, a Cinemeccanica Vic 5 and a Philips ST-200 platter (three very diffferent components). His solution was to fit micro-invertors to each machine, changing the Westrex motor to a 3-Phase synchronus motor with a toothed belt drive. In between the two machines he fitted a compensator which had two microswitches added at top and bottom. He then fine tuned the two microinvertors to balance the speed of the two projectors as close as possible to 24fps. In order to take into account any minor fluctuations he used the two microswitches on the compensator to switch to alternitive frequency settings on one of the micro invertors to either speed the machine down or speed it up. All that was then needed was some basic control gear to start and stop the machines, switch from individual to interlocked mode, interface with the platter and link the failsafe systems.

In practise the system works very well, with very litle need of any input from the comensator micro switches. The actual speed difference used for comensation is so small that no obvious effect is seen (or heard) on screen.

Of course this is a complicated solution for same machine instillations, but for this situation it worked superbly (and was relativly cheap to do).

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Martin Dean

During my days in television, for the station and outside clients, we could interlock four 16mm / 17.5 or 35mm multi follower heads, three 16mm double band projectors. One of three 16mm or three 35mm projectors with the picture was the master machine. If the Selsyn failed during a dubb, all hell would break lose,….. machines running forwards / backworks at incredible speeds. There would be a chorus of RUNAWAY as the picture projectionist dived to abort master control. It was always incredible that there was never any film damage!

I have bad memories from early booth experiences where there was always drama with an interlock. I don't know why but, for some reason, those two projectors weren't running at exactly the same rate and the accumulator always ended up "accumulating" extra film. We'd lose tension every couple of shows.

But those were crappy projectors. Curiously, all of our newer installations - which could probably handle interlocking much more capably - are not set up for interlocking.

I just thought of another question: Why do some movies (like Star Wars: Episode II) have a "no interlock" policy built into the contract?

~Manny (the fresh-maker)

Here's a hidden bonus on this limited DVD Edition...

Back in the late-1980s, I needed to interlock two Super-8 projectors for a home-made 3D movie I made back in the 1980s. I had somebody convert the inching knobs to have bicycle chain gears on them so they could be physically tied together. I ran the master with its own motor and the slave was in an "idle" mode, driven by the bike chain.

I have operated a lot of smaller theatres and found the need to interlock more often than most.

I have used selsyn with up to three screens.

I currently sometimes interlock two century machines with sync motors and one 5-deck strong platter between them. I do not use an accumulator...i just use the spare varic trolley for backtention on the failsafes.

There are two more century machines in that building that I interlocked for kidshows this year. They are most certainly NOT sync motors. I have a 8 foot, 5-roller accumulator to compensate. You can always try couple of machines if you have a large accumulator such as this one to see what happens. Start with the trolley in the middle and see what happens. If you are gaining film start next time with the trolley all the way up...If you are losing, all the way down. I've done this with up to 2hr prints on those two "mismatched" machines with no problems.

I have a simplex sh1000 with a regular ol' induction motor. I have noticed thatr when referenced to the DVD, the film will gain ground, and the DVD falls behind. Does this mean that I have a machine that runs faster than 24 FPS or isnt the DVD a very stable frame rate to compare to? The DVD is in my computer.

For a video signal to lock-up and display properly, the playback time-base must be a very accurate multiple of the recorded timebase. This allows normal as well as high-speed forward and reverse and still playback. Although the "picture" information is varied, the timebase is not.

Bottom Line: Your video image drifts relative to the film image because of the speed of the film projector not the video player.

>>> Phil

Also, for 3D dual-projector applications the left-eye and the right-eye shutters must be in sync. Both open together for the polarized method or each shutter opposed for the LCD method. The only way to make sure both 3D projectors are in sync is to use some sort of feedback control like a selsyn or tachometer/servo drive.

And for the reasons mentioned, an unresolved film projector or audio tape player would not remain in sync with a DVD player or any un-sychronized source. Even "crystal-controlled", "wireless sync" devices will eventually run out of sync due to the (although minute)inaccuracies of their timebases.

>>> Phil