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This topic comprises 2 pages: 1 2
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Author
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Topic: Synchronizing 2 projectors
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Brad Miller
Administrator
Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99
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posted 09-04-2016 04:48 PM
Sounds like it will be one of those 24/7 things where the projectors just run and run and run without ever being re-threaded like would happen in a normal multiplex.
Gordon's answer is correct for this application, but you could also rig up something on the cheap if you don't have the budget. Something that comes to mind without really putting much thought into would be to get two of those vertical sliding roller assemblies like people used to take up slight speed differences during interlocks, but rig a microswitch toward the bottom as well as toward the top of the accumulator roller's travel.
To try and paint an image, the film exits the left projector, runs through accumulator #1 and then enters the right projector. If the accumulator roller comes near the top of the vertical travel, the roller mount's bracket hits a microswitch that cuts power to the motor of the right projector. If the accumulator roller comes near the bottom of the vertical travek, the roller mount's bracket hits a different microswitch that cuts power to the motor of the left projector.
Then of course you would need to repeat that with a second accumulator so as the film exits the right projector, it would go through the second accumulator and then into the left projector.
So like I said, it's a "rig job", but if you're just trying to show film shuttling and such while using one loop between the two machines, that would do the trick. Assuming they didn't run EXACTLY in sync, but somewhat close, every so often one of the projectors would simply turn off for a couple of seconds. Just make sure you have a slow start kit on the motor so it isn't too jarring.
There are far more elegant solutions, but you would need to provide us with much more information. For example, what is the model of projectors in use? Does the motor have a slow start kit of some kind of inverter speed control on it? Does it have to be one loop running through BOTH of them in an interlock scenario? What is your budget for this?
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Paul H. Rayton
Expert Film Handler
Posts: 210
From: Los Angeles, CA , USA
Registered: Aug 2003
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posted 09-05-2016 02:33 PM
Hi, Jae -- Interesting concept. Your initial setup (as seen in the video) incorporates a bit of a homemade "loop accumulator" of sorts, though I can see how the frame-accurate sync would be important over a longish period of time. Those Bell & Howell projectors are not "synchronous", for sure.
The first model of projector that comes to my mind, and which has synchronous motors, is the classic Eastman 25. There are several reasons you would probably NOT want these: rather large, no built-in loudspeaker, and not manufactured for perhaps 50 years -- though some can still be found, since they were exceptionally well manufactured. Actually, considering the concept shown in your video, the built-in loudspeaker aspect seems to be really important, since the audio byplays, or "bounces", between the multiple projector sources of the installation.
Those Bell & Howells you are using are powered by a motor which drives a plain rubber belt. The belts, no matter how well matched at the time of setup, will ALWAYS have a differing degree of slip between multiple machines. It may be very small, but (as you've probably observed) over a period of time they will almost invariably drift apart.
To stay with the type of projectors you have, you could possibly open them up and replace the motors with something similar size that is a synchronous motor. PLUS, you'd have to convert all driven surfaces to "gearbelt" drives -- i.e., the driving pulley on the motor, and the pulley which advances the mechanism of the film through the projector. And, of course, use an appropriate toothed gearbelt to end any slippage of plain rubber surfaces.
Assuming those conditions are met, your two (or more) projectors will stay synchronized as you require. The rotational speed of sync motors is set by the frequency (Hz rate) of the power line, which in the US is 60Hz, so a set of identical motors started at more or less the same time will maintain their relative speeds precisely, regardless of any differing drag or loading the mechanism(s) may be presenting to the motor. Within design limits, of course...
I haven't opened up my own similar projector recently, but you'll probably have to do some searching to find an appropriately-sized motor that will fit in there, be the correct RPM, and be synchronous.
Or, possibly someone out there knows if there are any models of 16mm projectors made in years gone by which were already built to operate in a synchronous mode. There were some that have been built to work for television (telecine) purposes, but I doubt they'd have the built-in loudspeakers.
Good luck on your project!
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Paul H. Rayton
Expert Film Handler
Posts: 210
From: Los Angeles, CA , USA
Registered: Aug 2003
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posted 09-05-2016 04:26 PM
Yes, but the "frame accuracy" he is talking about is a bit over the top for what he essentially needs, and the term is a bit more demanding than what he needs. There is "lip-sync" frame-accurate, and a less critical, what I'd call "non-slip interlock" just so that interlocked machines all run at the same speed. Assuming his anticipated installations are similar to the one he showed in his illustration, there is a giant film loop circulating through the air, and once the two (or more) machines are up and running, they will stay that way -- if sync motors are driving the system with gearbelts. Again, that's assuming that there will be a bit of slack between the various machines, as per the illustration.
Trust me, we're intimately familiar with absolute syncing. We've got the ultimate "frame accurate" system, which uses e-tracs, shaft encoders, and a super hi-tech synchronizer which allows us to do absolutely perfectly-synced two-projector 3D, with even the shutters in absolute sync. That system is super expensive, but the concept he is looking for is just to keep the loop the same size, without drifting, over a run time of around 2 hours, or possibly more.
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