This is topic Basic question about interlocking in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by David Stambaugh (Member # 1102) on 08-07-2002, 09:24 PM:
One thing I've never really understood is how 2 or more interlocked projectors can pull film through at basically EXACTLY the same rate. I understand synchronous motors. But with gears, pulleys, and belts, how can you know for sure that one projector isn't running slightly faster or slower than the other? I realize there's an accumulator loop, but doesn't that only allow for a couple feet of "error"? If you're running a 10,000 foot movie interlocked, and one projector ends up pulling 2 more feet of film through it than the other projector over the film's running time (compensated for by the accumulator), that's only 0.02% difference in speed (if my calculation is right). Is the answer as simple as something like adjustable motor speed controls so you can get them exactly the same? Or is the 24fps speed really that precise across different projectors?
Posted by Steve Kraus (Member # 476) on 08-07-2002, 09:51 PM:
Yes, the speed really is that precise. Don't forget that we are talking about timing (toothed) belts and gears & shafts. So unless a timing belt jumps a tooth or there is a stripped gear somewhere (both of which would be causing problems quite apart from interlock operation) the speeds will match exactly. The fact that the medium is sprocket driven is important too--one turn of a sprocket advances so much film, regardless of tiny variations caused by shrinkage or expansion. If it were, say, mag tape being driven by a rubber pinch roller you'd have to have a control track with pulses and some sort of a servo feedback loop to control the speed which would be much more difficult. Film makes it so easy.The accumulator is really just to compensate for varation at startup as the machines come up to speed as there may be a difference from one to another. Once up to speed and locked synchronously to the power line the contents of the accumulator will not change.
Posted by Bobby Henderson (Member # 840) on 08-07-2002, 10:17 PM:
Interlocking is a pretty simple thing. But it is a practice many theaters do not do very often at all. In the 8 1/2 years the Carmike 8 has been in operation here in Lawton, I think they have interlocked films only 3 times (with two occaisions happening just last year). The idea works, but it has to be carefully tested before running a release print through it.And then you have the added task of burning an extra set of DTS CD-ROMs since only one set comes with a print.
Posted by Michael Schaffer (Member # 1204) on 08-07-2002, 10:35 PM:
Steve is right. At the prevoius theater I worked for, we interlocked (Kinoton FP30D) all the time, and the machines ran very accurately, with the exception of slightly different startup times. The Pennywise matrix which comes as the standard automation with Kinoton projectors ONLY sends motor start/stop signals over the serial bus with which the projectors (or rather the matrices) have to be connected to make interlocking possible.
Once we had a faulty matrix which failed to send a motor stop signal to the 2nd (master) projector, when the first (slave) stopped playing for no apparent reason, so the master kept pulling the film. The result was an incredible mess and a lot of damage on the two projectors.
Michael
Posted by Brad Miller (Member # 2) on 08-07-2002, 10:52 PM:
Did someone say interlocking? 
Posted by Darryl Spicer (Member # 711) on 08-07-2002, 11:02 PM:
>>The accumulator is really just to compensate for varation at startup as the machines come up to speed as there may be a difference from one to another. Once up to speed and locked synchronously to the power line the contents of the accumulator will not change.<<It is also to maintain tension on falesafes.
There is one problem with it and I encountered it at one of our theaters here. We were trying to interlock four machines. Three machines at this location ran fine but the lead payout machine in the line was feeding the film slightly faster than the other machines. So after a few minutes it would shut down and we would have to rush and shut down the other three. Well, it turns out after a little investigating that the lead payout machine had its motor changed out at some point. THe motor was only like maybe a year old or so. Everything was correct and matched but the newer motor was pulling just a bit faster. So I guess if you are still interlocking and you have to change a motor out on one of those machines. I would switch out a motor from another machine and put the new one on the other machine or invest in two new motors and replace both of them on the interlock machines.
Posted by Barry Martin (Member # 1376) on 08-07-2002, 11:14 PM:
I have never experienced any lost accumulation when running interlocks on the Strong Simplexes at my theater. Interlocks are rare, but we probably do at least 10 days of interlocking a year. Mostly the big releases like Star Wars, Spider-man, and Perfect Storm (big here in MA because of the location). They have all run pretty smoothly, but I have experienced a little delay when starting the projectors if only for a half a second. I just don't like the idea of running film over 100 feet of open booth space and across the hall requiring walking "through" the film in order to get to the other side of the booth. 
------------------
"The greatest thing you will ever learn, is just love and be loved in return."
Barry C. Martin
IATSE Local 182
Posted by Darryl Spicer (Member # 711) on 08-07-2002, 11:19 PM:
The projectors at that location are simplex 1050's. Since that motor was newer I guess it was just running just slightly faster than the others.
Posted by Steve Kraus (Member # 476) on 08-07-2002, 11:53 PM:
New or old shouldn't matter on a sync motor unless it is somehow defective and not locking up. A machine problem creating a greater load than the motor can drive could cause non-sync operation but in your case you're saying it's faster not slower. If the motor was running noticably hotter than normal for its type it may be that the start switch is not disconnecting the start winding. Not sure if that would make it go faster but I suppose it's possible. I presume it wasn't so blatant like a 50 Hz motor being run on 60 Hz. Sure that they are all sync motors?
Posted by Darryl Spicer (Member # 711) on 08-08-2002, 12:54 AM:
Actually, this projector was the first projector in the series that was being fed first. The motor on it was newer so that would mean that it was running correctly. The other three projectors have older motors probably 10 years old. Now they could be slowing down just a tad. Yes they are all the correct motors.
Posted by Gracia L. Babbidge (Member # 582) on 08-08-2002, 01:54 AM:
I've not had any serious troubles with my interlock setup. The machines in my interlock setup face in opposite directions (thus are almost back-to-back), one is a Christie P35C, the other is a Century JJ2, its quite an interesting rig to watch in action.
Posted by Alex Grasic (Member # 1371) on 08-08-2002, 02:57 AM:
I work at a fairly new theatre (only open for 8 months) and there has not been any setup made for any interlocks. Despite this, I'm always intriguied in the operation of an interlock and because of this, I want to try it at some point in the VERY near future. I have the capability of EASILY interlocking 9 although interlocking all 18 would be really an interesting experience!! What would be the simplest course of action that I would have to start with to setup my booth for interlocks (even if it's just two machines). The easiest two I could see it happening with would be my #3 and #7 projectors which are almost back to back and have the payout and rewind on the same side as the opposite projector (if that makes sense to you...or would a diagram help more??...I knew it's crude but better than nothing)
Any advice would be appreciated!!
|----------|
| || proj #3 |
|----------|
/----\
/plater\
\ /
\----/ /----\
/plater\
\ /
\----/
|----------|
| proj #7 | ||
|----------|
Posted by Frank Angel (Member # 248) on 08-08-2002, 03:06 AM:
All the interlocks I have seen in operation (someone else's operation, not mine, thank you) have been with synchronous motors. They actually interlocked 4 projectors together -- Gracia said it is an interesting rig to watch -- I thought it was downright scary! But my question is about selsyn motors. I know you can use them to interlock two projectors, but is there any way to get them to interlock more than two like you can with synchronous motors? Can you buss the control voltage across more than two motors?
Posted by Brad Miller (Member # 2) on 08-08-2002, 03:20 AM:
Frank, Scary, eh? May I direct you to the "videos" page of this website. Once you're there, you will find what you are looking for.
Posted by Peter Kerchinsky (Member # 1098) on 08-08-2002, 04:53 AM:
What about 3-panel Cinerama.
The "interlock", which I assume were sylsen motors were not only tied in with 3 projectors, but also the sound dubber.
It's been a while since I've seen it, but it works.
I've run interlock in two Seattle theatres and have never had a problem with it.
With all the prints they're producing these days and the fact bookers want to scatter showtimes, interlock may be a thing of the past.
Posted by Steve Kraus (Member # 476) on 08-08-2002, 07:39 AM:
Sure, selsyns were what were used to drive entire rooms full of dubbers in sync in the days before electronic drive. But those were driven by a master selsyn generator in turn driven by (if I have my facts straight) a switchable choice of a sync motor or a variable speed motor.
Posted by Rick Sanjurjo (Member # 1365) on 08-08-2002, 07:52 AM:
Darryl,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.
Hope this helps
Rick
Posted by John Pytlak (Member # 331) on 08-08-2002, 09:06 AM:
Yes, more than two selsyns can be tied together.
Selsyn Motors
Wiring Diagram
Use in Fantasound (Fantasia)
Use in Cinerama ------------------
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
Posted by Darryl Spicer (Member # 711) on 08-08-2002, 11:30 AM:
>>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.<<
The motor that was replaced was on a projector that was already set up for interlock. Unless someone in the past had changed that pully on that machine with the wrong one, they all should be the same. But since this theater was not the main theater I worked at I could not be for sure if that happened. Next time I am at that theater to do some preventive maintenance I will check that pully.
Posted by Richard Miller (Member # 1405) on 08-18-2002, 11:18 AM:
If the motors were not exactly the same then the movie would appear in slow motion and the sound would be slow as well. If this happens then you have to get it fixed anyway. Other than that it runs at a steady rate through the projector.
Posted by Richard Fowler (Member # 893) on 08-18-2002, 12:06 PM:
The 1/2 to 1% difference in speed between a sync and non-sync motor would not be noticable to most persons while viewing. Century projector, prior to Strong and ORC ownership, once shipped us four sets of equipment with 50 HZ motors with 60 HZ gearing...THAT was obvious due to the speed up on the screen.
Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com
Posted by Martin Dean (Member # 1416) on 08-18-2002, 02:21 PM:
The most interesting interlock system I have seen was put together as a home made project by the Chief Projectionists at one of the theatres I used to look after. I had some input in the design, but most of it was done by him. 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).
------------------
Martin Dean
martinidean@charter.net
Posted by Bernard Tonks (Member # 809) on 08-18-2002, 03:15 PM:
How about this for interlocking! It was quite common in the UK, to have a mechanical coupling bar between projectors 1 & 2 for dual projection 3D, which only could perfectly work. There was however Selsyn interlock in many cinemas such as the Warner, Leicester Square that ran two projector 3D “The House Of Wax” with four projectors & changeovers. There was also stereo sound which had to be run on separate follower soundheads! 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!
Life became easier when the job became just Telecine and video, film was replaced by ENG. Eventually the company offered a number of staff early retirement, which I accepted and concentrated on running my cinema full time.
Posted by Manny Knowles (Member # 1171) on 08-18-2002, 04:01 PM:
I hate interlocking.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.
Posted by Scott Norwood (Member # 30) on 08-18-2002, 04:24 PM:
Episode II wasn't supposed to be interlocked? Umm...oops... I guess what George doesn't know won't hurt him.(No problems at all with Century Projectors, Maxi-10 automations, and big, funky interlock box)
Posted by Wes Hughes (Member # 981) on 08-18-2002, 04:29 PM:
Interlocking is sometimes a mess because people have such different ways of setting up their interlock "systems."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.
Sometimes you gotta do what you gotta do: Last year a theatre (not one of mine)called and needed to interlock two machines that night (about 4 hours time). They had never interlocked, and did not have sync motors, selsyns, nothing. I took an accumulator and some rollers...set it up....BIG difference in speed between the two mcahines. Since it was only one show I solved the problem by creating another one...I loosed the belt on the faster machine until the accumulator trolley stopped moving! Worked great.
Posted by Josh Jones (Member # 447) on 08-19-2002, 01:08 AM:
This is a little off topic, but oh well,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.
Josh
Posted by Phil Hill (Member # 371) on 08-19-2002, 01:47 AM:
An induction motor's speed will vary with the load and therefore is not constant. Most modern film projectors use an AC sync motor that holds a constant and stable speed within it's voltage, frequency, and torque rating. Some of these motors are driven directly from the (50 or 60 Hz) AC line, and some from variable speed controllers that vary the operating frequency to the sync motor to control the speed.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
Posted by Frank Angel (Member # 248) on 08-19-2002, 02:12 AM:
So Wess, you are saying that you were able to get two projectors that had standard induction motors to run close enough so that you didn't need selsyn motors to force them to stay together, just the accumulator action was enought compensation? That's pretty impressive. And Phil, are you saying that if a projector is outfitted with a synchronous motor, that would stay in perfect sync with a second projector, or a DVD player? Very impressive. Now would that be accurate enough for 3D?
Posted by Phil Hill (Member # 371) on 08-19-2002, 02:47 AM:
Frank:
I'm not saying that at all. Even though a sync motor will stay "in-sync" at the operating speed, different motors will come up to speed at different rates because they usually operate in an induction mode for startup. Even if they did come up to speed together, it cannot be assumed that they would remain in sync. Variables due to minor differences in friction or other characteristics would allow the motors to drift and become noticed over a period of time. Mechanical variations between machines, all machines both film and video, is why there is a "control" track recorded on video tape and on other "to be sync'd" media.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
Posted by Manny Knowles (Member # 1171) on 08-19-2002, 12:13 PM:
This is one area where video wins. Video (digital or analog) will always run at the same rate. Videotape syncs up via a control track. I don't know if CD's and DVD's sync to anything like that or if they're just precise because they are (a-hem) digital. 
I know that you can sync a CD to a video with no problem because I have done many video recordings of concerts and live theatre shows where the audio was burned out to CD from the mixing board. There's never a problem maintaining sync.
Two videos of the same program -- once synchronized -- will remain in sync.
With film, there are real-world variables that interfere with tidy theories.
~Manny (operators are standing by)
Posted by Josh Jones (Member # 447) on 08-19-2002, 11:49 PM:
Also, a cd player uses a crystal reference for the sampling rate, so it will, in theory, not drift.Josh
Posted by Ken McFall (Member # 845) on 01-14-2003, 03:09 PM:
It's worth mentioning that if you use inverters to run your motors then there is no 'slippage' when you start and stop the projectors.
The inverters will be set to the same run up and slow down delays and the projectors will therefore start and stop at exactly the same rate.
With inverters you don't just turn the power on or off .... you start and power down the projectors in a controlled fashion. It's a very elegant solution and works remarkably well.
The loop absorber placed mid way is there to allow for the unexpected and the fact that the drag of the film will cause slight movement of the absorber. However, I've never had more than an inch or so, which was the film tensioning as it ran due to the drag over such a long distance.
You do need to use timing belts to eliminate any slippage on the projector drive itself. Apart from that you can use any combination of projector types. The inverters will power up the projector in such a fashion that there will be no slippage etc. Trust me it works and its very reliable.
The hardest part is hammering home good practice to your staff to ensure they don't 'create' problems. Also enforcing a minimum turnaround of at least 30 minutes when the manager does the time sheet so there is no rushing during lacing up and accidents happening.
I've run many shows now and have had no problems.
I should add that the last interlock, 'LORT Two Towers', was the first time I'd used Film Guard at the same time. The result was exceptional. With the film running over such a long distance there is going to be more abrashion and dust. Film Guard eliminated that and the print looked as good after three weeks running as it did on its first run.
Regards Ken.