This is topic Frames per second differences? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.


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Posted by Kara J. McVay (Member # 1285) on 01-12-2003, 10:50 PM:
 
I have 1 Century and 2 Simplex 35 projectors. I recently took over operations of my theatre after an 11 year absence, anyway the Simplex 35's were not there when I left. www.thestrandtheatre.net has pictures. ( I still haven't gotten them to film-tech [Frown] )

When we moved LOTR up to one of the Simplex 35's from the Century, one of my employees said "it will be over 3-4 min. sooner, because we moved it." I went WHATTTTTTTTTTTT [Eek!] ??? I am not that "motor savy", so my question is why does it run faster in my other booth? A little background on my booths. They are all separate locations (lots of fun), anyway, my balcolny & side theater booths are the Simplex 35's and the 1985 main booth is the Century. The balcolny is the original 1916 booth and the side was built in 1982. All I know about the electrical loads is that main & balcolny have 3 phase and side is still a single phase.

Just wondering
 
Posted by Aaron Sisemore (Member # 145) on 01-12-2003, 10:56 PM:
 
Do you notice a speeding up (or slowing down) of the audio when a print moves from one house to another? For the speed to be far off enough to shorten running time by 4-5- minutes the speed change has gotta be audible!

Its also possible that the guy is blowing smoke...

-Aaron
 
Posted by John Hawkinson (Member # 1135) on 01-12-2003, 11:00 PM:
 
It's a fact of life that not all projectors run at the same speed. Even two machines of the same model may not run quite the same. Oftentimes this is a function of belt wear and slippage. On a Century C with a poly V belt, you can speed-up or slow-down by at least 1 frame-per-second based on belt tension/slippage/wear.

3 minutes out of 178 (or 179, if you meant the Two Towers) is .1.7%, which is .408fps. That's well within the tollerances that you can expect.

This is why people who interlock regularly use shaft encoders and synchronous motors/selsyns to ensure precise synchronization (generally for a dubber), or accumulators/elevators to take up the slack if the projectors are close.

If you're sufficiently determined, depending on your equipment, it may be feasable to adjust for better accuracy.

--jhawk
 
Posted by Kara J. McVay (Member # 1285) on 01-12-2003, 11:00 PM:
 
I have never noticed it any audible difference. I have witnessed the film being shorter. LOTR is 3 hours almost on the money with the preview I have on it, and it is a bit shorter...
 
Posted by Mark Gulbrandsen (Member # 72) on 01-12-2003, 11:04 PM:
 
Kara,
Over that length of film...3 plus hours.... the difference in speed would be pretty much inaudible. 3 to 4 min sooner is not that much faster over than length of time. I wouldn't worry too much about it.....
Did you know that all British films were and may still be shot at 25 frames per second instead of 24??? All because of British Television being at a 50 cycle frame rate(interlaced)rather than 60 cycles like ours is. That way they don't need any special 2/3 pulldown movement like we have to use over here. Simplex 35's are a lighter load on the sound head/motor than a Century is, so the Simplex motors may run a bit faster.
Mark @ CLACO
 
Posted by Tim Reed (Member # 137) on 01-13-2003, 12:07 AM:
 
If you have a Century soundhead with a poly-v (serpentine) drive belt, it is either loose and slipping, or it is riding up on the outside of the large pulley (possibly from a folded-over belt "v"). Either one of these conditions will make the projector run slow. Not enough to notice sound variations, but enough for a 3-4 minute difference in running time.
 
Posted by Leo Enticknap (Member # 534) on 01-13-2003, 01:45 AM:
 
I agree with the 25fps/PAL theory - as Mark points out, because of the 50hz scanning rate, telecine with PAL is simply a case of running at 25fps and scanning each frame twice. It's certainly not that uncommon to find cinema projectors over here which run at 25, usually ones which were formerly part of a telecine setup and then recycled for theatrical use.

However, this would take a bit more than 3-4 minutes out of a three hour film. I'm still on my first coffee of the morning and not feeling up to complicated maths, but my memory is that the difference is 3-4 minutes in a two-hour film, 4-5 minutes in a three-hour film.
 
Posted by Paul G. Thompson (Member # 655) on 01-13-2003, 01:57 AM:
 
I think Tim has the answer. Poly-V belts do slip if you have that type of configuration.

Also look at the name ID tags on the motors, and see if there is any difference in the motor speed. It might give you some idea as to what is going on.
 
Posted by Jack Ondracek (Member # 1466) on 01-13-2003, 02:04 AM:
 
An indoor house in our state ran a set of Century "C" projectors with R3 soundheads. Don't know if this was the original setup, but they had twin V-belts between the motor and the soundhead pulley. Some brain trust decided to replace them with something from a local hardware store, or the like. The new belts rode high on the pulleys and created a fast enough speed to be audible but not offensive to the owner's "tin ear".

Ironically, when the machine was converted to those cog belts and brought back to proper speed, the owner's first response was disappointment that he wouldn't be getting out early anymore! [Roll Eyes]
 
Posted by Stephen Furley (Member # 1277) on 01-13-2003, 05:33 AM:
 
Mark wrote:

quote:
Did you know that all British films were and may still be shot at 25 frames per second instead of 24???
Mark, British films are normally shot at 24 fps, the same as 'normal' films in other countries. They are run at 25 fps for PAL video transfer, and live television transmission from film, in the days when that was done.

Film material which is intended only for this use is sometimes shot at 25 fps, to avoid this speedup, in the same way that film intended only for NTSC is sometimes shot at 29.97 or 30 fps to avoid the 3:2 pulldown issues.

The problem comes when you want to be able to transfer to both video systems, as is quite common, television distribution is no longer a national thing. Let's hope that if 625 PAL and 525 NTSC do get replaced by something better it will be by a single, worldwide standard.
 
Posted by Jean-Michel Grin (Member # 1360) on 01-13-2003, 06:12 AM:
 
For PAL technicians in Europe, NTSC means:

Never Twice the Same Colors ! LOL
 
Posted by Leo Enticknap (Member # 534) on 01-13-2003, 06:22 AM:
 
...and PAL means 'picture always lovely' [Smile] I was once told a witty alternative acronym for SeCAM, but cannot remember it.

As for a universal TV standard, you could argue that in some ways MPEG-2 has become that. Even though it is encoded in different pixel dimensions for PAL (720x576) and NTSC (640x480), the digital to analogue converters in DVD players and digital terrestrial set-top boxes are capable of displaying any sort of MPEG frame in a given TV standard. I'd guess that once analogue broadcasting is history, TVs will be like computer monitors: they won't be tied to any broadcast standard at all, and will display anything that the d/a converter chucks at it.
 
Posted by Brad Haven (Member # 954) on 01-13-2003, 10:33 AM:
 
We have the same problem, our century projector (MDAW2) runs too fast, i'd say we would gain about 3 mins for a 95 min film, i can hear the difference myself, more so if i have run the film at the correct speed on our ballantyne, but we have never had a complaint.
The next time we get some work done on our century i'm going to ask if we can get it corrected, it really bugs me, it sounds like the chipmunks at times! (exaggerated, but not far off!)
 
Posted by Scott Norwood (Member # 30) on 01-13-2003, 10:50 AM:
 
NTSC = Never Twice the Same Color
PAL = Problems Are Lurking
SECAM = System Entirely Contrary to the American Method
 
Posted by David Stambaugh (Member # 1102) on 01-13-2003, 11:17 AM:
 
Hmmm... [evil] [Wink] In this thread I posed the intellectually stimulating question "How can you be sure 2 interlocked projectors will run at exactly the same speed?" The replies basically said "They just do". But this thread is saying the opposite is often true. ?? Just curious. [Confused]
 
Posted by Dave Macaulay (Member # 813) on 01-13-2003, 01:25 PM:
 
I won't dispute the possibility of belt slippage but the answer may be simpler.
With synchronous motors (1800RPM hereabouts) projectors will run almost exactly the same speed, and sync is possible with just a loose loop in the crossover if you have to. An accumulator is still a good plan, starting and stopping and especially a fault can cause uneven feed.
The "standard" type motors don't have an exact speed. They're easy to detect as the label RPM is not an even multiple of the powerline frequency - but the actual shaft RPM depends on load.
Synchronous motors really want to run at the given speed and slowing down under load indicates overloading.
All the recent new projection systems I've seen have sync motors. Older installations almost always have standard motors as sync motors cost a bit more and there wasn't any pressing need for exact speed regulation.
I suspect that the popularity of multiplexes and thus interlocking has forced the change to sync motors. Odviously no practical accumulator is going to handle 3 minutes of film, and that difference wouldn't be unexpected with standard motors especially if the equipment is different.
Changing almost any projection package to a sync motor is possible, but fairly expensive - probably in the $1000 - $1500 range once the motor and drive components are purchased and installed.
 
Posted by Darren Briggs (Member # 1071) on 01-13-2003, 02:09 PM:
 
I think Dick at Bradford would confirm you can make projectors all run at the same speed on interlock no hassle. He has three doing it in cinerama no probs, think he uses selsyn motors.
I run Cinemecanica Vic 5's and an 8 all on Inverters and i can run all 5 on interlock and they stay in sync all day if you need it because of the inverters and toothed belts and all runing off the same phase power supply.
Never ran a film between then as the projection rooms are so far apart, but ran test loops to prove this point on a very quiet morning at work!.

Darren
 
Posted by John Hawkinson (Member # 1135) on 01-13-2003, 02:29 PM:
 
Dave, answers are twofold. One is that more modern technology than the Century C poly-V belt is a lot more reliable (and there's a timing belt upgrade for the Century C for a few hundred bucks). The other answer is "they are close enough to interlock with."

The place where you really notice speed differences is on changeover, especially with dts, because it crossfades the rate between reels on a changeover (30ms). We generally have the dts/1045 (flying disc) as the last item on our trailer reel. When we had a speed difference between left and right projectors, if the changeover happened while the final note was still playing, you could hear it pitch-bend as the dts 6D compensated and cross-faded. It really sounded bad [Frown]

For what it's worth, my technique for fixing this is as follows:
On a Century poly-V, the real adjustment is the tension of the motor, and most of it is in ensuring it does not move as you tighten down the 4 bolts that hold it in place.
Oh, one other thing. If you find a Century poly-V where the belt is not properly seated on the pulley, chances are you'll need to replace the belt after you re-seat it, if it has been badly seated for any length of time, as the belt gets deformed.
Darren: phase has nothing to do with it. All phases of your power grid had better be the same frequency, or all hell would break lose.

--jhawk
 
Posted by Jim Alexander (Member # 226) on 01-13-2003, 02:57 PM:
 
One thing, I learned along time ago, is, without synco-phase or external speed control, two motors will run at difference speeds; even those that are considered "high reliability" or "precision".

A number of years ago, the Canadian Armed Forces, in their infinite wisdom, purchased a flight simulator that had two video cameras mounted inside to record different things during a simulation. these two were recorded on two separate 3/4 inch industrial video tape recorders. The idea was that since they were recorded at the same time, they could be played back at the same time and they would stay in sync. Problem was, since they were two separate machine, it was impossible to sync them since one would always run slightly faster than the other.

Would you be able to hear the difference...even in the example above of a difference of 3-4 minutes? I don't think so, unless you can hear a difference of 1-1.33hz increase or decrease in pitch. And if you can hear that, then you are good and should be working for Audio Research or Bose.
 
Posted by Steve Kraus (Member # 476) on 01-13-2003, 03:05 PM:
 
If you want to check the speed you can use a solar cell (perhaps an old mono one out of your junk box, otherwise get a cheap one at Radio Shack), connect it to your DVM set for frequency measurement, and hold it in the light beam. With a 2X shutter (double blade or single blade, double speed) you will be looking for a deviation from 48 Hz.

I can't imagine someone wanting to fool around with interlock without sync motors. I saw it done once temporarily (motors were on order) and the operator had to hand brake the faster machine with a rag on the flywheel. It was lame.
 
Posted by Paul G. Thompson (Member # 655) on 01-13-2003, 08:15 PM:
 
If there is a small differential speed between the two projectors that you want to interlock, just build an accumulator. It works great! I built one about 14 years ago which was basically 10 passes at 8 foot each. However, you have to know which machine runs faster, so you can load the accumulator accordingly. Saved us lots of bucks not having to buy expensive sync motors. [Big Grin]

David, sync motors (if ganged) are supposed to run at the same speed. If they don't, one motor is developing magnetic slip, probably because of excessive load. If the slip is severe enough, the motor will lose all is torque, drop substancially in speed, and if left long enough running, it'll probably self-distruct.

Old selsyn motors from the 3-D days --- I never ran a two-projector 3-D setup, but from what I understand, only one projector motor was used, and a selsyn drove the other projector with the main motor free-wheeling. I understand that both drive motors were not used, because they would fight each other. If they ran at different speeds, they would both eventually overheat with a distinct possibility of burning up.
 
Posted by Dave Macaulay (Member # 813) on 01-13-2003, 09:28 PM:
 
My point is that even if the belt isn't slipping, a Century with a non-synchronous motor will probably run at a slightly different speed than a Simplex with the same motor just because the load is different.
If the Simplex has a sync motor some difference is almost certain. I can't be sure given the angle but the photo suggests a sync motor to me.
The speed difference is minor, maybe someone with perfect pitch could hear it (3 minutes in LOTR...) but I wouldn't worry about it.
Since he isn't trying to run interlock I don't see any reason to panic. I would check the belt drive though. A new belt and a thorough cleaning of the pulleys is usually needed on old machines as grease drippings are magically attracted to the area. The pulleys can even get so polished after years of wear that slippage is nigh impossible to eliminate, and the toothed belt conversion should be installed.
 
Posted by Gordon McLeod (Member # 33) on 01-13-2003, 09:53 PM:
 
Another area that is a problem with the poly v belts is the pulleys get polished and tyhen the belt starts to slip
Try a bit of belt dressing on it
 
Posted by Paul G. Thompson (Member # 655) on 01-13-2003, 09:54 PM:
 
The basic motor speed of the IR motor is determined by the number of poles and line frequency, and not the load. (unless it is overloaded.)
 
Posted by Dave Macaulay (Member # 813) on 01-14-2003, 10:45 AM:
 
A standard induction motor has a slip (RPM drop) of about 5% at rated load, the actual shaft speed depends on the load with less slip at no load. They will start from stall but often have a starting circuit for extra start torque applied via a centrifugal switch.
A synchronous motor has 0% slip unless overloaded. They will not start from stall without a starting circuit, again applied via a centrifugal switch. The motor has to be up to about 30% of full speed to manage much torque trying to lock up to synchronous speed. If your sync motor just hums and heats up instead of starting, the start switch or circuit is defective.
Larger motors usually have an external starting contactor as the centrifugal switch causes a lot of failures and changing out a 500+ pound motor for a $10.00 switch is not desirable.
 
Posted by Rick Long Jr (Member # 541) on 01-14-2003, 11:27 AM:
 
A quick way to determine if your projector is running at 24FPS is to make a loop exactly 7 and one half feet long (120 frames). When running, the splice should go through every 5 seconds. Time it over a minute or so. If the machine isn't running at the correct speed this test should confirm it.
 
Posted by Paul G. Thompson (Member # 655) on 01-14-2003, 01:16 PM:
 
I agree with the start-up slip, and I also totally agree with belt slip and pully wear.

Seems to me I have seen motors on projectors with speeds anywhere from name-plate data from 1725 to 1800 RPM. Now if there is a 5% Jesus factor and one motor on one machine has a 1725 rpm motor, that's now running ar 1638.75 RPM. An 1800 RPM motor under load would be running at 1710 RPM, and the differential speed would be 71.25 RPM, which is hardly even worth considering unless you want to interlock, and that's why they have interlock and sync motors, selsyns, and accumulators..

If one projector's running time was off by 2 minutes or more over a two or three hour run, it would make sense. But I still comsider a 1750 motor running at 1750 under nominal load. A small amout of slippage has never been a problem for me.
 
Posted by Steve Kraus (Member # 476) on 01-14-2003, 02:25 PM:
 
The 1725 RPM is a nominal figure net of slippage. The non synchronous motor chases sync (1800 rpm with the typical number of poles on 60 Hz.) but by its design cannot achieve it. As others have said the slippage varies with the load.
 
Posted by Paul G. Thompson (Member # 655) on 01-14-2003, 04:41 PM:
 
I was aware of a small tolerance of speed, but I didn't know what the percentage is.
 
Posted by Frank Angel (Member # 248) on 01-14-2003, 11:54 PM:
 
Paul G., although I never actually personally ran 3D interlock either, I have witnessed a system in operation and it seemed to me that each selsyn was engaged with its corresponding projector motor shaft via belt and pully. I may be remembering it incorrectly, but it seems that both projector motor shafts hand pullys with teeth and both were attached to their respective selsyns. I don't recall seeing either attached to the drive shaft with the projector motor disengaged. But your idea seems logical -- unless both projector motors were keeping nearly perfect sync, the selsyns would cause either a drag force or a "push" force on the motors, causing one or both to overheat. Interesting. What I find even more interesting is that with either selsyn or sync motor interlock, how do you start the whole thing and keep it all in sync from PICTURE START frame? I am not talking interlock for multiple screens, I am talking about 3D -- how do you start both machines which obvioulsy are not yet in sync when the motors are first tuned on. How is frame-for-frame accuracy maintained at startup? Interlocking for two or more screens is no problem; if any projectors is off even by a foot or two, accumulators will deal with it -- but I mean for 3D projection -- both machines must run frame-for-frame right from the very first second they are tuned on. Seems to me that this would be a freak show of a problem for the first few seconds. And if they are not frame-accurate, what can you do?

Frank
 
Posted by Steve Kraus (Member # 476) on 01-15-2003, 12:15 AM:
 
The selsyns keep it locked together, right down to the fraction of a rotation. One can drive the other so the other follows what it does or there can be a master generator (driven by a motor) that runs all three (two projectors and probably one mag sound dummy). With the selsyns engaged you can even inch them in unison.
 
Posted by Brad Haven (Member # 954) on 01-15-2003, 10:28 AM:
 
Paul said
quote:
I never ran a two-projector 3-D setup, but from what I understand, only one projector motor was used, and a selsyn drove the other projector with the main motor free-wheeling. I understand that both drive motors were not used, because they would fight each other. If they ran at different speeds, they would both eventually overheat with a distinct possibility of burning up.
I was lucky enough to run dual projector 3-D (dial m for murder) in the mid nineties and the above was the way we did it.
 




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