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Author Topic: Frames per second differences? Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 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.  |  IP: Logged Darren Briggs Master Film Handler Posts: 371 From: York, UK Registered: Dec 2001 posted 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  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 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 For what it's worth, my technique for fixing this is as follows: Take a twenty-minute reel of film and count it on a frame counter, and then run it through the projector with a stopwatch and time it carefully. This is your baseline. This is the only real way to know the actual speed, but takes a while to measure. Next, hook up a frequency counter to the output of the cinema processor. Run some fixed-frequency test film (1k tone works if you have enough, SMPTE MF test works reasonably well, since it is nice and long). Best not to use a loop, though, especially since you care about motor loading. Measure the frequency of the 1k tone on the counter. (Expect this number to not necessarily be near 1k, even if you are spot on at 24fps; 1k tone is not intended as a frequency reference, but is intended as a gain reference, and the frequency can be off by a lot.) Calculate the frequency you need to attain 24fps based on the measured speed in fps and the frequency reported by the counter. With the 1k tone running and the counter visible from your driving side of the projector, adjust the speed until you are as close to your target frequency as possible. Re-run your test reel of film and check and make sure you got it right. 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  |  IP: Logged Jim Alexander Film Handler Posts: 71 From: Greenwood, Nova Scotia, Canada Registered: Nov 1999 posted 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.  |  IP: Logged Steve Kraus Film God Posts: 4094 From: Chicago, IL, USA Registered: May 2000 posted 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.  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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. 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.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 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.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 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  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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.)  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 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.  |  IP: Logged Rick Long Jr Expert Film Handler Posts: 211 From: Toronto, Canada Registered: Jul 2000 posted 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.  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 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.  |  IP: Logged Steve Kraus Film God Posts: 4094 From: Chicago, IL, USA Registered: May 2000 posted 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.  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 01-14-2003 04:41 PM                     I was aware of a small tolerance of speed, but I didn't know what the percentage is.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 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  |  IP: Logged

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