Film-Tech Forum ARCHIVE: Accurate Projector Motor Speed?

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Posted by Joe Beres (Member # 678) on 03-07-2003, 03:18 PM:

I am running some silent films that require 20 fps (as per the archivists providing the prints) projection. The motors have basic rheostats that allow speed adjustment. I have run some film and guessed at the speed, but is there a definitive and/or accurate way to know when you've hit a particular speed?

Granted, when earlier silent films were shown, speeds were chosen by the projectionist, but I am wondering if there is a way to know what speed (in fps) you are running.

My predecessor passed on some photocopied sheets of "AS-1068 120 ~ STROBOSCOPE DISCS." These are small circular patterns that can be taped to one of the continuous sprockets and viewed under a 60 Cycle incandescent light source. When the pattern seems to be still, then supposedly you are running at 24fps (360rpm according to the sheet). Has anyone else used these? Judging by eye and ear, they seem to be quite accurate. Does anyone else have any for slower speeds?

Posted by Randy Stankey (Member # 64) on 03-07-2003, 03:58 PM:

Got an oscilloscope and a photo cell?

Hook the cell up to the scope, run the machine and point it at the light coming out the aperture, through the shutter. You should get what looks like a square wave at approx. 48 Hz. (Or 24 X the number of blades on your shutter)
Adjust the motor speed until you get 40 Hz. (Or 20 X the number of blades on your shutter, rounded off to the nearest whole.)

If you have a fequency counter you could do the same thing but you wouldn't have to muck around with figuring out freq. on the graticule.

Posted by Jeff Taylor (Member # 443) on 03-07-2003, 03:58 PM:

How about a tach? You'd have to calculate the rpm's, but it should be dead on.

Posted by Gordon McLeod (Member # 33) on 03-07-2003, 07:18 PM:

If you are using a invertor to drive the motor you could reprogram the display to display FPS or make up a chart of frequency or RPM(which ever it does display) cross referencing to fps

Posted by Peter Hall (Member # 695) on 03-08-2003, 08:48 AM:

We use a tacho on the sprocket - assuming a 6 frame sprocket the sprocket should run at (24 FPS * 60 = 1440 frames per minute, / 6 frames per sprocket) 240RPM or deriviatives thereof. I find that down to 18FPS is OK with a 2 blade shutter, however 16FPS flickers badly and a 3 bladed shutter and associated gearing should be considered. Drum shutters are better than blades at lower speeds. Also chances are you are running black and white stock slower (or with bigger lamps) than intended so consideration should be given to reducing the lamp current.

Posted by Leo Enticknap (Member # 534) on 03-08-2003, 10:56 AM:

A lower-tech solution would be that if you know the exact length of a reel in feet (most screening prints from major archives have this information on the can - we certainly put it there), time how long it runs at a given speed and then work out what that speed is using a footage/running time conversion chart.

Posted by Joe Beres (Member # 678) on 03-08-2003, 07:05 PM:

Thanks for the ideas guys. The tach idea is especially intriguing, Have any of you used any of the hand-held tachometers (contact or non-contact) on the market? Which might best suit this purpose. The non-contact/laser type sounds like it could work well.

Posted by John Anastasio (Member # 607) on 03-08-2003, 09:35 PM:

Paint or mark a small white line on the outside of the intermittent sprocket. When you shine a FLOURESCENT light on it 20fps will show you three equally spaced lines which seem to stand still. I they move clockwise, the machine is running fast. If they move ccw, the machine is running slow.

Addendum = make that 12 lines that stand still, since the sprocket makes one rotation every FOUR frames and a complete rotation five times per second at 20fps.

[ 03-09-2003, 08:03 AM: Message edited by: John Anastasio ]

Posted by Neil McGovern (Member # 1247) on 03-19-2003, 01:44 PM:

Our Victoria 5 has been modified to a variable speed projector. We can run at any frame rate from 1fps to about 50fps.

Sound Associates ( http://www.sound-associates.co.uk ) could give you details on how it was done.

[ 03-21-2003, 09:46 AM: Message edited by: Neil McGovern ]

Posted by Tim Reed (Member # 137) on 03-19-2003, 01:54 PM:

Good one, John! I was going to suggest a stroboscopic disc on the sprocket.

Posted by Matthew Bailey (Member # 601) on 03-19-2003, 03:03 PM:

If you have a laptop or desktop PC on hand,download the NCH tone generator software & set the generator to 48 cycles as a reference. use a stereo or mono cell aligator clipped to the processor cell inputs. I have a Motorola tone generator that I can set to 48 cycles.

Posted by Daryl C. W. O'Shea (Member # 1303) on 03-19-2003, 04:06 PM:

Can you run that one by again Matthew? Reference to what?

Posted by Phil Hill (Member # 371) on 03-19-2003, 04:32 PM:

I think he was referring to having the photocell pick up the shutter light interruptions and then beat that frequency against a "standard" frequency until a zero beat frequency was obtained thus indicating the projector's speed. But for 20 fps the reference freq would be 40Hz. (Assuming a 2-bladed shutter)

>>> Phil

Posted by Pete Naples (Member # 761) on 03-19-2003, 04:33 PM:

Like Peter Hall, I too use a tachometer on the sprocket. Never tried the optical pick up type, the contact type has always worked just fine for me.

Neil, that must be one bulletproof Vic5 that can run at 50fps! Whilst I'm sure that the invertor/motor is capable of that speed I wouldn't think that the intermittent would enjoy that kind of speed.

Posted by Ben Wales (Member # 92) on 03-19-2003, 04:34 PM:

Here in the UK the local Machine Mart sells either a hand held laser or contact Tachometers for about £70!, all you need is to measure the RPM and then work out the speed of the projector.

Posted by Neil McGovern (Member # 1247) on 03-19-2003, 10:03 PM:

quote:
Neil, that must be one bulletproof Vic5 that can run at 50fps! Whilst I'm sure that the invertor/motor is capable of that speed I wouldn't think that the intermittent would enjoy that kind of speed.

It certainly didn't enjoy it, we had to get a new one [Smile]

Posted by Ray Derrick (Member # 1001) on 03-20-2003, 08:46 AM:

Joe, there is a product available from ICS in Australia that is essentially a film cue detector with the addition of an accurate speed readout. The QS350 cue detector (which looks very similar to the FM35) calculates the speed from the sprocket holes and provides a digital readout in frames per second. The speed calculation is highly accurate and has a display resolution down to one tenth of a frame per second. If you would like more information just e-mail me and I can forward a brochure to you. The unit is priced about the same as an FM35.