Measuring Frame Rate of a Projector?

[Tony Ferrara]

From the archives:
https://8mmforum.film-tech.com/cgi-b...=009387#000022

The method I have used is very simple. I believe it is valid. With or without film load on the projector, place a small piece of masking tape near the circumference of one of the visible sprockets. Count how many teeth the sprocket has. Using your smartphone stopwatch, count the number of rotations the sprocket makes by looking at the tape marker for at least a minute or 2.
Frame rate =( (# of rotations) x (number of teeth in the sprocket)) / (the # of seconds of the duration you used to count the rotations)

Does this make sense, or am I missing something?

[Nantawat Kittiwarakul]

Short answer - definitely yes.

Or you can go even a bit nerdier by counting exact no. of turns, and time it instead. What you'll get is the exact time it takes for certain amount of rotation (therefore certain amount of frames), then work it back to actual frames for certain given time - fps in this case. A tad more calculation needed, but possibly more precise measurement.

I once went as far as comparing the projector's speed vs mains frequency, 50Hz in my case. The mechanism would appear as slowly creeping backwards (supposedly 24 vs 50 Hz, sort of) in certain lighting condition. I timed that "drift" then done a heck of calculation to get final fps. Pretty precise, but it wouldn't give me "on the fly" readout. I have to do the measurement, making adjustment, another measurement, then readjust, and so on. So yes, very time consuming.

Finally I gave myself a digital tachometer, and problem totally solved.

[Joseph Banfield]

I think with a belt driven projector you can be in the ballpark with accurately measuring speed but there will never be an accurate point where you can say this is it because there are too many variables to get a precise measurement. If you measure speed with a film fully loaded from supply reel to takeup you will get a different reading when measuring the speed with no film loaded at all. The takeup reel's slip clutch, the pressure pads pushing on the sound heads, the pressure in the film gate always introduces a slight drag when running. Also a cold machine should give a slightly different reading than one that has been running for 30 minutes or so.

With belt driven machines or those like the Eumigs that use drive discs there really is no way to determine precise speed because it's always varying very slightly. If they didn't vary then it would be simple indeed to record a perfectly synced soundtrack from beginning to end without the sound ever going out of sync and that doesn't happen in the real world without the use of special equipment that adjusts for slight speed changes during recordings.

An average speed is really all you can achieve and measure.

[Tony Ferrara]

Thanks. I agree with both of you. I think I wasn't clear in my wording, though.

Nantawat,

What you describe in your second paragraph is exactly what I meant. Count rotations; time with stopwatch.
Average Frame rate =( (# of rotations) x (number of teeth in the sprocket)) / (the # of seconds of the duration you used to count the rotations)

Obviously, the longer the duration, the better the accuracy of the measurement. I found that 1 minute or 2 is enough to determine how close you are to 24fps. I usually play a film with voices are music I know well to make sure the pitch sounds about right.

Joseph,

I meant this to be a reply to the post in the archived forum. Totally agree that the projector speeds are inconsistent. That being said, I believe my method will give you the same measurement results as using an optical digital tachometer. (I am not familiar with that technology.) The slippage of belts or discs should be reflected exactly the same way in the shutter rotations as in the sprocket rotations in most projectors, right? After the rubber/friction stage, gears sync the shutter rotation to the sprocket rotation. I am just using the KISS principle. A small piece of making tape, and a stopwatch that everyone probably has already (or even the "seconds" hand on an analog watch), is cheaper than buying a tachometer.

It is definitely an excercise in futility to get rubber driven projectors to run at exactly 24fps. But if it goes to 22fps or 26fps, the human ear would probably detect it. In reality, when installing a new belt on a machine with potentiometers, I don't even bother with the method I described. I just play a film and use my ears to determine if the pitch is too high or too low.

Do any of the really high-end projectors not use any rubber? I'm talking the Bauers, Fumeos, and Beauliaeuxs, etc. I have never seen any of those in person, but I know they cost big bucks. My highest-end proejctor is the Elmo ST-1200 HD, which uses a rubber belt to drive a rubber wheel to drive a metal shutter (unless you have rubber fitted around the shutter.) Never understood that design considering the lower capacity projectors in the Elmo ST series don't use that exact method (rubber wheel driving shutter), nor does the GS1200, I believe. Did Rube Goldberg work for Elmo?

[Osi Osgood]

I would think that a gear driven projector would end up being more accurate from for steady fps over time, as belts will inevitably lose tension over the years.

[Tony Ferrara]

But we can replace the belts.
I have a vintage Thorens turntable that uses belts. I bought it used about 10 years ago and have never changed the belts. It still sounds fine to me. At the time, I was debating whether or not to get a direct drive turtable, but read that belt driven don't contribute as much rumble as direct drives do. BTW, I have used a similar method to see if it is running close to 33 1/3 rpm. You don't even need masking tape. Look at the record label, count the rotations with a stopwatch handy. Simple yet effective.

[Nantawat Kittiwarakul]

After I bought myself that digital tach., only to find out that the actual projector speed can be quite off. However as long as the speed is CONSISTENT (always 24.5 fps no matter what, for example), it wouldn't be an issue in any way.

[Tony Ferrara]

Yes, but unless you have a direct drive machine (but I am not sure which machines have that), your speed will not be consistent. I have never had an issue, other than it being consistently slow or fast. I let my Elmo ST-1200HD run for 5 to 10 minutes in forward projection mode if more than a day has passed since I last used it. But that machine has the Rube Goldberg design. My Yashica 820 runs fine from the get-go, even after months of non-use.

You would think that with digital movies you wouldn't have issues, but I usually change the speed via VLC since sometimes there is a PAL speed-up to 25fps, even on some NTSC DVDs! You can even elect to change the pitch or not when changing speed. You need to know if they already changed the pitch though. Those darned M2K Chaplin DVDs I bought about 15 years ago, although mostly silent, have both PAL speed-up and ghosting. I have even played around with eliminating the ghosting with VLC, with mixed results, since everyone has a different idea of what setting to use. The thing is I have to play them via my laptop rather than my DVD player. So digital, whether files, streaming, or DVDs/Blu-Ray discs is still not a panacea without it's own problems.

Phil Johnson, on his 8mm and 16mm website, claims that TCM speeds up movies by 10%. I find that hard to believe since I have not noticed it. They certainly must use pitch correction if they do, since Bogart, Gable, and Grant don't sound like chipmunks. But with that type of speedup it would look like a Mack Sennett comedy.

At the end of the day, I like to watch silent movies on my variable speed projectors. At least I can change the speed in real-time, unlike VLC where you have to set the speed, close VLC, then re-start it for the speed change to take effect. New technology fixes some older problems, but introduces new ones. And don't get me started about digital cleanup! Film grain is your friend. Again, a topic for another day and another thread.

[Maurice Leakey]

Can I draw attention to an RPM checker used and sold by Van Eck.

RPM Check DT-2234C+ (measuring speed of projector) – Van Eck Video Services (van-eck.net)

RPM check - measuring speed device for filmprojector (e.g. for film transfer) - YouTube

[Phil Murat]

Hi Tony,

I have worked a lot on projectors speeds (Only Beaulieu and GS1200) . Both machines are very stable and and offers an accurate speed with 2 numbers after dot.
GS1200 and Beaulieu have standard belts (not timing belts). a light belts slippage is not a big problem in major case as regulation system responds imediately. Regulation is achieved with a sensor on main shaft or in film path (Beaulieu) associated to tacho-genrator(s).
There are different methods to adjust and control speeds (Pocket Tester setted on Frequency or "Special Low Frequency" frequencemeter device). It is a bit more difficult to measure very low frequencies (like 3 HZ) than high frequencies (20Mhz for exemple).
At this time I am working on an Arduino device which has been designed by a friend on a French Forum, the prototype is working pretty good with a high accuracy . Moreover, using Arduino , we expect results can be picked up and sent to a computer which displays frequency curves which allows to catch some fluctuations or some unwanted picks.In that case an I/R sensor is installed close to shutter blades . the device can be setted fo 1,2,3,4 blades.

GS1200 and Beaulieu (and some other modern projectors) are fitted with Tacho generators or sloted disks with I/R sensors which send informations to an I/C or a control circuit.Fot this application it is not necessary to introduce a Cristal in the circuit to get a reference , some I/C are designed to work in "Close Loop".

[Tony Ferrara]

Interesting stuff. But just for giggles -- let's modify my method to be described as follows:

Count the number of rotations of the sprocket for a given time frame (let's call it the "duration").

The number of frames projected during the chosen duration = (# of rotations) x (number of teeth in the sprocket)

Will any of those tools give a different answer than my simple method?

Divide by the duration (in seconds) if you want to get an average fps. The longer the duration, the more accurate the measurement. The key here is the accuracy of the "measurement"; not the accuracy of the projector maintaining a consistent speed for the duration. Just needed to stress that. So that fps will vary over the duration. The calculation will be the average fps for the duration. Hope I am being clear. Again, even with belt/rubber slippage anywhere in the projector, those more sophisticated tools will produce an accurate measurement of a potential inaccurate projector (meaning not maintaining a consistent fps.) I don't believe, unless the gears are broken, that the rotation of the sprocket would be different than the rotation of the shutter. If I am wrong, please correct me and explain. Whewhhh!!!!!


Maybe that Van-Eck device will be quicker and easier to use in real-time, while my method will take longer. Just me, but I like simple, inexpensive solutions. Like I said, I use my ear more than anything else. For silent films, I don't really care. If it has variable speed control, I adjust while watching until I think the speed is good for the type of film I am watching. In the silent days, there were no standards. You could go to the same theater multiple times, and if the projectionist and/or manager for the shift was different, you would see the film at different speeds. It's a big can of worms -- just saying.

The only time I did a simple telecine by pointing my camcorder at a projected screen image, the results were very satisfactory for my purposes -- to share about 3 to 5 minutes of home movies with some family over the net. I used a Bolex 18-5, set at 18. I eliminated flicker by adjusting the camcorder, not the projector. I forgot what the setting was -- some white balance adjustment or something. For a quick job that I didn't even plan out, and spent about 10-15 minutes doing including setup, I think it came out great. Not sure why people want to adjust the projector to eliminate flicker. Your family will be moving either too fast or too slow. But don't want to debate that here -- just saying. Another can of worms.