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» Film-Tech Forum   » Operations   » Film Handlers' Forum   » What is the acceptable tolerance from exactly 24 fps for motion pictures? (Page 1)

 
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Author Topic: What is the acceptable tolerance from exactly 24 fps for motion pictures?
Justin Hamaker
Film God

Posts: 2019
From: Lakeport, CA USA
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 - posted 01-28-2017 11:32 PM      Profile for Justin Hamaker   Author's Homepage   Email Justin Hamaker   Send New Private Message       Edit/Delete Post 
I was having a conversation today about frame rates and the differences between PAL and NTSC. It got me wondering about film projectors and whether the 24 fps was truly 24 frames per second.

Knowing that power output is not 100% constant or consistent, and that motors wear down over time, I assume there must be some acceptable tolerance. Since this would also affect the sound, I wonder how much variation from 24 fps is possible before it is noticed by the viewer.

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Frank Cox
Phenomenal Film Handler

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From: Melville Saskatchewan Canada
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 - posted 01-28-2017 11:53 PM      Profile for Frank Cox   Author's Homepage   Email Frank Cox   Send New Private Message       Edit/Delete Post 
Phase is constant, or electric clocks wouldn't be able to keep proper time. And you have a flywheel on the drive motor to make up for any momentary drops so I can't see why there would be anything different than 24fps.

But what I do know....

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Steve Guttag
We forgot the crackers Gromit!!!

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From: Annapolis, MD
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 - posted 01-29-2017 12:13 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
If they use synchronous motors, they will be as accurate as your line frequency (so they should be as accurate as most electric clocks). Asynchronous would depend on a many factors mostly relating to incoming power and load.

I'd have to look up what the SMPTE tolerance is on the 24fps speed is...my guess is that it would show up in one of the Audio Standards since sound/pitch is more affected by speed changes. Off hand, I'd say that it would be +/- 0.5fps but again, I'd have to check. I know that going to say 25fps has a noticeable pitch issue for 24fps material.

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Chris Markiewicz
Expert Film Handler

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From: Oakland, CA
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 - posted 01-29-2017 02:08 PM      Profile for Chris Markiewicz   Email Chris Markiewicz   Send New Private Message       Edit/Delete Post 
I'm not sure about the acceptable tolerance as such, but I read long ago that the Dolby readers had a certain frame-rate tolerance within which they would correctly read the data stream. Not sure about DTS or SDDS.
I got curious about our old drive-in Centurys years back and put a 1K test film through some of them. With the pulleys they had I was reading about 930 Hz.

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Mark Gulbrandsen
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From: Bountiful, Utah
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 - posted 01-29-2017 02:44 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
Crystal motors on film cameras are typically accurate to .05% plus or minus one frame in a given length of film...

Mark

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Justin Hamaker
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From: Lakeport, CA USA
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 - posted 01-29-2017 04:36 PM      Profile for Justin Hamaker   Author's Homepage   Email Justin Hamaker   Send New Private Message       Edit/Delete Post 
quote: Steve Guttag
I'd say that it would be +/- 0.5fps but again
What's interesting is +/- 0.5fps would mean a difference of 1800 frames an hour - or 75 seconds at 24fps.

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David Buckley
Jedi Master Film Handler

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From: Oxford, N. Canterbury, New Zealand
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 - posted 01-29-2017 05:33 PM      Profile for David Buckley   Author's Homepage   Email David Buckley   Send New Private Message       Edit/Delete Post 
Induction motors follow the frequency of the grid, and the grid is kept "close" to the nominal frequency at all times. However, grid frequency does vary a bit. So the grid, short term, isn't actually very stable. But as noted above, because clocks have to be right, the long term grid stability is amazing. The grid is corrected each day to bring clocks back to the right time.

Crystals work the other way up; short term, they are quite stable, but their long term accuracy is poor. One can use the mains to figure out how sharp or flat from it's stated frequency the crystal is.

Tone-wheel Hammond organs are also locked to the mains frequency, and in normal use, folks don't complain about them going out of tune.

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Richard Fowler
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From: Ft. Lauderdale, FL, USA
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 - posted 01-29-2017 06:46 PM      Profile for Richard Fowler   Email Richard Fowler   Send New Private Message       Edit/Delete Post 
I did a service call to several cinemas in Panama that where having problems with DTS. I found the Century projectors where running at 26+ fps due to wrong drive pulley parts. I have done many premieres of content shot at 25 fps which was a hassle during 35mm due to venue not having speed change options but not an issue with DCP projection.

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Donald Brown
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 - posted 01-29-2017 08:52 PM      Profile for Donald Brown   Email Donald Brown   Send New Private Message       Edit/Delete Post 
When I entered the field of exhibition with Interstate Theatres, the circuit had two theatres on Cape Cod that were still operating with booth equipment from the '30s: the Port Cinema in Harwichport and the Chatham Theatre in Chatham. Both of these had a set of four legged pedestals in the booth that included a mount for a variable speed drive motor. While the picture heads had been replaced, the sound heads were the originals, installed when the theatres were first equipped.
Joe Cook, the circuit's chief projectionist and a veteran from the days of Powers hand cranked machines, explained the utility of variable speed drives with a wry smile, indicating that long slow moving shows could be speeded up!

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Mark Gulbrandsen
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 - posted 01-30-2017 07:20 PM      Profile for Mark Gulbrandsen   Email Mark Gulbrandsen   Send New Private Message       Edit/Delete Post 
quote: David Buckley
Crystals work the other way up; short term, they are quite stable, but their long term accuracy is poor.
Actually no true in modern film cameras as they are TCXO type crystals that are extremely stable. Plus or minus a few PPM typically. The crystal oven is always on as long as power is applied to the camera. While this is mainly for colder temperatures the TCXO is specified for the right high temperature will maintain it's accuracy over a very wide temperature range.

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Bobby Henderson
"Ask me about Trajan."

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 - posted 01-30-2017 07:55 PM      Profile for Bobby Henderson   Email Bobby Henderson   Send New Private Message       Edit/Delete Post 
quote: Chris Markiewicz
I'm not sure about the acceptable tolerance as such, but I read long ago that the Dolby readers had a certain frame-rate tolerance within which they would correctly read the data stream. Not sure about DTS or SDDS.
That sounds more like the number of bad frames that can be skipped before a fault occurs, dropping the audio out to the back-up optical analog track.

Dolby Digital could handle up to 4 frames of bad data before dropping to analog. SDDS could (in theory) miss more frames due to the back-up digital track with data staggered different from the primary digital track. On the other hand the printed data spots of the SDDS sound track were so tiny in comparison to Dolby Digital they could be a source of problems on their own. DTS could miss up to 48 frames of bad time code before failing to analog.

These digital sound formats do not read and play in the same manner as an analog track, where if you increased the fps speed it would increase the pitch and speed of the audio. The digital data is packeted. It has to be read and decoded in chunks and then played according to that data, which is set in a fixed time domain. If the system is running properly then those data packets are played back seamlessly. If the projector is running too slow or too fast the audio playback will be choppy and frequently fault to analog, if it even runs at all.

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Scott Norwood
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 - posted 01-30-2017 08:18 PM      Profile for Scott Norwood   Author's Homepage   Email Scott Norwood   Send New Private Message       Edit/Delete Post 
If I remember correctly, SRD works fine at 25fps. I have never tried it at 30fps or at slower speeds.

As for crystal-controlled cameras, I believe that the specification for the Nagra crystal recorder is +/- 1/4 frame per 10 minutes. I have shot full 400' rolls of 16mm film with Arriflex cameras and Nagras, DATs, and Sound Devices hard-disk recorders, and can confirm that all of them will hold sync perfectly well over a ten minute interview.

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Sam D. Chavez
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From: Martinez, CA USA
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 - posted 01-30-2017 11:50 PM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
SR.D will run 11% fast and 7% slow or the other way around. There were plenty of slow running Century with aluminum drive pulleys. I measured one at 21FPS.

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Simon Wyss
Film Handler

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From: Basel, BS, Switzerland
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 - posted 01-31-2017 02:14 AM      Profile for Simon Wyss   Email Simon Wyss   Send New Private Message       Edit/Delete Post 
An old subject

There is no international standard that defines 35-mm. film projection tempo.

We have ISO 23 that specifies 24 frames per second and tolerates 25 for 35-mm. film (camera usage). ISO 25 says 18 or 24 f. p. s. for 16-mm. film, 25 tolerated (camera usage). ISO 26 (projector usage of 16-mm. film) says 16 f. p. s. minus 0,5 plus 2,0 and or 18 ± 1 and 24 ± 1.

With motors running by the grid AC frequency you have about ± 0.1 Hz short term variation but in the long it’s very precisely 60 Hz or 50 Hz.

Personally, I find it ridiculous that projectors are not run with crystal accuracy since the introduction of crystal control for camera motors. On the other side there’s nothing more exciting to me than theatre projectors hand cranked, for silents, of course.

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Sam D. Chavez
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From: Martinez, CA USA
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 - posted 01-31-2017 12:22 PM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
Crystal control? That's a hoot. American projectors were less sophisticated than a manual washing machine, especially the motors. The only precise mechanism is the intermittent movement and even that leaked like a British motorcycle.

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