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Author Topic: What is the acceptable tolerance from exactly 24 fps for motion pictures? Justin Hamaker Film God Posts: 2253 From: Lakeport, CA USA Registered: Jan 2004 posted 01-28-2017 11:32 PM                        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.  |  IP: Logged Frank Cox Film God Posts: 2234 From: Melville Saskatchewan Canada Registered: Apr 2011 posted 01-28-2017 11:53 PM                        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....  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-29-2017 12:13 PM                     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.  |  IP: Logged Chris Markiewicz Expert Film Handler Posts: 209 From: Glenaviegh, County Tipperary, Ireland Registered: Aug 2002 posted 01-29-2017 02:08 PM                     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.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-29-2017 02:44 PM                     Crystal motors on film cameras are typically accurate to .05% plus or minus one frame in a given length of film... Mark  |  IP: Logged Justin Hamaker Film God Posts: 2253 From: Lakeport, CA USA Registered: Jan 2004 posted 01-29-2017 04:36 PM                        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.  |  IP: Logged David Buckley Jedi Master Film Handler Posts: 525 From: Oxford, N. Canterbury, New Zealand Registered: Aug 2004 posted 01-29-2017 05:33 PM                        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.  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 01-29-2017 06:46 PM                     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.  |  IP: Logged Donald Brown Expert Film Handler Posts: 131 From: Lincoln, DE Registered: Sep 2009 posted 01-29-2017 08:52 PM                     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!  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-30-2017 07:20 PM                     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.  |  IP: Logged Bobby Henderson "Ask me about Trajan." Posts: 10973 From: Lawton, OK, USA Registered: Apr 2001 posted 01-30-2017 07:55 PM                     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.  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 01-30-2017 08:18 PM                        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.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 01-30-2017 11:50 PM                       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.  |  IP: Logged Simon Wyss Film Handler Posts: 80 From: Basel, BS, Switzerland Registered: Apr 2011 posted 01-31-2017 02:14 AM                     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.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 01-31-2017 12:22 PM                       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.  |  IP: Logged

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