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Author Topic: Ramping up projector speed to get more Sessions in... Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 12-02-2003 12:04 AM                     The sound at the projector should be a bit early to compensate for the distance from the screen to the audience, because the speed of light is rather faster than the speed of sound. If I'm 50 feet from the screen, the sound has leave the speakers a bit "early" to arrive at my ears when the light from the accompanying picture gets to my eyes. We can safely approximate that the light arrives instantaneously. The speed of sound is about 1000 ft/sec, so this is about 1/20 second for a 50 foot distance... the sound should be 1.2 frames advanced at 24fps. Speeding up the projector - which won't change the 1.2 frame sound advance - will reduce the time represented by that 1.2 frames. The sound will be that much earlier at my seat than it sould be. This will probably be imperceptible for any speed-up that I wouldn't notice otherwise. But speeding up the projector will be affecting the sound sync.  |  IP: Logged Warren Smyth Expert Film Handler Posts: 158 From: Auckland ,New Zealand Registered: Aug 2003 posted 12-02-2003 12:39 AM                     In the days of Western Electric universal bases, the power to the motors was controlled to maintain consistant voltage to ensure correct sound speed of 24fps. This was accomplished by a control unit for each motor which had a generator on the motor shaft. This provided feed back voltage by which the control unit was able to monitor and control output voltage to the motor, to mainain consistancy. The line voltage could vary, but these wonders of electronics for their day, would ensure that Jeanette Mcdonald and Nelson Eddie were right on key. Now these units had a switch where by the projectionist could switch from REG (regular) to VAR (variable). You've guessed it, by swiching to variable, control of the speed of the motor could be incremented by a potentiometer mounted at the top of the unit. I cannot say it was a wide spread practice, but I know that five minutes or so was carved off sessions at times so that patrons could catch their buses after the performance at night. These were the days when shorts were run on the first half and one auditorium would hold one or two thousand patrons. Incidentally these control units produced facinating results when a vacuum tube failed. The motor would race and the projectionist would dive across the room to switch into variable control to bring the speed back. The teeth on the large sprocket of the Ernemann provided a strobescope for speed. When the teeth appeared to move in reverse slowly the machine was at about 24fps. If one of the other set of tubes in the unit failed, the motor would run slow. I have often marvelled at the sophistication these control units at such an early period.I think they date back to the late twenties - at the start of sound  |  IP: Logged Leo Enticknap Film God Posts: 7474 From: Loma Linda, CA Registered: Jul 2000 posted 12-02-2003 01:34 AM                        Ken: I raised this very question (how does digital sound deal with slight variations in film transport speed) on another thread without any reply, and raised it with Darren over a beer on Saturday. The answer is that Dolby digital will tolerate a +/- 2fps variation in speed, and will reproduce the track by 'stretching' or 'squeezing' the duration of playback in the digital to analogue conversion, without affecting the pitch at all. If the speed goes above 26 or below 22, digital sound dies and drops to SR. In that case you would, of course, hear a change in pitch and I'd have thought you would hear the change very clearly, given that it would be the equivalent of a 12% speed change. I don't know what the margin of error is for DTS or SDDS, but I imagine it would be something similar.  |  IP: Logged Stephen Wilkinson Film Handler Posts: 17 From: Kincumber, NSW, Australia Registered: Oct 2002 posted 12-02-2003 07:14 AM                        From my reading, DTS can only increase to 25 f/s before reverting back to SR. It has something to do with the timecode being 30 f/s. As mentioned SRD can speed up to 26 f/s, anymore than this and it will drop out, reverting back to SR. From my tests when SRD runs at 26 f/s the pitch does increase, and of course the gaps between words are shorter but only because of the speed up, which you would get in analogue, not due to a higher data rate. SRD can still "WOW" like a tape machine. If you have a sound drum on a projector that has a bit of drag on it the SRD will "WOW" just as in analogue. (not as much, but will) Most NEW projectors have speed controllers, especially Kinotons, 24 f/s, 25 f/s and 30 f/s settings. Most that I have come across in servicing are set to 25 f/s! Generally due to the crazy session requirements put on exhibitors (independent ones) by the film distributors here in Australia! (but that's another story for a different forum thread) So if you've got a 3 phase motor on your projector it will probably have a speed controller. stephen  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 12-02-2003 07:56 AM                        Dave Macaulay wrote: quote: This will probably be imperceptible for any speed-up that I wouldn't notice otherwise. But speeding up the projector will be affecting the sound sync. Thanks. But the implication was that the sound sync was off enough to be noticeable if they ran "at the upper limit" for speed. A fraction of a frame sync difference is not significant.  |  IP: Logged

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