Re: Roll Playback Speed
By Mark Fontana
I (Mark Fontana) originally asked:
> > Are the rolls generally encoded such that when played on a standard-sized
> > takeup spool, the tempo will remain as intended despite the effect of
> > othe increasing layers of paper accelerating the transport speed?
BJELE@telxon.mis.telxon.com (JELEN BILL) responded:
> ... I was perplexed until I read Zoltan
> Janosy's thesis. He noted that this increase of tempo is countered
> somewhat, "since the increasing mass (of paper on the take-up spool)
> slows down the low torque pneumatic winding motor." Although he noted
> that these forces won't exactly cancel each other out.
This is interesting-- I hadn't thought of that effect as being desirable-- it would seem far easier (and more accurate) to encode the master roll with the correct acceleration and require a steady playback speed.
But I've noticed this exact effect in my scanner. Until recently, I was using a 16mm projector motor- geared down majorly- to drive my spoolbox. There was so much torque that if the transmission stuck halfway, the delicate pins involved would easily be damaged. In fact, there was so much torque that the takeup spool was guaranteed to turn at the exact same rate, despite tension from a roll (or misplaced fingers...).
I recently replaced the drive assembly with a small 12V motor salvaged from one of those VHS videotape rewinders... a solid-state model train transformer/rectifier provides a power supply and means to control the speed.
This works, but this small motor doesn't provide nearly the torque of the projector motor. I found that the tempo would get irregular toward the ends of rolls (when more force is required to turn the payout spool against the tension the transmission provides, due to its decreasing diameter). It is probably THIS effect that counteracts the speedup we've been discussing; that is, it becomes harder to pull the paper off the payout spool, so this sort of counteracts the speed increase from the takeup spool's increasing diameter.
I guess these contraptions were never meant to be very exact... it's just hard to grasp this in the modern world, where precision in music and mechanical devices is taken for granted!
Mark
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(Message sent Wed 14 Jun 1995, 07:21:32 GMT, from time zone GMT-0400.) |
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