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