```Hello, J.D.,

I tried thinking this problem through a while ago and it hurt my brain.
You've heard from three experts who all said basically the same
thing--with which I concur.

What I thought I would add is a little thought experiment.

These machines did not have a capstan and they pulled the tape by a
motor attached to the takeup reel. For some reason it has been referred
to as "rim drive" perhaps because the motor often contacted a rubber
surface on the rim of the reel table to provide a "gearing" ratio from
the motor to the reel table, rather than being directly attached to the
spindle.

So, let's assume that we have a constant speed motor that can deliver
infinite torque (bear with my and hold your "tsk tsks"). If that were
the case, you could calculate the actual speed by knowing the radius of
the tape pack at time of recording, the speed of the motor and pi. As
the takeup reel filled up, the tape speed would increase, so if you're
listening on a capstan driven machine, the pitch would drop. Let's call
this calculation (A).

BUT, the motor does not have infinite torque. As the radius of the
takeup reel tape pack increases, the moment arm gets longer meaning that
the same amount of holdback tension may cause the motor to slow down a
bit as it can't deliver all the torque that would be required. So this
adjustment factor could be figured out, but the speed at the end of the
tape will fall short of what you calculated in (A) above. Let's call
this adjustment factor (B).

So, at this point, the speed at any given point is (A) - (B).

Now, some odd things enter in that will change this. As the (gasp zinc
carbon flashlight) batteries discharge, the motor will not be able to
deliver as much torque as the batteries will deliver less current and
their voltage will drop compared with fresh batteries, so we have a
third factor.

And if this unknowable was not enough, the holdback tension was provided
by pressure pads--usually against the heads--so this creates multiple
additional factors for affecting speed:
--the calendaring/polish of both tape surfaces will affect the friction.
--the environmental temperature and humidity will affect the friction.
--the cleanliness of the guides/heads will affect the friction.

So, there you have a quick snapshot of the dynamics of this system and
why fixing it in post in software is the best alternative.

I reiterate what Ted Kendall said, "there were myriad reel-drive
Japanese recorders around at the time, all of which were different
between samples, never mind designs." And I might add, there were
differences with the same recorder based on tape type, climate
conditions, and battery condition. A fair number of these recordings
originated in Vietnam in my experience, but thankfully a large number of
the ones I have received were recorded with capstan machines. the Craig
212 was a classic of the era with a capstan.

Cheers,

Richard

On 2019-07-12 3:53 p.m., Scott Phillips wrote:
> These would have been rim drive recorders, without a capstan drive roller. There is no fixed speed, it was determined by reel motor torque and the diameter of the amount of tape on either reel at any moment. Good luck with that, software is about your only available practical tool I know of..
>
>
> Best regards,
>
> Scott Phillips
>
>
> -----Original Message-----
> From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of J. D. Mack
> Sent: Friday, July 12, 2019 1:30 PM