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
> To: [log in to unmask]
> Subject: [ARSCLIST] Reel-to-Reel tape problem
> 
> I'm looking for some advice/info.  I sometimes transfer reel-to-reel tapes to CD or digital files for my customers.  Frequently, I receive 3 inch tapes from the 1960s that start at one speed and gradually speed up or slow down substantially as the tape plays. The speed range is usually between 1 7/8 and 3 3/4, but never landing on either speed.  I can correct for this using Adobe Audition's gliding stretch, but it takes quite a bit of trial and error.  What sort of tape player would I need to hunt down to play these tapes correctly without having to resort to a software solution?  My customers never have any idea what brand and model was used to make the recordings.
> 

-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.