On the Emulation of TU58s

Johnny Billquist bqt at update.uu.se
Tue Jun 16 13:16:51 CDT 2015

On 2015-06-16 19:51, tony duell wrote:
>> And yet another (possibly the most common one on computers) is to have a
>> small drive wheel that pulls the tape at constant speed across the
>> heads, and then have some other construction that drives the tape reels
>> depending on tape tension or length. Think vacuum columns or spring
>> loaded arms.
> That is closely related to the capstan and weakly-driven take up spool I think.

I would disagree. There is no slipping clutch. Instead, the motors (on 
both sides) are driven by the uptake available. When there is too 
little, the motors run to give out some more tape. When there is too 
much, the motors run to pick up the excess.

So, motors can actually run in both directions, as needed. They are 
actually controlled by a pretty dumb closed loop system.

The actual tape movement as such, is all done by the small wheel next to 
the head, which just runs the tape past the head.

>> And then we have drives like the TU80/TU81 which do not seem to fit into
>> any of the mentioned categories. I'm not entirely sure how they work,
>> but I think they are similar to the DECtape in this sense.
> I don't know. I can think of several ways it could work....

I can also think of several. I just don't know for sure what it does.

> 1) There is a tacho on an arm that rests against the takeup spool (as in the Cipher F880). The control
> system drives the spools to keep that speed constant.

There can definitely be a tacho on the motor, but since you do not know 
how much tape is on the reel, this will actually not tell you the tape 
speed. But there is no tacho on the tape itself, unless my memory fails me.

> 2) Similar, but the tacho is a rotating tape guide that the tape goes round

See above.

> 3) It assumes the takeup spool is intially empty. It then drives forwards, and measures the
> speed of the supply spool. From that, you can work out the diameter of the supply spool, and
> thus I guess the amount of tape. You can then (a) determine where you are on the tape by the
> relative speed of the 2 spools, this gives you the amount of tape on the takeup spool and thus
> how fast you need to turn that to get the right speed.

That would be very hard, as you do not know the thickness of the tape, 
nor the diameter of the tape reel center.

I've been wondering if there is some sensor of tape tension/pressure at 
the heads, and this will allow the drive to figure out how much 
faster/slower the reels must run, relative to each other, in order to 
keep the tape tensioned. Then you can figure out tape speed across the 
heads (if you care) by just observing flux changes.

> One of my VHS video recorders does something similar to work out how much recording space is
> left on the cassette.

That can't be very precise... :-)


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