On the Emulation of TU58s

tony duell ard at p850ug1.demon.co.uk
Tue Jun 16 11:10:57 CDT 2015

> As long as we're talking philosophy, what do y'all think about emulating the TU58 drive, vs. emulating the TU58 > *tape*?
> I cannot properly express my opinion of that tape cartridge design even if I violate list rules about use of 
> profanity. But the drive itself isn't all that bad, aside from not having enough motors to manipulate a 
> well-designed tape cartridge mechanism.

The tape cartridge (along with QIC cartridges, etc) is an interesting design. Of course the general problem is 
to run the tape at constant linear speed past the head while winding it off one reel onto the other. In general
there have been 3 solutions to this.

One is to drive the tape itself using a capstan and maybe a pinch roller. Then have the takeup spool
driven too fast with a slipping clutch in-line. And a weak brake on the supply spool. This of course is what
was done in audio recorders (reel to reel and cassette), Video recorders (ditto), etc.

Another is to simply drive the takeup spool but to measure the speed of the outside of the tape wound on
it. thus keeping a constant tape speed. I had forgotten this one until I realised that's how the Cipher F880 
works. Or maybe to run the tape round a rotating guide that has a tacho disk on it. I've not seen that used,
but I bet it was somewhere.

The last is to drive the outside of the tape wound on the spools. Of course this keeps constant tape speed.
Mechanically it is by far the simplest. All you need is one motor driving a belt that is tensioned against the 
outside of the tape spools. This is how the TU58, etc, work. The problem is due to the physical properties
of the tape and belt, which leads to tape wear and damage and a short life.
There are at least 2 types of TU58 cartridge (DEC called them 'metal base' and 'plastic base' I think. Is
one any better than the other?

Incidentally, given the fact that a constant motor speed -> constant tape speed, it should be possible to
make a device to put the timing track on a blank tape for the TU58. Has anyone done that?

> But what about emulating the tape cartridge, instead? Imagine a gizmo in the form factor of a TU58 cartridge, 
> containing a wheel for the capstan roller to engage, but connected to an encoder instead of the ******* 
> ************ ***** ** **** belt drive of an original cartridge? Where the tape would normally be exposed, there is 

Now that I want to see!

Even a module containing coupling head and encoder disk that slots into a TU58 drive and which links
to an external box of electronics containing the SD card, etc, would be interesting.

You would, of course, not know which track it was reading, so you would have to output 2 blocks, one on
each track, at once. And how would you detect it was writing? Look for an extra signal at the coupling head
or something?


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