On the Emulation of TU58s

[tony duell]

> 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.

> 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....

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.

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

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. 

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

-tony