On the Emulation of TU58s

[Johnny Billquist]

On 2015-06-16 20:34, Johnny Billquist wrote:
> On 2015-06-16 20:30, Henk Gooijen wrote:
>> -----Oorspronkelijk bericht----- From: Johnny Billquist Sent: Tuesday,
>> June 16, 2015 8:22 PM To: cctalk at classiccmp.org Subject: Re: On the
>> Emulation of TU58s
>> On 2015-06-16 20:20, Chuck Guzis wrote:
>>> On 06/16/2015 11:08 AM, Henk Gooijen wrote:
>>>
>>>> The round (approx 2" diameter) "bearings" of the TU80 do not rotate.
>>>> Instead, the metal has tiny holes. Air pressure pushes the tape away
>>>> from the bearings. As the tape moves, due to movement pressure, the
>>>> tape comes closer to the air bearing. That is measured. To get the
>>>> air pressure back to some sort of equilibrium the pickup or supply
>>>> reel speed is adjusted.
>>>> That is how I understand the mechanism ... now writing this down, I
>>>> think I should read up the Theory of Operation chapter ...
>>>
>>> Pretty much the standard CDC vacuum-column design since the 1960s.  The
>>> TU80 is, after all, a CDC product.  Two counter-rotating capstans--tape
>>> movement is determined by applying positive or negative pressure to
>>> ether or both (i.e. positive pressure on both results in the tape
>>> "floating"
>>
>> Uh... The TU80/TU81 do not have vacuum columns...
>>
>> Johnny
>>
>> -----
>> well, the tiny space between the tape and the air bearing could
>> be regarded a vacuum column, but it's a very short one :-)
>> The two air bearings are immediately above and below the R/W
>> and erase head. And they do *not* rotate. The tape glides on
>> an air cushion over the bearing.
>
> Hmm, I guess it could. But it is a very different construct compared to
> a vacuum column drive. In this case we're talking about overpressure to
> act as a cushion, and not underpressure to act as a takeup/buffer
> mechanism.

So I decided to read up on the TU81 operation, and it does work by 
creating an overpressure to act as a bearing before and after the head.
The overpressure is also used to detect tape tension so that a constant 
tension can be maintained.
The supply reel is controlled by the air pressure sensors, so that 
tension is maintained, while the pickup reel is directly controlling the 
speed. Both reels have tachos as well.
There is no capstan, but constant linear speed can be maintained by the 
fact that the takeup reel is of a known size, and you have the tachos, 
and you essentially can estimate good enough how much tape is on the 
takeup reel.
(Lots of computations in here... :-) )

	Johnny