> On Jun 16, 2015, at 3:48 PM, Eric Smith <space...@gmail.com> wrote:
>
> Johnny Billquist wrote:
>> Then you can figure out tape speed across the
>> heads (if you care) by just observing flux changes.
>
> Tony Duell wrote:
>> Only if there is something on the tape. These computer tape drives could
>> surely record on a
>> totally blank tape and get the right number of bits per inch. So the thing
>> can't use the data rate
>> at the head as a speed measurement.
>
> Not for something that can write a blank tape (e.g. a 7-track or
> 9-track), but there were other tape systems that required a formatted
> tape, and servoed the reel motors to the flux transition rate.
> Obviously the factory had to use a drive with a different tape speed
> control system to format the tape.
DECtape has a timing track, but that can be written on site and simply runs the
tape takeup motor at its design speed, as far as I can tell. The reel
dimensions of DECtape ensure that the speed doesn’t change all that much (min :
max diameter ratio is rather modest).
paul
