Tape reel data recovery from MERA-400 polish computer

[Paul Koning]

> On Mar 10, 2017, at 1:39 PM, Al Kossow via cctalk <cctalk at classiccmp.org> wrote:
> 
> 
> The next extension is to track the tachometer values so that you can detect and compensate for tape stick/drag
> which is absolutely critical for formats that don't self-clock, like NRZI.

NRZI is not self clocking if you consider an individual track in isolation.  But it IS if you consider all the tracks at the same time, provided either (a) the data is recorded with odd parity, or (b) the all-zeroes data character isn't used.  (a) is the case for 9-track tapes.  7 track tapes may be even parity, but that case seems to apply by convention only to text data (not binary data) and there, (b) applies.  You do have to correct for track skew in this process, but that applies in any case even if you have an independent authoritative bit clock.

It clearly can help to have tach signals as a way to improve bit framing, but I don't see that it's mandatory.

	paul