On 03/10/2017 12:58 PM, Paul Koning via cctalk wrote:
On Mar 10, 2017, at 1:39 PM, Al Kossow via cctalk <cctalk@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.
Yes, but the problem is 800 BPI is about the upper limit on
density due to tape skew. I don't know for sure, but I
strongly suspect that skew gets worse as the tape gets
older. I've seen some tapes that were only a few years old
where the skew got very bad. You could put a scope on the
skew alignment test point and see the skew pattern pulsing
as the supply reel rotated, meaning the tape got
differentially stretched while sitting on the reel.
But, glad they were so successful in their effort!
Jon
