Tape reel data recovery from MERA-400 polish computer

Jon Elson elson at pico-systems.com
Fri Mar 10 20:42:32 CST 2017

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 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.
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!


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