Tracy Johnson wrote:

> I'm reminded once an Engineer told me a 1600bpi "only" drive could
> theoretically read 800bpi written tapes.  The procedure was to force
the
> machine to only read every other bit.  All one had to do was discard
the
> checksum at the end.

That's a good one!  :-)

As I recall, the drives always used NRZI (Non-Return to Zero, Inverted)
encoding for 800 bpi, and PE (Phase Encoding) for 1600 bpi.  With NRZI,
tapes were written with odd parity, so that each frame would have at
least one 1-bit.  (Otherwise, a string of NUL characters would look like
a section of unrecorded tape.)  With PE, I think even parity was
generally used.  

I'm skeptical that a drive designed for PE would be able to make any
sense of an NRZI-encoded tape.

Walter  

Walter J. Murray

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