Substituting DSHD for DSDD disks (or DS2D if you prefer)
cclist at sydex.com
Wed Jan 27 17:06:05 CST 2016
On 01/27/2016 02:07 PM, Eric Smith wrote:
> The double-density RX02 data fields are in a modifed MFM, which is
> what M2FM or MMFM stands for. It doesn't use the same encoding
> rules as are most commonly used for M2FM, though.
...and that was exactly my objection. This becomes a case of the Monty
Python "Basingstoke in Westphalia" and "our Cole Porter". The RX02 MFM
(and I believe I touched on the rationale) may be "modified MFM", but
it's not modified in the standard (1,4) RLL encoding that the rest of
the world uses/used. I can conceivably change a couple of encodings for
FM, but that doesn't make it MFM. For example, not allowing 4
consecutive clock and data 1 bits in a row (I could, for example, leave
out a clock bit.). Come to think of it, that's exactly what standard
IBM 3740 address marks do, but nobody calls that "MFM".
If you insist that the 3740 format *defines* FM, then you're leaving out
a lot of recording formats that use (0,1) RLL, but not 3740 conventions.
Honestly, you DEC guys need to get out more.
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