Substituting DSHD for DSDD disks (or DS2D if you prefer)

[Chuck Guzis]

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.

--Chuck