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

[Eric Smith]

On Wed, Jan 27, 2016 at 9:49 AM, Chuck Guzis <cclist at sydex.com> wrote:
> That's exactly what I was asking.  But you implied that the RX02 was MMFM,
> which, in my experience is not the case.

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.

DEC's M2FM uses the same encoding as MFM except for the case of a run
of exactly four consecutive one bits.

data:  011110

MFM encoded:

        DCDCDCDCDCD
        00101010100

DEC M2FM encoded:

        DCDCDCDCDCD
        01000100010

DEC claimed that this was done because using normal MFM encoding, the
data field could contain a pattern that matches a preample fllowed by
an ID mark. I am dubious of that claim; I did an exhaustive search and
was unable to find any sequence of data bits which, MFM-encoded, would
match any of the RX02 mark patterns (standard 3740 patterns for index,
ID, single-density data, and single-density deleted data, and
non-standard patterns for double-density data and double-density
deleted data).