On Thu, 3 Nov 2022, Mike Katz via cctalk wrote:
GCR is Group Code Recording, used on the Apple II, Commodore 1541 drive and
Amiga (and others) use a different encoding scheme than the normal FM
(Frequency Modulation) or MFM (Modified Frequency Modulation) encoding
formats used by a majority of floppy disk controllers.
There is a very good description of floppy encoding formats here:
https://extrapages.de/archives/20190102-Floppy-notes.html
Amiga is not GCR; it is MFM, but without the IBM/WD sector/track
structure. WD 179x controllers can read tracks. It is too bad that NEC
chose to implement traack read as multiple sector read, rather than raw
track.
But, you are right about the others.
Apple still used GCR on the 400K/800K Mac format.
PROBABLY also on the Lisa Twiggy disks.
Victor/Sirius 9000
and many others, . . .
