On 6/6/2013 3:57 PM, ivan@ivanexpert.com wrote:
Can it be fairly said that, apart from "certification", SD floppies and DD floppies are the same thing, and that the terms "single-density" and "double-density" don't apply to Apple II 5.25" disks, because they refer to whether a drive encoded a disk with FM or MFM, whereas an Apple II drive encodes disks with GCR, which, in terms of data density, is in between?
Yes. SD and DD really distinguish between FM and MFM encoding techniques, and do not apply to Woz's methods of GCR encoding. 48TPI is the thing that all of these notions of "single/double density" share in common.
Was GCR chosen because MFM encoding wasn't a viable or economical option at the time, and it was a higher-density method than FM? And why not 40 tracks (which would have yielded 160K per side with DOS 3.3) instead of 35? Same reason?
Economy of chips in the Disk II subsystem, like pretty much all decisions that Woz made. It was, electronically speaking, radically simpler than contemporary Shugart-based designs.
Remember too that the Disk II design went through a couple of firmware revisions (13 vs. 16 sector) that implemented different GCR encoding techniques (5 and 3 vs. 6 and 2). We could probably call those "single density" and "double density," except they were not a doubling of capacity... ;-)