OT, Apple II . Re: Cassette to MP3 Converter
- --- In firstname.lastname@example.org, "joshbensadon" <joshbensadon@...> wrote:
> I've already read "Beneath Apple DOS". Very good book on the floppy disk software, but falls a little short in the ordering of the 5/3 nibbilizer.And _Beneath Apple DOS_ is flat-out wrong when they claim that there are clock bits between the bits of the nibble. The whole point of using GCR is that it is self-clocking, so it doesn't need to throw away half the bandwidth on clock bits the way that normal single-density (FM) encoding does. That's why Apple gets 13 or 16 256-byte sectors per track where competing systems using single density only got 10 256-byte sectors.
The 4+4 nibblization of the address mark contents is FM, a degenerate case of GCR.
Normal double-density (MFM) encoding is even more efficient that Apple's GCR, for a given channel bandwidth, but requires more hardware than the Apple design, and more precise discrimination of the time between flux changes. To be any good, MFM requires a phase-locked-loop for recovery. The state machine of the Apple floppy controller includes a crude state machine, but not good enough for reliable MFM.
For many years, hard drives usually used (1,7) or (2,7) RLL, which is a form of GCR that is even more efficient than MFM, but requires an even better data separator. Now most hard drives use even more complex coding than that.