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OT, Apple II . Re: Cassette to MP3 Converter

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  • jdripper
    ... 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
    Message 1 of 13 , Jul 15, 2013
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      --- In cosmacelf@yahoogroups.com, "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.

      Eric
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