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Old 22 December 2013, 13:03   #22
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Join Date: Aug 2004
Posts: 2,481
Some floppy questions...

Would GCR support basically be the same as 4μs/bitcell support, at least as far as the lower-level emulation is concerned? (To support something like Apple II disk image files the higher-level code would need to GCR-encode the data.)

Do some copy-protections change between 2μs↔4μs mid-track?

For writing tracks which are all or partially 4μs bitcells, can that be achieved by writing the entire track at 2μs/bitcell, but "doubling" bits in the 4μs regions (i.e. if you want to write 10101 at 4us, actually write 1100110011 at 2μs)? For reading though, I'm guessing reading 4μs-written regions at 2μs is unreliable?

The Apple II disk format uses some kind of GCR encoding. disk2file in the Apple2000 package can read Apple II disks, so presumably reads at 4μs.

The Commodore 1541 drive uses GCR encoding. The 1541 package on Aminet can read C64/1541 disks (only part of the disk unless you reduce the drive RPM slightly). Disk-2-Disk by Central Coast Software (thread where I asked about that a few years ago) can (very slowly) read entire disks in an unmodified drive. It does something strange with the drive motor to achieve that.

Maybe someone can explain a little about precompensation? MFM vs GCR precompensation (the MFMPREC bit in ADKCON) and the various precompensation settings (none/140/280/560ns). I don't think trackdisk.device uses any precompensation?
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