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Forums > C64 Coding > Optimal bit rates for 40 track disks
2017-11-27 20:51
ChristopherJam

Registered: Aug 2004
Posts: 1409
Optimal bit rates for 40 track disks

I've just been doing some bit density calculations, assuming an outer radius of 2.5inches, 48 tracks per inch, and 300rpm.

Any 40 track disk is going to be relying on the bit density on the innermost track still being ok, so it would make sense to use the same threshhold for the rest of the disk too.

So, a 40 track floppy has a bit density of 4717 bits per inch on track 40, assuming the lowest bit rate. That's only 5% higher than the maximum used by the standard format, and waaay under the media spec of 5870bpi. If we use a density threshold of 4740bpi to rezone all 40 tracks, we then get the following:

Zone 1 (21 sectors per track): tracks  1-21 (maximum density 4702bpi)
Zone 2 (19 sectors per track): tracks 22-28 (maximum density 4694bpi)
Zone 3 (18 sectors per track): tracks 29-35 (maximum density 4738bpi)
Zone 4 (17 sectors per track): tracks 36-40 (maximum density 4716bpi)
764 non-directory track sectors per side


Does anyone know if something like the numbers above were ever a thing? Or did all the 40 track productions leave the first 35 tracks at their original densities, and just extend zone 4 out to track 40?
 
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2018-03-07 20:26
tlr

Registered: Sep 2003
Posts: 1791
Quote: iirc on the 1541 the bam/dir link is simply ignored. directory cant leave track 18 either.

The link seems indeed to be ignored. Loading a file starting at sector 18,0 or 18,1 works in practice however.
2018-03-07 20:37
chatGPZ

Registered: Dec 2001
Posts: 11391
of course, files can be anywhere
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