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Forums > C64 Coding > Optimal bit rates for 40 track disks
2017-11-27 21:51

Registered: Aug 2004
Posts: 731
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?
2017-11-27 23:21

Registered: Mar 2009
Posts: 1469
I don't know if this helps but 40-track formats are described here in the d64.txt in the zip: https://ist.uwaterloo.ca/~schepers/DOWNLOAD/FORMATS.ZIP

Also here (DolphinDos etc): http://unusedino.de/ec64/technical/formats/d64.html
2017-11-27 23:36

Registered: Dec 2001
Posts: 8367
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?

slightly "higher than normal" bitrates have been used on all tracks for protection purposes, no idea if on the higher tracks in particular, but wouldnt surprise me at all.

the "normal" formats (speeddos etc) of course simply extend zone 4, since thats the only thing a regular drive could write.
2017-11-28 06:27

Registered: Aug 2004
Posts: 731
Yes, I'd noticed that *.d64 just extends the track count without doing any rezoning. Makes sense that it would be less work for DOS extensions, too. Still wasteful ;)

Hmm. I've just realised that you'd need to keep track 18 the standard density if you wanted to be able to load a loader with the built in routines. You'd still get the same extra 100 blocks from the track extension and rezoning the others, at least.

Thanks all.
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