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

Registered: Aug 2004
Posts: 788
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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2017-11-28 05:27
ChristopherJam

Registered: Aug 2004
Posts: 788
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.
2018-03-06 17:43
Repose

Registered: Oct 2010
Posts: 138
I dreamed of the same ideas after reading 1541 Internals. There's also some gaps and extra bytes you can remove, as well you can probably read a track with one sync but random access isn't so good that way. Bear in mind that motor speed can vary a lot so you have to account for that, it's not that the media can't record it, it's that the higher density timings can having missing bits at the end of a byte due to the speed too high, I'd guess about half of 1/8 or 6% speed tolerance.

There's compatible with your drive and compatibility from other drives too, so you have to look at total variance.
2018-03-07 06:06
lft

Registered: Jul 2007
Posts: 353
Quoting ChristopherJam
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.


Technically only the BAM, the directory block(s), and at least one block for the first loader stage. The rest of track 18 can be high-density.
2018-03-07 07:56
ChristopherJam

Registered: Aug 2004
Posts: 788
Quoting lft
Technically only the BAM, the directory block(s), and at least one block for the first loader stage. The rest of track 18 can be high-density.


:-O
2018-03-07 08:11
tlr

Registered: Sep 2003
Posts: 1167
Quote: Quoting ChristopherJam
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.


Technically only the BAM, the directory block(s), and at least one block for the first loader stage. The rest of track 18 can be high-density.


Maybe the loader can be fitted in the directory block? Could it even be possible to reuse the BAM as directory too? What happens if it links to 18,0 instead of the regular 18,1?
2018-03-07 09:22
Count Zero

Registered: Jan 2003
Posts: 798
AFAIR there was a program published in the german magazine 64'er which formatted tracks 36+ and installed a loader into the directory track somewhere.
The loader was probably using some &-magic on the filename to execute directly in drive ram - can't remember that part exactly.
2018-03-07 09:43
lft

Registered: Jul 2007
Posts: 353
Actually, now that I think more about it, you could put the loader on one of the tracks that use the standard bit rate, e.g. track 1. That leaves only the BAM and directory on track 18, and tlr's intriguing question.
2018-03-07 18:52
Groepaz

Registered: Dec 2001
Posts: 8543
iirc on the 1541 the bam/dir link is simply ignored. directory cant leave track 18 either.
2018-03-07 20:26
tlr

Registered: Sep 2003
Posts: 1167
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
Groepaz

Registered: Dec 2001
Posts: 8543
of course, files can be anywhere
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