--
Bill @ GarberStreet Enterprises
http://garberstreet.netfirms.com
So by limiting yourself to 35 track single-sided FM (4+4), I think you
might
be able to have a disk readable and writeable on both Apple and IBM 5.25
drives.
Just imagine the impact such a program would have had 20 years ago!
3.5 drives-I don't know. I was never really familiar with the workings of
Apples 3.5 drive. Clearly the IBM can't read Apple's 800k. But could the
Apple read a 720k IBM. I don't know. I thought the increase in data
density
on Apple's 3.5 drive was due to putting more blocks per track on the
outer
blocks, rather than the encoding format. But I seem to remember that the
IWM
was used in the 3.5 drive controller as well, so I'm not really sure.
You are correct about where the extra capacity comes from. But Apple
3.5" 800KB disks are still GCR, using essentially the same encoding as
the 5.25" disks.
In the "parent" of this thread, Eric Smith suggested that it might be
possible
for an Apple II to read an FM single-density disk by reading each track
two
or more times, and correlating the bit-shifted results to determine the
original
content. I replied with a proposal that might be able to do it in one
rotation
per track by sampling the controller's shift register three times per
"nibble".
I think that the Apple II could do this with 5.25" disks, but 3.5" disks
would
be too fast for it to work.
There would have to be some heuristics to determine correct nibble sync,
but it should be possible. Of course, a SSSD (90KB) disk is not the most
practical data interchange medium for either a PC or an Apple II. ;-)
-michael
Check out amazing quality 8-bit Apple sound on my
Home page: http://members.aol.com/MJMahon/
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