Re: Did Commodore cheat with the quad density floppies?

On Fri, Jan 4, 2019 at 8:19 PM <afachat@gmx.de> wrote:
>

> Thanks for pointing that out, that is my understanding as well. And thus my
> question, why the drives still work with 5900bpi media....
>
> Probably, as mentioned in another post here, quality of the media improved
> over time so more bpi were possible.
>
> > Again, I suggest reading the fine informations contained here:
> > http://www.retrotechnology.com/herbs_stuff/drive.html
>
> This does unfortunately not answer my question...

well well.... difficult questions need studying, I don't think in this
list there's any of the original 8x50 disk drives designers, otherwise
that was the quickest method to answer your question.
In that link, one can read a lot of things, and reason over the data
that don't directly answer your question, but probably can give some
clue to reach an educated guess.
I will point out a few facts (or at least, they seem hard enough facts to me).
300 oersted media was used on most 8-inch floppy disks and on all the
non-HD 5 1/4-inch floppy disks (SD/DD/QD 48 and 96tpi certified
media).
BPI on 300 oersted media varies between 8 and 5 1/4 inch drives, also
it's *always* quoted either for FM or MFM modulations, never for any
other, like GCR. It seems to me that it must depend
on the modulation/bit encoding and it's not a magnetic-media intrinsic limit.
8 inch drives have another interesting characteristic: they need to
reduce the write current when writing on the upper half (tracks 43-77)
of the media. 8-inch floppies are huge, linear velocity of the actual
media under the R/W head varies a lot between track 0 and track 77, so
(I'm assuming) using the same current when the linear velocity of the
media gets too slow (on inner tracks 43-77) produced probably too
large flux areas and needed to be reduced in order to maintain small
enough magnetic areas. If someone has an official explanation why the
8-inch drives needed to reduce the write current, I'd appreciate to
know it.

The limit obviously seem to depend on head dimension/construction AND
rotational speed, otherwise 8 inch drives (360 rpm, 48 tpi) would have
the same BPI figures as 5 1/4 inch drives (300 rpm, 48 tpi) but
they are different, and the limit is also dependent on the write
current, albeit in a rather difficult to estimate (without official
documents) way.

Other facts: 96tpi MFM formats merely doubled the number of tracks per
side, but sector count per track is the same, so MFM 40 tracks 48 TPI
has 360Kbytes formatted capacity and MFM 80 tracks 96 TPI goes to
720Kbytes (double side of course). This seems to me a bad choice, but
probably was forced because of a limitation perhaps of the standard
FDD controllers.
CBM engineers seem to have had a different idea here, but I start
guessing from now on: if the R/W head on a 100 tpi mechanism is
narrower (and of course it is in the "track" dimension), maybe it's
also thinner in the other dimension (or maybe they just verified it
really is smaller in the other direction). So why not trying to use a
higher flux transition frequency and see what happens with these 100
tpi mechanics regarding the bits per track on different zones?
The head difference clearly allowed much more higher flux transition
rate, otherwise they had used higher frequencies also on the 4040/1541
format. 4040 format also was slightly reduced as bytes per track on
one zone (as we all know) respect the original 2040 format, and I
guess it has been reduced probably because not all the 48 TPI drives
that CBM was using were reliable enough on that particular bit density
in that zone.
I guess (and it seems reasonable) that CBM engineers used the highest
amount of bytes per tracks in the various zones that allowed reliable
(by a large margin I'd say) operations. I can still read most of the
disks that were last written around 1989 or so.
If we agree that they found the optimal sector per tracks in the
various zone by probably experimenting (and the 2040/4040 format
difference seem to suggest that), then I don't see why the same
engineers would not try the same method using the 100 tpi mechanics:
see what they can do, maybe tweak a bit the head write current and/or
pulse shape and see what can be reliably done on the same 300 oersted
magnetic media, just with better mechanics and smaller heads.
I never had the luck to have a 100 tpi drive, so I don't know if the
disks written around 198x are still readable nowadays, but if the
failed disks have the same percentage of the ones written with 48 tpi
mechanics, then probably the reliability of the write format was as
solid as the one used on 48 tpi drives. If, on the other hand, 100
tpi-written disks are much less readable (and by a large amount, I'd
say) then we could conclude that CBM engineers went too far pushing
the limits of the 300 oersted media and the 100 tpi mechanics
combination.

Frank