Re: Did Commodore cheat with the quad density floppies?

Den Thu, 3 Jan 2019 15:15:43 +0100 skrev Francesco Messineo
<francesco.messineo@gmail.com>:
> On Thu, Jan 3, 2019 at 2:51 PM Mia Magnusson <mia@plea.se> wrote:
> >
> > Den Wed, 02 Jan 2019 15:34:24 +0100 skrev André Fachat
> > <afachat@gmx.de>:
> > > Hi there,
> > >
> > > I was looking at floppy disk recording schemes and I am wondering
> > > if the 8050/8250/1001 floppy disk format with over 500kB per side
> > > was actually out of spec of even the Quad Density disks?
> > >
> > > The recording frequency was increased from 250kHz to 375kHz (×
> > > 1.5, for the innermost i.e. most critical track/speed zone). That
> > > resulted in a much increased number of bits per inch. See here:
> > > https://extrapages.de/archives/20190102-Floppy-notes.html
> > >
> > > What do you think?
> >
> > Afaik the 8050/8250/1001 drives are supposed to use "QD" disks,
> > which seems to be a format that's supposed to handle a higher
> > density than DD.
> 
> "QD" disks have the very same 300 oersted magnetic media as SD/DD
> disks, it was only the mechanics and R/W heads that allowed to use
> more effectively the storage media.
> Later HD drives used different R/W heads (or different currents) and
> required 600 oersted media, doubled bitrate to 500 Kbps and changed
> speed to 360 rpm.
> 
> At the university, we had a few Olivetti LSX-3005 that were equipped
> with 96tpi 5 1/4" drives (QD, not HD obviously),
> I remember nobody ever tried to find "QD" disks, normal DD 48TPI disks
> were used (albeit I remember good quality brands were purchased
> usually, like 3M, Olivetti).

As I recall, some 48 TPI disks actually caused problems when used as 96
TPI.

A qualified guess is that once 96 TPI DD disks became rather common,
they just made that kind of disks and labeled some of them as 96 and
some as 48 TPI for market / pricing purposes.

Later when the market settled for 40 TPI DD and 96 TPI HD the
manufacturers could well have switched back to media that only support
48 TPI, if there were ever any kind of issue with track-to-track crosstalk.

> > It seems common for people to think that QD was a marketing thing
> > used for 96TPI DD disks, but I've seen so many 96TPI disks marked
> > DD and
> 
> again, 96TPI DD is just the same media as 48TPI DD, just maybe tested
> better (or just advertised as 96TPI, who knows).

Well, with narrower tracks, the signal to noise ratio will be worse
with all other parameters the same, so 96 TPI disks might actually
differ from some 48 TPI disks.

> > only a few (like one or two, and it was last summer that I first saw
> > them) disks actually labeled QD. (They contain a book keeping
> > software package, in Swedish, from the Swedish Commodore importer
> > Datatronic. Will be preserved as soon as I get my 8050 up and
> > running, which has been waiting a while for me to find my stash of
> > IEEE cables :) )).
> >
> > It would be really strange if floppy media didn't evolve the same
> > way as magnetic tapes did. With media good enough for 250kHz at
> > track 35 when the 5.25" floppys were new, and soon good enough for
> > 250kHz at track 40, it seems reasonable that some years later the
> > media used for those drives were actually good enough for 375kHz at
> > the 48TPI equalient of track 35, which almost is where the highest
> > track number on a 77 track 100 TPI drive will end up.
> 
> I really think the better Kbps rating is due to the use of the more
> efficient GCR code instead of the MFM.

Why wouldn't Commodore had used that in the
2040/3040/4040/2031/2031LP/1540/1541/1551/1570/1571 drives too then?

> > (At some point in time a market for cheap rather crappy disks seems
> > to have evolved though, but those were probably anyway nothing
> > people used in their 8050/8250/1001 drives).
> >
> > (Everyone who's been around long enough to remember cassette tapes
> > from the 70's and the 80's remember that before tapes like Maxell
> > UD and similar the standard / ferro / type I tapes did really sound
> > crap with a high noise level and muffled treble. Then something
> > happened in the late 70's and early 80's, resulting in more and
> > more kinds of tapes getting a lot better, and at the start of the
> > 90's basically almost all tapes had a decent sound even though
> > there were of course still differences between them).
> 
> There're two different issues on compact cassette:
> 1) different (really much different) magnetic media, type I (Fe2O3,
> iron oxide), Type II (CrO2), type III (FeCr), type IV (metal), these
> media required different equalization and different recording
> currents. Type I are usually very bad sounding and noisy, type IV have
> the best quality, but the recorder really NEEDS to know what type of
> tape it's trying to record into, otherwise you wouldn't get much
> better results, unless maybe a bit less noise if you use a type IV
> tape on a old, low quality recorder.
> 2) Dolby NR pre/de-emphasys. These noise reduction techniques have
> been introduced starting from 1965, last one afaik was introduced in
> 1986. Not all recorder were equipped with these circuits. A type I
> with the best NR circuit could sound really better than a type II with
> no Dolby.
> So, all in all, nothing in common with floppy disks :)

In practice the cassettes did differ rather much between models and
manufacturers. There were even cassette decks like AKAI CS-707D which
had two different tape positions for "type I" tapes, called LN and LH,
which were intended for usage with older/"European" (Usually
Philips, Agfa, BASF and similar from the 60's and 70's, and the crappy
American tapes like Ampex, Scotch/3M and similar) v.s.
newer/"Japanese" (usually Maxell UD and similar from the 70's, and most
types from the 80's and newer) tapes.

Have a look at any decent cassette tape test in some serious consumer
electronics magazine from back in the days, and you'll find that the
tapes differed a lot within each type.

This must surely have happened on diskettes also, but as the media is
used in a different way the only important things would be that the
noise is under a certain threshold and the "treble response" is good
enough so data won't get lost at higher bit rates, and of course drop
outs.



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