Re: Did Commodore cheat with the quad density floppies?

On Thu, 3 Jan 2019, Mia Magnusson wrote:

> 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.

What they were labeled as was determined by what they were tested as.  The 
testing is the most expensive part of the manufacturing process.

> 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.

It's not more efficient. It's more accurate. In MFM, if you have a long 
string of zeros, reading accuracy is limited by the accuracy of your 
clock. GCR eliminates this problem.

> 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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