Re: Did Commodore cheat with the quad density floppies?

On Thu, Jan 3, 2019 at 5:52 PM Mia Magnusson <mia@plea.se> wrote:
&gt;

&gt; &gt;
&gt; &gt; "QD" disks have the very same 300 oersted magnetic media as SD/DD
&gt; &gt; disks, it was only the mechanics and R/W heads that allowed to use
&gt; &gt; more effectively the storage media.
&gt; &gt; Later HD drives used different R/W heads (or different currents) and
&gt; &gt; required 600 oersted media, doubled bitrate to 500 Kbps and changed
&gt; &gt; speed to 360 rpm.
&gt; &gt;
&gt; &gt; At the university, we had a few Olivetti LSX-3005 that were equipped
&gt; &gt; with 96tpi 5 1/4" drives (QD, not HD obviously),
&gt; &gt; I remember nobody ever tried to find "QD" disks, normal DD 48TPI disks
&gt; &gt; were used (albeit I remember good quality brands were purchased
&gt; &gt; usually, like 3M, Olivetti).
&gt;
&gt; As I recall, some 48 TPI disks actually caused problems when used as 96
&gt; TPI.

Probably in the 96TPI HD drives, the ones that need 600 Oersted media.
QD drives need 300 Oersted media, they can't write to 600 Oersted "HD"
media (try yourself)
300 Oersted media is always the same, regardless on what is printed on
the box (SD/DD/QD, 48/96TPI).
HD drives need 600 Oersted media to use the 1.2MB format, but they can
read/write to 300 Oersted media in the FM/MFM formats.
HD drives will usually fail to write 1.2MB formats on 300 Oersted media.
I can assume some manufacturer produced better media than others, but
that was largely independent on what was advertised on the box :)
When I went through my whole lot of Apple II disks, I've found a few
brands of disks that were so degraded that they would leave a lot of
magnetic stuff on the heads, and I had to clean the drive's head after
trying to read such a disk.
On the other hand, some unmarked generic floppies were as good as new
after all these years.


&gt;&gt; &gt; I really think the better Kbps rating is due to the use
of the more
&gt; &gt; efficient GCR code instead of the MFM.
&gt;
&gt; Why wouldn't Commodore had used that in the
&gt; 2040/3040/4040/2031/2031LP/1540/1541/1551/1570/1571 drives too then?

In facts, CBM used the more efficient GCR in their drives :)
LSX-3005 QD MFM drives stored 720Kbytes (double side), using 80 tracks
per side, while the 8250/SFD-1001 GCR drives stored 1Mbyte on the same
disks and using only 77 tracks per side :)
MS-DOS stored 360Kbytes on 40x2 tracks using MFM, 4040/1541 GCR stores
340Kbytes (on two sides) but only on 35x2 tracks (so, using 10 less
tracks!!)
Yes, of course CBM drives also use variable bitrate to store more
sectors in the larger tracks, but this is possible because of GCR
encoding.
All this on 300 Oersted media.

>
> 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.

well really magnetic media is used quite differently on audio tapes VS
floppy disks.
On audio tape you would be interested in dynamic range, frequency response etc.
Floppy disks write at a high enough current level to almost saturate
locally the magnetic media, that is also a good measure against
magnetic memory of the old data written in the same spot.

Frank