Re: Did Commodore cheat with the quad density floppies?

----- Original Message ----- 
From: "Mia Magnusson" <mia@plea.se>
To: <cbm-hackers@musoftware.de>
Sent: Thursday, January 03, 2019 8:50 AM
Subject: Re: Did Commodore cheat with the quad density floppies?


...
> 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.

I've never seen a reference to 'QD' disks in any material from Commodore; they usually say you can use SD in the lower capacity drives (15xx, 20xx 40xx etc.) and recommend DD diskettes for the 8xxx higher capacity ones.

> 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 

Interesting; if they were marked DD then how do you know that they were '96TPI disks'? Do you mean (48TPI) DD disks *recorded at* 96 TPI?

Most people consider 'Quad Density' a misleading term not even used consistently by all manufacturers; as Frank points out, the media material and therefore the maximum recording *density* is the same as DD, but with more (narrower) tracks. The only difference aside from marketing hype is that quality improved in the early years and in some cases the diskettes *may* actually have been certified for 96TPI recording. 

Just like 3.5" diskettes it really comes down to only two types of 5 1/2" diskettes: 300 Oersted *Low Density* (SD, DD, 'QD', 48TPI, 96TPI, 100TPI) and 600 Oersted HD diskettes (although the coercivity of 3.5" diskettes does not differ quite as much between DD and HD and unlike 5 1/4 diskettes they can often be used interchangeably)

FWIW I've used all sorts of diskettes in my 8050s and never had a consistent problem (other than the usual occasional error) with any type, SD, DD or 'QD', even hard sector diskettes (since most CBM drives don't have an index sensor).

Interesting side note: hard sector diskettes were dirt cheap back in the day since relatively few systems could use them, so I bought quite a few; they came in very handy a few years ago when I acquired some CP/M systems that *did* require them, since by then they were almost unobtainable.

I wonder if part of the answer to Andre's original question may be the fact that Bits per inch is not necessarily the same as Flux transitions per inch/mm...