Re: CBM-900 floppy disk format/encoding

From: MikeS <>
Date: Wed, 26 Oct 2011 03:12:51 -0400
Message-ID: <3C5FB3E36DF44A54A8276E1CF2EE333A@vl420mt>
----- Original Message ----- 
From: "William Levak" <wlevak@SDF.ORG>
To: <>
Sent: Wednesday, October 26, 2011 12:19 AM
Subject: Re: CBM-900 floppy disk format/encoding

> Quad density has the same capacity as HD, but supports the same magnetic
> coating used in DD, except finer grain particles.  HD uses a magnetic
> coating with a stronger magnetic field, and therefore, not campatible with
> DD.


Actually, that's a little misleading; so-called quad density disks/drives
use exactly the same magnetic medium as DD disks/drives and, as mentioned
previously, are recorded at the same linear density as DD disks but with
twice the number of tracks, fixed sector MFM formats giving roughly 720K
compared to DD's ~360K. Although they were the same disks, in the early days
of poor manufacturing yields QD disks were indeed QA selected for the higher
quality required for the narrower tracks.

High density disks on the other hand do use a higher coercivity medium and
rotate at a higher speed, which permits a greater linear density and a
greater number of sectors/track, yielding 1.2MB per disk compared to QD's
~720K; by switching the strength of the write current and/or double-stepping
most HD drives are actually capable of writing (and reading) DD and QD
diskettes, although there will almost certainly be errors if the different 
formats are used on the same disk.

A good overview of the 'industry-standard' formats and capacities:

      Cyl Sides Density oe  RPM Capacity
      40   1        SD     300 300   80 KB
      40   1        DD    300 300  180 KB
      40   2        DD    300 300  360 KB
      80   1        DD    300 300  360 KB
      80   2        DD    300 300  720 KB ("QD")
      80   2        HD    600 360 1200 KB

With the possible exception of the CBM-900 disks that started this
discussion, Commodore and some other manufacturers of the day only used
DD diskettes but took advantage of their 'intelligent' drives to squeeze
more capacity out of the higher capacity disks than the 'standard' fixed
sector/track MFM disks by using GCR encoding and variable number of
sectors/track (more sectors/data on the longer outer tracks); this is how
they managed to get >500 KB per side on 8050/8250/SFD1001 disks compared to
the 'standard' ~360KB/side.

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Received on 2011-10-26 08:00:03

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