Re: CBM-900 floppy disk format/encoding

I have checked a number of sources on the internet.  There does not seem 
to be any agreement on what the track density of quad density is.  Some 
say 48 tpi, others 96 tpi.  96 tpi seems to be in the majority.

      SD, DD    48 tpi
      QD, HD    96 tpi
      all 3.5"  135 tpi

Magnetic coating grain size has varied over the years.  When DD disks were 
first produced, they required a smaller grain size than the existing SD 
disks.  As the DD coating was produced in quantity, the old SD formulation 
was discontinued and DD coating was used for all disks.  The same thing 
happened as the newer formats were developed.  All disks made today 
have the same grain size. Only HD requires a separate formulation.

Testing the disks is the most expensive part of the manufacturing process.
Therefore, the label on the disk (SD, DD, or QD) was determined by which 
density the disks were tested at.  These days SD disks are not available, 
as there is not enough demand for a separate test process.  DD can always 
be used in their place.  All disk manufactured today can be used at the 
hughest density, they are just not certified to write at a density higher 
than the label on the box.

On Wed, 26 Oct 2011, MikeS wrote:

> ----- Original Message ----- From: "William Levak" <wlevak@SDF.ORG>
> To: <cbm-hackers@musoftware.de>
> 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:
>
> http://www.3480-3590-data-conversion.com/article-floppy-disks.html
>
>
>     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.
>
> http://www.commodore.ca/manuals/commodore_1541_4040_8050_8250_comparison.htm
>
>
>      Message was sent through the cbm-hackers mailing list
>

wlevak@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org

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