Re: CBM-900 floppy disk format/encoding

From: MikeS <>
Date: Fri, 28 Oct 2011 01:27:00 -0400
Message-ID: <946C0508AE3D414AAEE216E0D874B653@vl420mt>
Well, Quad Density is by definition roughly twice the capacity of Double
Density (i.e. ~360KB/side vs. ~180K/side) and even with techniques like GCR
and variable sectors/track it isn't really feasible to squeeze twice as much
data into the same number of tracks, so until better coating formulations
came along the only way to double the disk capacity was to double the number
of tracks by making them narrower and closer together.

Most QD (and HD) disks were indeed recorded as 80 tracks at 96 tracks per
inch, but some systems used 100 TPI drives instead; AFAIK all of Commodore's
'Quad Density' drives used 100 TPI mechanisms so unfortunately you can't use
the much more common 96TPI versions. Although they only used 77 tracks
instead of 80 they more than made up for it by putting more data in the
tracks as the tracks got longer toward the circumference of the disk, thus
achieving a capacity of >500KB/side vs. the 'normal' QD capacity of ~360KB.

I still suspect that the CBM-900 used 1.2MB HD disks though...

Good points about coating formulations and quality control & selection.


----- Original Message ----- 
From: "William Levak" <wlevak@SDF.ORG>
To: <>
Sent: Friday, October 28, 2011 12:07 AM
Subject: 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: <>
>> 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.
>>      Message was sent through the cbm-hackers mailing list
> SDF Public Access UNIX System -
>       Message was sent through the cbm-hackers mailing list

       Message was sent through the cbm-hackers mailing list
Received on 2011-10-28 06:00:03

Archive generated by hypermail 2.2.0.