Re: CBM-900 floppy disk format/encoding

On 2011-10-28 07:27, MikeS wrote:
>
> 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.

That is correct

CBM-8050:   SS/QD/100TPI (single drive casing)
CBM-8250:   DS/QD/100TPI (dual drive casing)
CBM-8250LP: DS/QD/100TPI (dual drive casing - low profile)
SFD-1001:   DS/QD/100TPI (single drive casing - low profile/1541 style)

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

I can't say anything for sure (meaning no definitive conclusion) since I 
had very little time yesterday evening.
But I did have time to open up the CBM-900 and had a look inside.
Also I opened a SFD-1001 just to verify my theory that the CBM-900 uses 
drive that is mechanical identical with the one inside SFD-1001.

And I was right the drive in the CBM-900 is completely identical with 
the one inside a SFD-1001.

 From that I'll dare to conclude that the CBM-900 floppy disks are 
DS/QD/100 TPI.

/Uffe

>
> ----- Original Message ----- From: "William Levak" <wlevak@SDF.ORG>
> To: <cbm-hackers@musoftware.de>
> 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: <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
>>
>> Message was sent through the cbm-hackers mailing list
>
>
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