Re: CBM-900 floppy disk format/encoding

From: MikeS <>
Date: Fri, 28 Oct 2011 13:48:45 -0400
Message-ID: <287864A4C3C241C0BA9B354C9AF08DF7@vl420mt>
----- Original Message ----- 
From: "Groepaz" <>
To: <>
Sent: Friday, October 28, 2011 10:42 AM
Subject: Re: CBM-900 floppy disk format/encoding

> On Friday 28 October 2011, you wrote:
>> 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.
> 100tpi


Only in the closed world of Commodore and a few other vendors (e.g. Vector

The QD 'standard', such as it was, was the same as the more modern HD
drives, i.e. 96TPI, which meant narrower heads and stepper motors/mechanisms
that took twice as many steps as DD drives for the same distance but with
the read/write electronics and RPM the same as DD drives (300); thus, unlike
100TPI drives, there was backward compatibility since both types could also
read and write 48TPI DD disks by simply double-stepping the head(s).

The difference between 96TPI QD and HD drives is that HD drives can use
either DD/QD or HD by selecting two different read and write parameters to
accommodate the two different diskette types and their different magnetic
properties (usually with pin 2 of the interface); many HD drives also have
dual-speed (HD's 360RPM vs DD/QD's 300RPM) capability, but 300RPM can also
effectively be emulated by changing the rate at which data is transferred
to/from the diskette (as in the PC for instance).


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Received on 2011-10-28 18:00:03

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