Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!zaphod.mps.ohio-state.edu!mips!prls!pyramid!athertn!paul
From: paul@athertn.Atherton.COM (Paul Sander)
Newsgroups: comp.sys.apple
Subject: Re: 5.25 floppy disk formats
Summary: Bit cell sizes are not the problem
Message-ID: <15682@athertn.Atherton.COM>
Date: 22 Dec 89 20:12:30 GMT
References: <8520.infoapple.net@pro-generic>
Organization: Atherton Technology, Sunnyvale, CA
Lines: 46

In article <8520.infoapple.net@pro-generic>, sb@pro-generic.cts.com (Stephen Brown) writes:
> In-Reply-To: message from ericmcg@pro-generic.cts.com
> 
> I think the problem with getting a 5.25 running double density would be more
> than just the heads being too big. You'd have to double the clock rate, and then the
> bit cells would be too small (especially at the outer edge of the disk where
> the rotational velocity is the greatest).

As it turns out, the bit cell sizes on single-density and double-density
diskettes are the same!  They use different encoding methods to store
the bits, however.  Single-density diskettes use an encoding method called
FM (Frequency Modulation) that stores a clock bit and a data bit.  The
clock bit it always present when data are stored (but not always in
headers) and the presence or absence of the data bit signifies a 1 or 0.

Double-density drives typically store their data in a format called MFM
(Modified FM) and stores clock bits only in the absence of data bits,
relying on the phase of the bits to distinguish them.

There is a second double-density format called MMFM (Modified MFM) that
appears to have some advantages over MFM as far as simplifying timing
requirements, but I have yet to see any drive controllers that advertise
this feature.  I know that it is not a standard format.

Apple's Disk ][ controller, as I understand it, encodes bits in neither
FM nor MFM formats.  Further, the encoding algorithm is burned into a
PROM on the controller, and is not easily changed.

Here are some references for further information:

Worth and Lechner
Beneath Apple DOS
1981, Quality Software

Worth and Lechner
Beneath Apple ProDOS
1984, Quality Software

SA800/801 Diskette Storage Drive Theory of Operations
1977, Shugart Associates


-- 
Paul Sander        (408) 734-9822  | If you must describe both quantity and
paul@Atherton.COM                  | quality of someone else's code, try
{decwrl,pyramid,sun}!athertn!paul  | "awful lot."