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From: hans@umd5.umd.edu (Hans Breitenlohner)
Newsgroups: comp.sys.atari.8bit
Subject: Re: 1050 disk format
Message-ID: <2028@umd5.umd.edu>
Date: Wed, 11-Nov-87 12:15:41 EST
Article-I.D.: umd5.2028
Posted: Wed Nov 11 12:15:41 1987
Date-Received: Sat, 14-Nov-87 02:49:51 EST
References: <8711031041.8580.Makey@LOGICON.ARPA> <1049@cg-atla.UUCP>
Reply-To: hans@umd5 (Hans Breitenlohner)
Organization: University of Maryland, College Park
Lines: 32
Summary: More than you ever wanted to know about recording formats.

The 1050 disk drive (as all Atari drives) uses industry standard (or is it
IBM standard) Floppy disk format (except that the index hole is not used).
In single density FM encoding is used, in double (or enhanced) density
MFM encoding is used.  

FM encoding records a transition in the middle of each bit cell, and an 
additional transition between equal bits.
MFM encoding records a transition in the middle of bit cells containing
a one bit, and additional transitions between bit cells which both contain
zero bits.
Thus transitions are one-half or one bit cell apart in FM mode; and one,
one-and-a-half, or two bit cells apart in MFM mode.  Note: Since the bit 
density is twice as high in MFM mode, the actual minimum and maximum spacing
between transitions is the same in both modes.

The lowest density of recorded transitions in either mode happens for 
alternating 0 and 1 bits.
The highest density of transitions in either mode occurs for sequences of
all zero bits or all one bits.
The intermediate transition density in MFM mode occurs for sequences of
the form 001001001 etc.

A good test pattern probably should contain some of all
recording densities.  IBM uses data bytes of E5 to fill empty sectors
in FM mode, which qualifies on that count.  Similarly the pattern 6D B6 DB
which was suggested alternates between high and low density transitions in
MFM mode.  I would think, however, that a good MFM test pattern should also
contain a good supply of medium-density signals, to verify that the three
spacings can be distinguished reliably.

An additional complication you need to know about is the fact that all data
bytes are inverted before they are written to disk, and after reading.