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From: dillon@CORY.BERKELEY.EDU.UUCP
Newsgroups: comp.sys.amiga
Subject: MFM format and possible improvments
Message-ID: <8702050359.AA01021@cory.Berkeley.EDU>
Date: Wed, 4-Feb-87 22:59:13 EST
Article-I.D.: cory.8702050359.AA01021
Posted: Wed Feb  4 22:59:13 1987
Date-Received: Sat, 7-Feb-87 10:02:45 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: University of California at Berkeley
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	Unfortunetly, I'm no expert on disk media and hardware.  This is my
understanding:

The raw bit data which goes into drive represents the existance or lack of
a flux reversal on the media.  0=no flux reversal, 1=flux reversal.  You
need to encode the data such that:

	(A) You don't go too long without a flux reversal or the phased locked
	loop will loose it's lock.  This means no more than 3 '0' bits in a 
	row.

	(B) You can't have the peaks too close to each other or you get
	intersymbol interference due to data shifting.  Thus:

	bit	0101011000110
	flux	_--__-____-__

	This seems to indicate that you cannot have more than 2 '1' bits in
	a row, but I'm not sure.  Does one also have to enforce a 'No more
	than N peaks in T time' rule also??

MFM encoding works as follows.  Each bit is encoded like this:

	databit		bits-to-disk
	0	->	*0		(where * is the complement of the
					previous bit in the stream)
	1	->	01

MFM thus guarentees no more than 3 zero's in a row, and no more than one one
in a row.  It takes exactly two clocked bits to disk for every bit in the 
data.

	1  0  1
	01 00 01

My question is, is it allowed to have two one's in a row, and does one also 
have to take into account a 'N peaks over T time' limit?  Because if two
one's in a row IS allowed, you can use the following method to encode the
data:

	8 data bits	-> 11 'disk' bits. the encoding ensures no more than
	3 0's in a row, and no more than 2 1's in a row.

	If you *can* do this, then you have just given yourself about a 30%
	increase in storage.


	Current MFM:
		11 sectors * (512+32 bytes/sector) =  5984 bytes/track.

	New method which may or may not work:
		16 sectors * (512+32 bytes/sector) = 8704 bytes/track.

	Making for a 1.25Meg floppy.

So is it possible?  If so, I'll write a trackdisk driver for the baby.

------------------------
Note on RLL: Can't be done on the Amiga.. it requires you to boost the
bit clock for the drive.

					-Matt