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From: charles@hpcvcd.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: MFM format and possible improvments
Message-ID: <4410003@hpcvcd.HP>
Date: Sun, 8-Feb-87 22:08:09 EST
Article-I.D.: hpcvcd.4410003
Posted: Sun Feb  8 22:08:09 1987
Date-Received: Wed, 11-Feb-87 07:16:07 EST
References: <8702050359.AA01021@cory.Berkeley.EDU>
Organization: Hewlett-Packard Co., Corvallis, Oregon
Lines: 59

It has been several years since I worked with floppys, but I'll give
this a try.  My experience is with 8 inch drives.

>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.
Yes.

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

Bit shifting is a significant problem.  But consider that the head is
this mechanical thing.  It has a physical size that is significant in
relation the the track and bit cell size.  I don't know how close
floppys run to this limit, but there certainly is a limit to how
small a flux region can be made.

>	This seems to indicate that you cannot have more than 2 '1' bits in
>	a row, but I'm not sure.

That is one way to look at it.  I usually think of it in terms of
time.  On an 8 inch floppy, the encoded bits can be 2us, 3us, or 4us
apart.  With FM and single density, the timing was 2us or 4us.  Thus
in the shift from single to double density, the minimum and maximum
bit timing to the drive did not change.  All that changed was the
resolution required of the system.  Notice that this implys the early
single density drives could run double density if they were adjusted
well and if you used floppys that didn't shift the bits much.

>	Does one also have to enforce a 'No more
>	than N peaks in T time' rule also??

Never heard of such a thing.

>	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.

Won't work.  With MFM on 8 inch floppys a continuous stream of data
ones produces encoded bits with ones 2us apart.  That is the closest
together you can put the flux reversals.

It turns out a continuous stream of zeros also produces encoded bits
with ones 2us apart.  The only way to tell the difference is to
remember from the start of the data which encoded ones represent data
ones and which represent zeros.

Altho I explained this in terms of 8 inch floppys, I am pretty sure
the only thing different with smaller floppys is the actual timing.

I understand GCR is more dense than MFM.  Personally, I would rather
give up a couple hundred KB to have a commonly readable disk format.

While reading the Hardware Reference Manual, I got the impression
that the decoding of the floppy bits to real bits is done in
software.  I hope that is not true.  Perhaps that is part of the
explanation for why the A1000 is so infernally slow.  With several
floppy controller chips on the market, this seems counterproductive.

	Charles Brown
	hplabs!hp-pcd!charles