Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!sharkey!edsews!uunet!microsoft!w-colinp
From: w-colinp@microsoft.UUCP (Colin Plumb)
Newsgroups: comp.sys.amiga
Subject: Re: Getting 1.5 MB on a standard 880K floppy without compressio
Message-ID: <1215@microsoft.UUCP>
Date: 3 Apr 89 06:56:29 GMT
References: <3631@sugar.hackercorp.com> <94323@sun.Eng.Sun.COM> <96103@sun.Eng.Sun.COM> <3669@sugar.hackercorp.com> <16362@cup.portal.com>
Reply-To: w-colinp@microsoft.uucp (Colin Plumb)
Organization: very little
Lines: 26

Gator@cup.portal.com (Walter Thomas Reed) wrote:
> Well, according to Amiga Disk Drives Inside and Out (by Abacus)
> the amiga cannot handle several one bits following each other, so the
> recording density is changed to half speed (4ms/bit instead if 2ms.)
> 
> Does this mean that GCR is less effecient than MFM and you actually get less
> storage?  The book was unclear about that.

As my original article mentioned, it's a limitation of the oxide on the
floppy.  Given that 4us between one bits on the innermost track of a 90
mm floppy spinning at 300rpm is the minimum, we can translate this into
distance.  Assume the innermost track has a diameter of 50mm, that gives
you 50*Pi mm of track to fit the 0.2s/4us = 50,000 1 bits on to, that gives
you 50*Pi/50,000 mm = Pi um of track per flux change.  Since the 50mm is
only a guess, round it to 3um.

Tha Amiga hardware will let you write consecutive 1 bits at 2us times,
the floppy just won't remember it.

Anyway, yes, the traditional 5/4 GCR (4 bits encoded as 5) format is
less efficient than MFM.  It can fit 4 bits into 20us, while MFM can
fit 5.
-- 
	-Colin (uunet!microsoft!w-colinp)

"Don't listen to me.  I never do." - The Doctor