Path: utzoo!attcan!uunet!samsung!usc!ucsd!nosc!logicon.com!trantor.harris-atd.com!charybdis!sonny
From: sonny@charybdis.harris-atd.com (Bob Davis)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: What (exactly) are MFM and RLL Modulation Techniques?
Message-ID: <4320@trantor.harris-atd.com>
Date: 17 Sep 90 15:04:48 GMT
References: <4304@trantor.harris-atd.com> <1453@gold.GVG.TEK.COM> <4313@trantor.harris-atd.com> <177@thor.UUCP>
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Reply-To: sonny@trantor.harris-atd.com (Bob Davis)
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Organization: Advanced Technology Dept., Harris ESS, Melbourne, FL
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	My heartfelt thanks to Larry Jones for assembling one of the
most lucid groups of words on FM, MFM, and RLL digital modulations for disk
drives I have ever read.

In article <177@thor.UUCP> scjones@thor.UUCP (Larry Jones) writes:

  [REALLY Good Stuff deleted (Go back and read it or at least save it)]

>
>However, even though the transition density doesn't increase, the
>location of the transitions becomes more important.  For MFM, we
>need the location twice as accurately as FM and for RLL three times.
>How accurately the transitions can be recorded and detected depends
>on both the recording medium and the record and playback electronics.

	So, the density of flux transitions on the disk is exactly
the same for FM, MFM, and RLL, it appears. Thus it seems that stressing
the magnetic medium data storage capabilities is NOT the problem at all
in getting an MFM (or FM) disk to take RLL formatting.
	For ALL three schemes, it is necessary to be able to say whether
or not a flux transition occurs in the center of a bit time. Thus, it seems
that the bit clocks for all three drive types need to be able to define a 
window to look for the transition, and the window is of the SAME 
FRACTIONAL BIT TIME WIDTH ACCURACY for all three drive types.
	The real problem appears to be that, for this SAME flux transition
density on the disk, the transfer bit rates on and off the disk are in 
the ratios 1, 2, 3 for FM, MFM, and RLL, respectively. Thus, even though
the required transition window accuracies are of the same fractional bit 
time width, the ABSOLUTE WINDOW TIME WIDTH ACCURACIES required are in the 
ratios 1, 1/2, 1/3 for FM, MFM, and RLL, respectively.

	What I have learned is this: When an MFM drive is pressed into
RLL service, the magnetic properties of the platters is probably OK. The
electronics must pass 26/17 = 1.53 times the bit rate on and off the
disk in RLL service compared to MFM. The transition time window is
2/3 as wide. The drive motor speed control needs to hold the speed more
accurately so the transitions stay in the transition window.
	Some MFM drives can hack these tighter requirements of RLL service.
        Some can't. 
	I have had a cheap MiniScribe 3425 20Mb MFM drive in 
32Mb RLL service for a ZERO trouble year. Now and then I scan for sectors 
going bad. None have popped up. There are zero bad sectors on the entire
drive. Sometimes you get lucky.




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