Path: utzoo!attcan!uunet!zephyr.ens.tek.com!gvgpsa!gold!grege
From: grege@gold.GVG.TEK.COM (Greg Ebert)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: What (exactly) are MFM and RLL Modulation Techniques?
Message-ID: <1456@gold.GVG.TEK.COM>
Date: 14 Sep 90 16:53:32 GMT
References: <4304@trantor.harris-atd.com> <1453@gold.GVG.TEK.COM> <4313@trantor.harris-atd.com>
Distribution: na
Organization: Grass Valley Group, Grass Valley, CA
Lines: 36


[This response also addresses another question about 'non-RLL' drives]

First of all, I goofed in my original posting. 'RLL' is actually "2,7 RLL"
which means that the minimum spacing is 2 zeros, and the maximum is 7 zeros.
I should have examined the patterns more closely because I stated IBM had
a maximum spacing of 8, and XEROX 6. They actually are both 7.

I believe the reason there are RLL drives is because the read-data timing
(also known as drive-margin) needs to be tighter for RLL than MFM because
there is a higher probability that a PLL will fall 'out-of-sync' when the
spacing between adjacent 1's is 7 zeros. For comparison, the maximum gap 
between adjacent 1's in MFM corresponds to a run of only 5 zeros in RLL.

The design of phase-locked loops (PLL's) is an art in itself. The PLL has
a profound effect on data reliability, but there are limits to it's
capabilities. Thus, an effort is made by the drive manufacturer to build
a drive with a better drive-margin. Factors such as spindle-speed variation,
signal-to-noise ratio, and write current contribute to bit shift.

However, I would feel comfortable using RLL formatting on an 'MFM-only'
drive if I had an error-correcting controller.
>
>P.S. I do not yet understand why this stuff is called Frequency Modulation.
>     Do you?
>
Nope. My guess is that its sloppy nomenclature, just like calling DOS an 
operating system (HA!).

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