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From: sonny@charybdis.harris-atd.com (Bob Davis)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: What (exactly) are MFM and RLL Modulation Techniques?
Summary: MFM, RLL, hard drives, modulation
Message-ID: <4304@trantor.harris-atd.com>
Date: 12 Sep 90 19:54:00 GMT
Sender: news@trantor.harris-atd.com
Reply-To: sonny@trantor.harris-atd.com (Bob Davis)
Distribution: na
Organization: Advanced Technology Dept., Harris ESS, Melbourne, FL
Lines: 53


	I know that MFM stands for "Modified Frequency Modulation"
and that RLL stands for "Run Length Limited". Beyond that, things
in my mind (and in those of all authors I have thus far found)
begin to get unbelievably fuzzy unbelievably fast.

	Would someone who KNOWS details about these hard drive
data modulation techniques, please explain them to someone who
understands digital modulation techniques, or point him to a reference?

	For example, is MFM simply binary Frequency Shift Keying (FSK)?
How is bit synchronization derived? In what sense is MFM "Modified"?

	On the RLL side, I am not so much interested in details of
the Run Length Limited coding as I am in the modulation scheme and
bit synchronization details. Is the basic modulation scheme still
FM as in MFM? Is it Binary FSK?

	I gather from what I read that MFM puts out some kind of pulse
at the beginning of EVERY bit time to aid bit sync. Is that true?

	I also gather that RLL's improved packing density comes
because it doesn't have to insert the "bit sync pulse" each bit time,
and therefore the average number of transitions per bit time is reduced.
Consequently, I can squeeze in more bits per track and still have
the same transition density per linear inch of track. Is this right?

	I gather that RLL drives must derive bit sync clock from
the data transitions themselves without help from MFM's "bit sync
pulses". Is that right?

	I gather by implication from stuff I read about RLL that the
coding limits long runs of zeros or long runs of ones PRIMARILY (solely?)
so that the bit synchronization circuitry does not have to "coast" too 
long between data transitions. Is that true?

	How do the transition densities per INFORMATION bit (including 
all synchronization pulses and coding overheads) compare for MFM vs. RLL?

	How do the transition densities per linear inch of track on the
media compare for MFM vs. RLL?

	If you know a reference that answers such questions as these,
or if YOU know answer(s), please tell me.

	Thanks.

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