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: <4313@trantor.harris-atd.com>
Date: 14 Sep 90 14:05:04 GMT
References: <4304@trantor.harris-atd.com> <1453@gold.GVG.TEK.COM>
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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
Lines: 61

Thanks, Greg, for your response.

In article <1453@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Greg Ebert) writes:
>
	[Stuff deleted]
>figured out that bit shift is predictable. You guessed it! They slightly
>altered the intervals at which bits are written, depending upon the
>pattern and the density. [Remember that bit density increases as you
>move toward the inner tracks]. This is called write-precompensation.
>When the bits are read-back, they pop-up when they are expected to do so.
>

	From my experience in high rate digital modems, it sounds like
"write-precompensation" is what I would call an Equalizer applied
*before* the channel (i.e., the disk) rather than the more often encountered
case of *after* the channel (i.e., at the demodulator) for the purpose
of controlling InterSymbol Interference (ISI) occurring in the channel.

	[Deletions]
[Continuing, discussing RLL:]

>
>Noting that the MINIMUM spacing between adjacent 1's is at least 2 zeros, and
>that the encoded data stream is twice the rate of the incoming data, it
>follows that the frequency (ie, flux-transitions/unit length) is actually
>LESS than the data rate; 2/3 in this case. Remember that MFM had a worst-case
                          ^^^ [Emphasis by Bob Davis]
>flux-transitions/time (or length) equal to the data rate. Since the flux-change
>rate for RLL is LOWER than the data rate, we can get MORE data stored for the
>same flux-transitions/length.

	So, if the flux transition rate for RLL is 2/3 that of MFM, then
when RLL increases the sectors/track from 17 for MFM to 26 for RLL, doesn't
the density of flux transitions ON THE MEDIA wind up almost precisely
the same for MFM and RLL?
	And if that is true, why do they recommend a special drive for
RLL? Why isn't an MFM drive good enough?

	Thanks...

P.S. I do not yet understand why this stuff is called Frequency Modulation.
     Do you?
     Is the signal recorded on the medium actually a carrier which has
     been FM'ed with these data transitions that everyone speaks of?
	Or is the baseband data itself some form of FM. (The MFM baseband
     data stream looks a lot like Serial Minimum Shift Keying, or SMSK,
     which is a binary FSK scheme that has attracted a lot of interest
     in high speed digital modems of late).

 Surely there must be a definitive technical reference article or book 
that someone can recommend to me that deals with these details and I can
adjourn my Learning from the Net. But I do know that ALL personal computer
books I have read treat this technical area with an abominably bad hand-waving
chapter or section.

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