Path: utzoo!mnetor!uunet!steinmetz!davidsen
From: davidsen@steinmetz.steinmetz.UUCP (William E. Davidsen Jr)
Newsgroups: comp.sys.ibm.pc
Subject: Re: RLL on a ST-225
Message-ID: <9692@steinmetz.steinmetz.UUCP>
Date: 25 Feb 88 21:13:30 GMT
References: <5037@dhw68k.cts.com> <16800212@clio> <1017@neoucom.UUCP>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: General Electric CRD, Schenectady, NY
Lines: 55

In article <1017@neoucom.UUCP> wtm@neoucom.UUCP (Bill Mayhew) writes:
| In article <16800212@clio>, berger@clio.las.uiuc.edu writes:
| 
| > Most drives intended for MFM work fine with RLL.  Seagate is one of the
| > only manufacturers to advise against it,...
| 
| The makers of RLL controller cards are also pretty cagey about
| naming drives that will work with their cards too.  There are
| reasons.  The bit rate from a standard MFM encoded drive is 500,000
| bits/sec.  The bit rate for RLL is 750,000 bis/sec.  The reason for
| this is obvious.  Since you can't change the RPM of the drive, and
| the drive has more bits/track, then the data has to move at a
| hgiher bit rate for RLL.  The higher bit rate requires that the
| bandwith of the head amplifiers be sufficent first of all;
| secondly, the media of the disk platter itself has to be able to
| handle the proper bit density.

Let me see if I can clarify this a bit... when RLL comtrollers put data
on a disk, they don't just send it 50% faster, they preprocess it.
Somewhat like data compression. While the *bit rate* goes up, the number
of flux changes per inch (fpi) doesn't. For every combination of disk
media and head there is an upper limit on how close two flux changes may
be. This is why the write precomp changes on some disks, because the
diameter of the inner tracks is smaller and the flux changes are closer
together.

Let me try to say it pictorially:

no RLL		0..1..0..1..0..0..1..1
50% more fci	0.1.0.1.0.0.1.1
RLL 2,7		D...D...D...D...D...D

How this works is that the timing of the flux changes is now more
critical, and the number of bit per second is up 50%, but the flux
changes stay an acceptable distance apart. They are often farther apart
than with MFM.

The reason some disks may fail is that the timing of the flux changes
must be quite precise, and if the media doesn't yield a good clean
transition the timing will "jitter" enough to give CRC errors.

BTW: MFM is just a term used for one type of RLL compression, I believe
RLL-0,2 but please don't quote me. Back in about 1978 floppies made the
change from "single density" to "double density" using this newfangled
thing "MFM recording. There is another RLL being tested which gives 100%
increase in data on a disk, and I believe it's called RLL-4,9 (again
don't quote me).

Now some real hardware hacker can post a real rebuttal to this, telling
me I've glossed over the detail, and explaining it in a way meaningful
only to other hardware types.
-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me