Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!ames!amdcad!diablo!phil
From: phil@diablo.amd.com (Phil Ngai)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MFM as an RLL drive?
Message-ID: <28523@amdcad.AMD.COM>
Date: 22 Dec 89 22:08:03 GMT
References: <887@crash.cts.com> <28373@amdcad.AMD.COM> <44809@bu-cs.BU.EDU> <423@hitech.ht.oz>
Sender: news@amdcad.AMD.COM
Reply-To: phil@diablo.AMD.COM (Phil Ngai)
Organization: Advanced Micro Devices, Inc. Sunnyvale CA
Lines: 21

In article <423@hitech.ht.oz> clyde@hitech.ht.oz (Clyde Smith-Stubbs) writes:
|Would someone who knows what they are talking about please stand up
|and clear this up: to my knowledge the difference between MFM and RLL is NOT
|that the clock rate is different, but that RLL has less redundant information,
|i.e. to record one MFM bit takes two transitions while to record one
|RLL bit takes only 1 point something transitions. If this is the case, then
|the drive electronics surely are unaware of the difference? The only thing
|that should matter is the stability of the drive (i.e. motor flutter etc.)
|because of the reduced redundancy.

I think this is an accurate description of the situation. Note that
for ST506, the data separator is on the controller, not the drive,
meaning the drive electronics only sees the transitions and it is the
controller that extracts the clock and turns the transitions into data
bits. Moving the data separator to the drive so the analog information
didn't have to travel over a noisy cable was one of the advantages of
ESDI. 

--
Phil Ngai, phil@diablo.amd.com		{uunet,decwrl,ucbvax}!amdcad!phil
Ann Landers says "Let's talk about legalizing drugs."