Path: utzoo!attcan!uunet!cbmvax!grr
From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.periphs
Subject: Re: GCR (was Re: RLL: ..... )
Message-ID: <3806@cbmvax.UUCP>
Date: 18 May 88 10:25:16 GMT
References: <11508@mimsy.UUCP> <116@lakart.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 23

In article <116@lakart.UUCP> dg@lakart.UUCP (David Goodenough) writes:
> From article <11508@mimsy.UUCP>, by chris@mimsy.UUCP (Chris Torek):
> >>... 2,7 RLL is a variable length code (e.g. 0011 maps to 00001000 but
> >>010 maps to 100100); I don't have a simple formula for the 2,7 RLL code!
>  
> So where do we draw the line between GCR (group code recording) and RLL?
> I was under the impression that GCR does a similar thing to RLL in that it
> takes (say) 4 bit patterns and expands them to (say) 5 or 6 bit patterns
> on the disk. I have always heard GCR described as self-clocking, for the
> reason that it does not require "explicit" clock pulses a la FM or MFM,
> instead the data pulses are never far enough apart to cause a loss of sync.

GCR is one particular implementation of an RLL code, in that there are
coding rules that define the minimum and maximum distances between flux
transitions.  MFM is also a RLL code for that matter, however one can
claim that codes like GCR and (2,7) RLL don't have explict clock bits
or slots, just a coding scheme that insures an appropriate transition
distribution.

-- 
George Robbins - now working for,	uucp: {uunet|ihnp4|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@uunet.uu.net
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)