Xref: utzoo comp.sys.ibm.pc:15627 comp.periphs:953
Path: utzoo!attcan!uunet!cbmvax!grr
From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.ibm.pc,comp.periphs
Subject: Re: RLL: an intuitive (and somewhat silly) explanation
Message-ID: <3791@cbmvax.UUCP>
Date: 16 May 88 23:15:44 GMT
References: <1255@kodak.UUCP> <638@mccc.UUCP> <216@octopus.UUCP> <650@mccc.UUCP> <218@octopus.UUCP> <11508@mimsy.UUCP> <223@octopus.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 29

In article <223@octopus.UUCP> pete@octopus.UUCP (Pete Holzmann) writes:
> In article <11508@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
> }In article <218@octopus.UUCP> pete@octopus.UUCP (Pete Holzmann) writes
> 
> As far as I know, some self-contained drives (ESDI, SCSI, etc) use an
> entire surface formatted with a clocking pattern; the dedicated surface
> provides clocking, which enables higher density data encoding, since you
> don't have to worry about embedding clock wiggles in your normal data areas.

You are confused here.  Many high-dollar drives do have a "servo head"
that is used to maintain accurate head position and incidentally to
derive a "servo" or "write" clock. 

However in all cases, the "read" clock is derived from the data stream,
not the servo clock.  The servo clock does not represent abosolute bit
position - at the linear track densities used by modern drives this
would be a most difficult feat...

Other drives dispense with the separate head.  The lower density drives
can get by with either open-loop (stepper motor) head positioning or by
using an optical encoder to approximate track position.  Some higher
density drives use specially coded markers and/or differential detection
in the read heads (embedded servo techniques) that yield an error signal
when the head is not centered over a data track.

-- 
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)