Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!apple!uokmax!occrsh!fang!alfred!tous!tarpit!bilver!bill
From: bill@bilver.uucp (Bill Vermillion)
Newsgroups: comp.arch
Subject: Re: CD-ROM documents (was Paperless Office)
Message-ID: <1990Dec8.174708.2391@bilver.uucp>
Date: 8 Dec 90 17:47:08 GMT
References: <1990Dec5.105743.25693@actrix.gen.nz> <1990Dec6.154348.5206@d.cs.okstate.edu> <1990Dec7.020857.18469@amd.com>
Reply-To: bill@bilver.UUCP (Bill Vermillion)
Organization: W. J. Vermillion - Winter Park, FL
Lines: 52

In article <1990Dec7.020857.18469@amd.com> phil@brahms.amd.com (Phil Ngai) writes:
>In article <1990Dec6.154348.5206@d.cs.okstate.edu> norman@d.cs.okstate.edu (Norman Graham) writes:
>|But suppose we reverse the situation and use a constant rotation speed
>|(i.e. constant angular velocity (CAV)). Then the distance between 
>|transition A and transition B becomes a more complex problem. Remember
 
>Well, you ought to know that in the magnetic storage field, there
>are lots of products being shipped with more bits stored on the
>outer tracks than on the inner tracks, and they have very good
>seek times. I just saw a Seagate 89 megabyte IDE disk drive with
>Zone Bit Recording (ZBR) technology. It ranges from 44 sectors/track
>to 30 sectors/track. I assure you they are NOT varying the angular
>velocity!
 
>This kind of thing is both quite doable and is being done, in volume,
>at low cost. It is certainly more complex but that does not mean
>it can not be put into production in commodity products, it just
>means you need clever engineers to develop it.

Except the big difference in the magnetic media is there are many tracks,
with a high tpi density.  In a CD-ROM there is essentialy ONE track, which
is a spiral.

In magnetic media you can say at track xx we will go to an xxxx bit rate
and when we reach track yy we can go to a yyyy bit rate.

The imbedded or dedicated servo controls will ALWAYS position you to the
correct track, that's what they are there for.

Ignoring for the moment that a CD-ROM uses one track, imagine counting the
tracks out on a CD.

In a magnetic drive the position is dependant on the drive itself, and to
take a plater from one drive to another would yield strange results.

In CDs the tracks are independant of the drives.  And if you examine the
specifications for CD mastering you will find that there is a range for
spacing the "tracks".   Therefore you can not position a CD-ROM the same
way .  You could seek to an approximate postion, and then read the data
stream to see where you are, and re-seek according to that.   In magnetic
media you can seek to the exact track each time (of course you do read the
id to make sure!).

Basically it boils down to two facts.  Magnetic drives were designed for
highly dense data storage and are made with considerable precision while CD
media were designed to be highly transportable over less than precision
products.


-- 
Bill Vermillion - UUCP: uunet!tarpit!bilver!bill
                      : bill@bilver.UUCP