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From: lauren@RAND-UNIX.ARPA
Newsgroups: net.micro
Subject: CD-ROM speeds...
Message-ID: <9@brl-tgr.ARPA>
Date: Fri, 19-Jul-85 04:04:11 EDT
Article-I.D.: brl-tgr.9
Posted: Fri Jul 19 04:04:11 1985
Date-Received: Sat, 20-Jul-85 12:39:12 EDT
Sender: news@brl-tgr.ARPA
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Actually, "head" motion is not that major a problem in terms of
overall speed, but is still significant.  Consumer CD machines are
spec'd for about four second access, while Sony's professional
machines are spec'd for around two seconds.  Part of the way Sony
achieves this is by moving the disc, not the optical head assembly,
on their professional units.  There is a lot of very finely 
aligned optical equipment (lenses, mirrors, prisms, etc.) that must
sit very close to the disc at the exact track being played.  They
(or the disc) can only be moved so fast safely.  Also very important
is the significant settling and search time associated with these
units.  CD's are actually amazingly "imprecise" in some ways.  The
center spindle hole is always somewhat eccentric from the exact
center of the disc.  The surface of the disc is much less uniform
than is actually required for the primary lens to be within the
correct focal distance at all times for playback.  There are a variety
of other variations as well.  

To get around all this, there are all sorts of servo loops that are
constantly riding the lenses and head/disc assembly up and down
and right and left to keep the disc in "focus."  Whenever you move
the assembly more than one track, it takes a finite period of
time to re-achieve optical lock.  Even more critical is the fact
that to seek on a CD you basically must take the time to read the
disc as you move along--there's no simple servo track you can
latch onto to exactly determine your position independent of the
normal reading cycle.  There are tricks that are pulled to try
speed up this process--but those very tricks are what brought
us to the point of 2-4 second seeks, instead of much longer ones.

The whole CD system was based on the concept that you didn't need
the kind of high-speed positioning accuracy of magnetic media
systems, since you could instead take your time and read the disc
as you went along for positioning and address info.  The limitations
of the optics are very significant.

I don't doubt that it's possible to improve somewhat on the two
second seeks of the Pro players.  But I don't expect the kind of 
speeds that would be required to make CD's really competitive, in terms
of seek times, with conventional hard-surfaced magnetic media.

--Lauren--