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From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: CD-ROM documents (was Paperless Office)
Message-ID: <3019@crdos1.crd.ge.COM>
Date: 7 Dec 90 20:54:34 GMT
References: <1990Dec5.105743.25693@actrix.gen.nz> <1990Dec6.154348.5206@d.cs.okstate.edu> <1990Dec7.020857.18469@amd.com>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 37

In article <1990Dec7.020857.18469@amd.com> phil@brahms.amd.com (Phil Ngai) writes:

| 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!

  Your facts are correct, but they don't apply here. There is a, as in
one, as in single, track on CD-ROM, and the linear read rate is
constant, because that's the standard.

  In ZBR, if you keep the RPM the same, and put more data on the outer
tracks, your data rate changes, at least at the head.

  When you look for track N on ZBR it is always the same distance from
the hub, because it's circular. In CD-ROM you seek to the right
distance, more or less, then wait until *the* track comes squarely under
the head, then slide along the track to the right data. And since the
time between marks is used for encoding, and it's written with constant
speed and varying RPM, it must be read that way.

  I believe you're impying that there's a better way, and you're right,
but the fact that these things are already out, have a standard, and are
produced in mass quantities to make them cheap is an argument for
staying with them, or one of the other established optical standards
which are closer in performance (and price) to magnetic disk.

  Victor used ZBR on it's floppy disks, which was inovative, but
Shugart got rich with tired but cheap tech. Design can't ignore cost. A
new design *now* would have to be very good to get people to pay more
for it, or to make the startup (low volume) cost competitive.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
    VMS is a text-only adventure game. If you win you can use unix.