Xref: utzoo comp.arch:6763 alt.next:226
Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!uw-beaver!cornell!rochester!pt.cs.cmu.edu!cat.cmu.edu!ns
From: ns@cat.cmu.edu (Nicholas Spies)
Newsgroups: comp.arch,alt.next
Subject: Re: The NeXT Problem
Keywords: Curie point, local heating
Message-ID: <3376@pt.cs.cmu.edu>
Date: 22 Oct 88 16:55:08 GMT
References: <26435@ucbvax.BERKELEY.EDU> <7774@gryphon.CTS.COM> <9287@bigtex.cactus.org> <1359@cpoint.UUCP> <1490@imagine.PAWL.RPI.EDU>
Sender: netnews@pt.cs.cmu.edu
Organization: Carnegie-Mellon University, CS/RI
Lines: 37

In article <1490@imagine.PAWL.RPI.EDU> brazil@pawl18.pawl.rpi.edu (Timothy E. Onders) writes:
>The Cannon drive is not a true Optical drive. It is an optically enhanced
>magnetic storage device. Through the use of lasers, the tracks can be made as
>small as the width of the beam. The actual reading a writing is done with
>a magnetic field, much the same as it is done in a normal hard drive. ...

In fact, opti-magetic discs are written by raising the disc coating to its
Curie point in a magetic field, which alters the polarization of light
reflected off (and/or transmitted through?) the coating according to the
direction of the magnetic field is was written under.  Reading is done
optically, not magnetically, by noting the changes of polarization.

>a magnetic field, much the same as it is done in a normal hard drive. So
>far, Tandy has been the only manufacturer to come close to a true erasable
>optical media. Theirs seems to be based on phase change media, which changes
>from clear to opaque and back depending on the power and wavelength of the
>laser beam it is exposed to.
...
>years ago. Then, as now, the phase change medium seems more promising, since
>it allows for a lighter R/W head since it is not necessary to have a magnetic
>coil, as well as the optics, not to mention the fact that, as Tandy suggested,
>Phase change media could be read by present read-only devices, since the
>data is stored in much the same way as on a conventional CD.

Conventional videodiscs and CD's store data as a series of pits of varying
length, of a constant depth of 1/4 wavelength deep, so that when they are
illuminated by the reading laser, light reflected from the bottom of the
pits is out of phase by 1/2 wave, yeilding destructive interference so
the pits appear black (or light against a black background?). In any case,
current laserdisc technology is reflective and does not depend on the
relative opacity of the medium.


-- 
Nicholas Spies			ns@cat.cmu.edu.arpa
Center for Design of Educational Computing
Carnegie Mellon University