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From: cprice@mips.COM (Charlie Price)
Newsgroups: comp.arch
Subject: moving laser beams, not media
Message-ID: <40535@mips.mips.COM>
Date: 31 Jul 90 21:10:38 GMT
References: <2635@mindlink.UUCP> <10048@pt.cs.cmu.edu>
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Reply-To: cprice@mips.COM (Charlie Price)
Organization: MIPS Computer Systems, Inc.
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In article <10048@pt.cs.cmu.edu> lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes:
>
>Beats me.  However, I would certainly hope that the access mechanism
>involves fewer moving parts than it does now.
>
>Specifically, it would be nice if we scanned a laser beam over the
>media, rather than rotated the media.  Or, at least, did the "head"
>movement that way.  There was hope for this sort of thing, a decade
>ago, and somehow it never happened.  I believe that the best spatial
>modulators had very limited angular effect: perhaps we should be
>trying to shrink the CDROM disks, just as we've been shrinking the
>magnetic disks.
>-- 
>Don		D.C.Lindsay

Do you have any reason to think that "scanning" a memory media at
varying angles could be made to work (economically)?

Don't CDs at least, pretty much *depend* on the read beam being
delivered orthogonally to the media since the effect used
to sense bits is interference in reflected light, not the
amount of light reflected?

A second problem is knowing whether you are really looking at the
information that you think you are.  A disk has a lot of "servo"
data that lets you position to read and lets you know that you are
in the right place.  If you don't have a separate servo surface,
you need some servo data embedded in the track every now and then
to correct on-track positioning, make sure that you are on the
track you think you are on, and maybe resynchronize to a data bit rate.
An embedded servo system depends to some extent on the mass of the
head arm  to keep it from drifting very far off track over short periods.
It also depends on the mass of the rotating spindle to keep it
from changing velocity too quickly and thereby changing the data
bit rate too much over short periods.
A system that depends on mass is immune to some level of vibration
because you don't move the masses quickly enough that you
can't compensate for them.

Suppose that orthogonal illumination isn't a problem,
and you can compensate for light delivered and reflected at an angle.

You have to LOOK at the reflected light, right?
I can't think of any way to do this without moving something
that is at least as large and massive as the optical head itself.
I can imagine an arrangement that would allow you to scan
"straight lines" at high speed, but would require something to
move to get to another line.
Moreover, if you end up with the illuminating beam and the
sensor not rigidly attached together, you have introduced
another spatial relationship that must be managed to make the drive work.

Other problems are storage density and precision.
If you want high density, you need small recording domains
(spots) packed very close together.
Illuminating the media at an angle changes the spot size at
varying angle and changes the size of the spots.
It also increases the absolute position error on the media
surface, so the spots are both larger and spaced further away from
each other.
You get fewer spots per surface.
This is OK, just a tradeoff to be made (and it is made in mag drives).

You need some way to either compensate for or render unimportant
defects in the media (nothing is perfectly flat, perfectly smooth,
identically transparent, ...).

This is my linear thinking speaking.
Probably this will be solved by doing something that is
a posteriori entirely obvious.

Anybody for the optical christmas tree ornament?
You could make spheres (or partial spheres)
that were rigid enough and speherical enough that you could
position an illumination source and sense cluster in the middle
and you would illuminate/reflect orthoginally without moving
the media or moving the laser very much.
You just snap the new media over the laser head and away you go.

I can also imagine spinning a disk like you do today, but
doing a very fast scan along a slighty curved line on the
surface by scanning a laser beam across a radius of the disk.
You would use the disk surface something like a VCR uses the
recording surface of a tape.  You would still have latency
to an individual track, but you could potentially read the entire
disk surface out in a few revolutions.
-- 
Charlie Price    cprice@mips.mips.com        (408) 720-1700
MIPS Computer Systems / 928 Arques Ave. / Sunnyvale, CA   94086