From: rhoades@ll.mit.edu (Captain Chaos)
Newsgroups: rec.audio
Subject: Re: questions on new digital formats
Date: 16 Dec 92 16:33:48 GMT
Organization: MIT Lincoln Laboratory

In article <lisfn9INNgm@exodus.Eng.Sun.COM> drp@reed.Eng.Sun.COM (Darrell Parham) writes:
>I have some questions about the new recordable digital formats
>(MD, DCC, DAT):
>
>- The claim is that you can play analog cassettes in DCC
>  machines. Can you also record analog cassettes in DCC
>  machines? I would like the capability of recording
>  cassettes for friends who might not have DCC yet?
>
	I don't think so, though I would image that it could be added.

>- I am under the impression that a magnetic format like
>  tape will degrade over time, while this isn't much of
>  a problem with an optical format like CD's.
>  Do people generally agree with this?
>  How do the Mini-Disks fare in this respect?
>  I hear they use a combination of magnetic and
>  optical recording (which I have a poor understanding of)
>  Could one expect them to degrade over time very much?
>  Would this differ between pre-recorded and home-recorded
>  mini-disks (is the same mechanism used) ?
>  Is degradation less of an issue with any digital format
>  than it is with an analog format?

	I think magnetic degradation, which I don't know a lot
	about, is indeed more significant to analog recordings
	than digital.  A weak bit equals a strong bit in the
	eyes of a computer -- imagine if over time you lost
	some of the information in a computer file!  With an
	analog recording, however, I think you can actually lose
	some of the signal.  With magneto-optical disks, the magnetic
	material is protected (encased in plastic) and never comes
	in contact with anything, which should improve its longevity
	significantly.

	I'll add a quick (?) description of how magneto-optical (MO) works
	since it's pretty neat.  The capacity of conventional magnetic
	disks is limited by how much a magnetic field can be "focused",
	which limits the density of bits on the disk.  A laser can be
	focused into a smaller spot, on the other hand.  So an MO disk has
	data stored magnetically but read optically.  To read a bit, polarized
	laser light is shined on the disk at an angle.  This light then
	bounces off at one polarization angle for a plus field and one for a
	minus field.  The returning light passes through a filter which either
	passes or blocks (depending on polarization angle of the light)
	the light and volia!, you have an optically detectable difference.
	The breakthrough in this technology was making the difference in
	the two returning angles big enough to be detected outside a lab
	(it was tenths of a degree, now is degrees).  Writing the data
	also relies on optical properties.  The magnetic material in the
	disk will only change if it is at a high temperature (300+ C I think),
	unlike computer disks (so you can put a magnet safely on an MO
	disk so long as it isn't in your oven at the time).  To write a bit, a
	big unfocused magnetic field is applied near the disk, and a separate
	laser is used to briefly heat up the bit you want to write and
	voila!, you've written a magnetic bit much smaller than your
	hard-drive could.

	Note that since this technology is complicate (ie expensive), Sony
	went to the trouble of making an MD player capable of reading
	conventional optical disks (pits and lands like a CD), so pre-recorded
	stuff can be made cheaper.  Making a player which reads both, rather
	different formats, was a bit of a trick.

Andrew
rhoades@ll.mit.edu