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12/05/2002
Pitless Compact Discs
Mark Twain reputedly said, "The report of my demise is greatly
exaggerated." However, on the twainquotes.com Web site, I
found a copy of Twain''s note of May 1897 in which the original
quote appears. The note referred to the serious illness of a cousin
and Twain wrote "The report of my illness grew out of his
illness, the report of my death was an exaggeration." I''m sure
Twain would agree that the modern version of his quote is more
eloquent and what he would have written in his second draft.
Either quote would have applied to situations we''ve encountered
recently. My wife was told that a member of one of her women''s
groups had died some time ago and was surprised that she hadn''t
heard the news earlier. Imagine her greater surprise and shock
when, at a concert in Lincoln Center, we met the "deceased",
quite alive and looking better than ever!
You can also imagine my confusion last week when a college
classmate called. My wife answered the phone and said Jake
seemed unusually concerned about our well being. When I got
on the phone, he paraphrased Mark Twain, telling me that
apparently the report of my death was greatly exaggerated. I, of
course, was completely unaware of my demise although, as I
approach my 75th birthday, I admit to not being as swift as I
once was. Could I have let my expiration date go by unnoticed?
The next day, a gal from the college called and apologized
profusely for the wrong asterisk after my name in a mailing that
listed donors to the annual giving fund. The correct asterisk
would denote a record of consistent giving, not my demise! I
was greatly relieved to find that my faculties were still intact.
Having been resurrected, I''m free to continue my search for the
answers to life''s big questions. Take the compact disc (CD).
When computers came out with the option of burning your own
data on a CD, I could accept this new advance. However, I was
quite surprised when the rewritable CD appeared. Now you can
not only write on a CD but also write over the old data, just like
on a floppy or your VHS tape. To explain my surprise, let''s
review the recording and playing of a CD, a topic we discussed
in an earlier column. The recording of data, music or software
on a CD is accomplished by using a laser to burn tiny pits in an
aluminized disc. These pits, hundreds of times smaller than a
pinprick, and the spots without pits encode the digital 0s and 1s
to store the music or other information.
In your computer or CD player, a laser beam is sequentially
focused on each pit or absence of a pit. Actually, this laser looks
at the other side of the disc, so a pit is now a bump. A
photodetector looks at the laser light reflected from each spot as
the laser beam moves along its track on the disc. The amount of
light reflected is different for a bump vs. a no-bump spot. The
reflected light beams are converted to electrical signals that are
processed to reconstruct your music, video game or whatever.
What has bugged me is that once you burn a pit in the disc, it
should be quite difficult to get rid of that pit and start over again.
With the VHS tapes in our VCRs, we can use the same tape over
and over to record our favorite TV shows because we store the
information as patterns of magnetization. These patterns are
easily changed by recording over them, once you''ve mastered the
art of programming a VCR.
The answer to my question about rewritable CDs is given
succinctly by Gordon Rudd in the "Ask the Experts" column in
the December issue of Scientific American. Visits to the Web
sites cseserv.engr.scu.edu, disctronics.co.uk and boisetech.com
fleshed out my knowledge of the subject. It turns out that I''ve
been behind the times. For a number of years, there have been
alternate approaches to making CD recordings that do not use
pits to encode the information. One method is the so-called dye-
polymer approach. The disc has a smooth layer of a polymer that
incorporates a photosensitive dye. When the dye is heated with a
laser the dye turns from being translucent to dark. Thus, instead
of pits, you have patterns of dark and translucent spots through
which the playback laser light beam can pass. Once again, you
encode the dark and light spots as 0s and 1s in digital format.
This is all well and good and the dye-polymer process is used in
CD-Recordable (CD-R) discs, on which you can record data
from your own computer if you have a "writing" laser. However,
once you''ve written on the disc, the information is there for
keeps. For the CD Rewritable (CD-RW) discs, it''s another
ballgame. Now you''re getting into fancy materials, the so-called
phase-change compounds (let''s call them PCCs). Rudd, in the
Scientific American column, identifies one PCC as an alloy of
silver, indium, antimony and tellurium. Let''s form a thin layer of
this PCC on a CD. The alloy has the interesting property that, if
it is crystalline, it reflects light. However, if it is amorphous,
that is, non-crystalline, it is dark and light is not reflected. If
we hit a spot of the alloy with a laser beam, we can melt that spot
(700 degrees Centigrade) and when it quickly cools it is amorphous.
As before, we use the laser to burn light and dark spots on our
CD. But how to overwrite this light-dark pattern? Here''s where
the neat trick comes in. These amorphous spots will crystallize if
you heat them to around 200 degrees Centigrade, a temperature
high enough to jog the atoms into moving around and ordering
themselves in a crystalline manner. If you have a second, "erase"
laser in your computer, that laser can be used to heat up the
amorphous dark spots to this temperature. With the dark spots
now crystallized, you can melt a new dark spot pattern onto your
disc. You''ve got your read-write disc.
Now that we understand the various CDs, what about the digital
versatile disc (DVD)? (I didn''t know until writing this column
that DVD does not stand for digital video disc - did you?) I find
that there is the DVD, the DVD Video, the DVD-ROM, the
DVD-R, the DVD+R, the DVD-RW, the DVD+RW and the
DVD-RAM. And while a CD stores roughly half a Gigabyte, the
DVD can store anywhere from 4.7 to 17 Gigabytes depending on
whether it''s a single sided, single layer DVD, a single sided,
double layer DVD or a double sided, double layer DVD.
I''m sure you techies out there think all this is pretty simple but
for one about to turn 75, all this is getting beyond my feeble
brain''s capacity to handle. I do think I understand why a DVD
can store so much more information. As I understand it, the dark
spots are much smaller than are those in the everyday CD. You
can pack more of these smaller pits or dark spots on a DVD.
Because of the smaller spot size, the laser light used to read the
spots has to be of a shorter wavelength. This means that, instead
of the infrared lasers used in our conventional CD players, lasers
emitting light in the visible red range will be needed.
This makes me happy since I spent a long time working at Bell
Labs on red light emitting diodes. At least I understand how the
diode lasers used to read the CDs and DVDs work. OK, you''re
right, I''d probably have to go back and read my old papers to
refresh my memory. Isn''t it great that we have these discs and
the Internet to store all this information that we once thought we
would never forget? It''s a great time to be alive!
Allen F. Bortrum
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