AOH :: G12.TXT|
Some stuff on CD-Rom
SUBMITTED BY: THE WIZARD
TITLE: CD-ROM Quality Control StillProblem, Say Manufacturers
"Reliability is having a disk that works when you need it to,"
says Tom Brown, whose company stamps out CD-ROMs. Unfortunately,
Brown hasn't found this maxim to be the case in the CD-ROM
industry: "We did a test of 12 different CD-ROM disks and 10
different drives," he says. All were supposed to be High Sierra-
standard CD-ROMs, and every disk workedsome of the time, but
according to Brown, no disk worked on every drive, and no drive
worked with every disk. Clearly, the bg CD-ROMs
still has a long way to go.
According to Brown, who's the CD-ROM development manager at Shape
Optimedia (Sanford, ME), all those CD-ROM drives should be
compatible. Most of the problems show up because of the design
differences between different brands of drives. Brown told an
audience at the Rothchild Optical Media Manufacturing Conference
in San Francisco that even though all the drives were designed to
read CD-ROMs meeting the same specification, "there's still a
sensitivity of certaidives at particular ends of the spec."
Since there's no way to prevent the drive manufacturers from
trying different reading techniques, it's up to CD-ROM
manufacturers to avoid those ends, says Brown. "In order for CD-
ROM to become a thriving medium, it has to be reliable. We're
looking for a disk that gives a low number of data seek retries,
and we don't want any uncorrecable errors or failures."
Part of the reliability problem stems from the common view that a
CD-ROM is really just another kinf audio compact disc,
according to Michael Dunn of Klockmer Ferromatik Desma (Erlanger,
KY). Dunn says that's wrong; because CD-ROM disks must be
created to much more exacting specifications, it's time to start
viewing CD-ROM "as an extension of advanced optical disk
specifications rather than audio disk specifications."
Dunn says the biggest quality-control problems involve the level
of bifringence--optical imperfections caused by stress in the
molding process. A CD (or CD-ROM) is typically made cing
plastic into a mold that carries the pattern of pits that form a
digital signal. Too much stress in the molding process can
create irregular density patterns in the disc, or a surface
that's irregular or simply isn't flat.
A poorly made conventional LP might have poor sound quality
because of a cheap molding process. A CD-ROM with the same
problems would simply be unreadable on some--or all--drives.
Fortunately, many of the processes developed for making high-
density WORM and other optical diare migrating to CD-ROM
manufacturing. By carefully controlling such variables as the
injection pressure and the temperature of the mold, manufacturers
say they are reducing the bifringence problem.
That can't happen any too soon. Apple, Tandy, and other
manufacturers will have CD-ROM drives on store shelves by the end
of 1988 and, as Brown put it, "the number of manufacturers gets
bigger every week."
microbytes/features #159, from microbytes, 300chars, Mon Jul 25 15:39:23 1988
TITLE: Will Tandy's Erasable CD Make the CD-ROM Grade?
In April, Tandy announced that its researchers had developed an
erasable optical disk that was compatible with standard compact
discs and CD-ROMs. With its news of the THOR-CD (Tandy High-
Intensity Optical Recording Compact Disc), Tandy succeeded in
upstaging its own announcement of a PS/2-compatible computer.
But some analysts immediately expressed doubts about the Tandy
system. Rumors began to circulate that a THOR-CD could only be
erased and rerecorded at most a half-dozen times, and that Tandy
had simply licensed the technology from Optical Data, Inc., of
However, according to Donald Mattson, president of Optical Data,
it simply isn't so. Mattson told an audience at the Rothchild
Optical Media Manufacturing Conference in San Francisc that
Tandy licensed Optical Data's patents, but the Tandy researchers
also created their own work. "It didn't appear they wing
much in the industry," says Mattson, "but they were working with
us for two years before the THOR-CD announcement in April. They
were also the first to link rewritable CD with computer data, and
to design a system that will play on any other CD player. To my
knowledge, no one else can make that claim."
On a conventional CD, information is represented by a series of
pitsthat are stamped into the disk. The pits are then read by a
laser as digital information. Mattson explained that the THOR-CD
system can be read the same way, but is fundamentally different:
rather than a pit, a 7 milliwatt laser creates a bump, which can
then be read with a lower power laser (between 1 and 2
The disk's two-layer dye-polymer structure has differentially
dyed layers; the top layer allows laser light at a wavelength of
830 nanometers to pass through to the bottom layer. When the 7
milliwatt laser hits the surface, it passes through the top layer
and heats the elastomer bottom layer, which expanand creates
the bump. A lower-powered laser in the same range will simply
read the bump. Erasing is done with a 780nm laser, which heats
the top layer; the surface relaxes, and the bump disappears.
Mattson says the dye-polymer system was originally designed to
compete with magneto-optical media, and that may be where rumors
of limited erasability come in. While magneto-optic disks can be
erased and rewritten almost infinitely, the dye-polymer system is
limited to somewhat less than 10,000 erase-anrite cycles--a
very real limit, but a far cry from the half-dozen erasures that
some skeptics suggested. (According to Mattson, the dye polymer
system is much less expensive and is much more resistant to
corrosion than magneto-optical disks, but offers slower read
times and is about two years away from commercial availability.)
Once rewritable CDs are available, both Optical Data and Tandy
believe that they will become the medium of choice for short-run
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