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."
 
--Frank Hayes
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microbytes/features #159, from microbytes, 300chars, Mon Jul 25 15:39:23 1988
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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 
Beaverton, Oregon.
 
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 
milliwatts).
 
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 
CD-ROMs.
 
--Frank Hayes