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A Detailed Overview
CD-ROM, CD-R, CD-RW
Compact disc computer
media comes in three principal forms: CD-ROM,
CD-R & CD-RW. All compact discs are made of optical grade
polycarbonate that is injection molded into a disc 120mm in
diameter—then coated with a reflective metal layer which enables a
laser
beam to “read” a digital data stream encrypted to the disc. Compact
disc
media are very similar but each is uniquely different and designed to
serve different purposes.
CD-ROM is a pre-recorded disc, which is to say the data which the CD
contains has been permanently embedded into the polycarbonate, in the
form of “pits and lands” at the time it is injection molded. This data
cannot
be changed or appended at a later time.
CD-ROM begin as a “glass master” from which a “father” is made by an
electroforming process—the father is a reverse of the glass master.
From the father a “mother” is made and subsequent to the mother one or
more ‘stampers’ are produced. The stampers are used for injection
molding the polycarbonate into discs which are sputtered in a vacuum
chamber with an aluminum reflective layer, barely 50nm thick, which is
then sealed with a protective lacquer spincoat and then decorated with
a
silk-screen printed label.
The most familiar form of prerecorded compact disc is the CD-Digital
Audio format which, in fact, launched the CD revolution in the early
1980’s after its development by Philips and Sony for consumer
electronics products. These CDs are mass produced, in minimum lots of
1000 units in a manufacturing process that culminates in the delivery
of
finished discs about three weeks after the order is placed and the
pre-mastered data set is delivered to the factory. Because of long
lead
times & large manufacturing quantities, these discs are relatively
inexpensive, often less than $1.00 each, depending on quantity and
lead
time.
CD-R is a recordable compact disc and sometimes referred to as a
Write-Once CD. CD-R differs significantly from CD-ROM (Read Only
Memory). Although both are manufactured using the same process,
CD-R is subject to more stringent Quality Control procedures at a
plant
that adheres to very tightly controlled environmental standards. The
result
is a disc suitable for both data distribution & archiving, with a
useful life of
75-100 years, compared to that of stamped CDs which have a design life
of just 3-5 years.
The principal difference between a CD-Recordable disc and CD-ROM is
the addition of an amorphous organic dye between the polycarbonate
substrate and the reflective metalized layer. This is the recording
layer; it
is typically a shade of blue or yellow-green depending on the
formulation
and the reflective metal above it. Until recently gold—99.995% pure,
has
been used because it is an inert metal and resists the corrosive
effects of
the organic dye. However, advances in dye formulations have resulted
in
stabilized dyes that permit the use of less expensive silver, also
99.995%
pure, which yields a bonus of increased reflectivity.
Uniformity of the dye layer, which is spin-coat applied to the disc
and
level of refinement in manufacturing processes determines the optical
recording characteristics, durability, read interchangability and
useful life
of the disc. Data is encrypted on CD-R by a recording drive or
“burner”
which uses a laser diode that burns an optical pit (as opposed to a
physical pit as in prerecorded CDs) on the disc. CD-R are sometimes
referred to as a “one-off” due to the fact that they are a first
generation
disc. Stamped CD-ROM are typically fifth generation discs and could
actually be referred to as a “five-off disc”—but are not.
CD-R is an enabling technology, more so than stamped CDs, because
they allow an end-user to produce them one at a time or a CD
production facility to burn several thousand on demand–eliminating
waste, obsolescence and loss. Due to it's flexibility and convenience,
CD-R is the fastest growing optical disc media, averaging more than
100% yearly growth since 1994. This high growth demand has pushed
manufacturers to produce CD-R at a faster pace and reduced the cost of
high quality blank CD-R to less than $1.00 per disc.
CD-RW is principally a short term
storage product. It requires both a
different type of burner than CD-R & a different type of reader than
either
CD-ROM or CD-R. As a result, most CD drives cannot read a CD-RW
disc which in essence makes this type of media impractical for either
portable data archive or distribution applications.
CD-RW media is manufactured using much the same process as CD-R
with one notable exception: the “dye” layer used in CD-RW is actually
a
dual layer combination of dielectric crystalline/amorphous dye
compounds. This compound changes its physical attributes when heated
to specific temperatures by the laser beam in the recording process to
form the requisite optical pits and lands or be “erased” and
rewritten. To
do this, the CD-RW recording system utilizes a more flexible laser
device
than those used in CD-R writers.
The difference in media types is further distinguished by their
reflectivity,
which enables a disc to be read. Due to the RW compound’s general
characteristics & recording method, it reflects less light than CD-ROM
and CD-R which in turn requires special multi-read drives. Multi-read
drives read all three CD formats and as a result are generally more
expensive than standard CD readers. Nonetheless, this difference alone
has prevented CD-RW from receiving universal adoption since most of
the 500 million drives currently installed in computers can't
accommodate
its low reflectivity—about 25% that of CD-R & CD-ROM.
Interestingly, CD-RW drives can burn both CD-R & CD-RW whereas
CD-R burners cannot write to CD-RW media. This is credited to the
drive’s capability of changing the wavelength of the laser recording
beam
when the drive senses which type of media has been inserted into it.
Conversely, CD-R drives do not possess this capability & the drive’s
optical pickup cannot recognize (“see”) the CD-RW disc due to it's low
reflectivity.
CD-RW media is rewritten by one of two different methods, both require
that the disc is first initialized just as CD-R must be initialized
for Direct
CD usage with the UDF format. The first and least desirable, requires
that the entire disc be reformatted prior to rewriting. The other
allows
modification to the VTOC and lead-in area of the disc which enables a
rewrite similar to the method used by a computer’s hard disk drive.
The
practical constraints of using CD-RW is the media/drive’s slow seek
and
access performance compounded by the fact that the installed base of
CD-RW readers is essentially nonexistent
On the bright side, CD-RW media costs have plummeted from $25.00
per disc when they were introduced in 1997 by the joint efforts of
Ricoh &
Yamaha. Today, certified data grade CD-RW is readily available at less
than $8.00 per disc in low unit quantities depending on brand &
packaging configuration. However, it is safe to assume that CD-RW will
languish indefinitely at the periphery of the mainstream swell created
by
CD-R.
In conclusion, compact disc publishers use only CD-ROM & CD-R. By
design, CD-ROM is suited for long runs of thousands of discs (in the
big
picture 10,000 discs is a short run) and long turn times (usually
about two
weeks or more) with the benefit of low unit costs. However, as the run
length and turn times shorten, unit costs increase dramatically. As is
the
case with any type of manufactured product the “time is money” axiom
applies.
CD-R, due to its high versatility, low cost and value as an on-demand
publishing media, has all but supplanted CD-ROM in many instances.
And because it can be silk-screen printed and replicated in a matter
of
hours, as opposed to days for CD-ROM, publishers can take delivery of
a few hundred to several thousand discs in just 24-48 hours at costs
that
are in line with those of their stamped brethren.
CD-R has shed its glittery tech-toy image and when it is duplicated by
an
established and reputable service bureau can deliver performance on
par with, and in many cases better than, stamped CD-ROM. In order to
minimize any risk of poor disc performance or media interchangability
issues, compact disc publishers need to learn everything they possibly
can about prospective vendors, i.e. how and what kinds of tools they
use
to qualify, handle, duplicate, label and QC both blank and duplicated
CD-R media. It is imperative that you do this before engaging anyone's
services—this also applies to CD-ROM vendors. |
