CHAPTER 13

752 Chapter 13 Optical Storage
CD-RW
Beginning in early 1996, an industry consortium that included Ricoh, Philips, Sony, Yamaha,
Hewlett-Packard, and Mitsubishi Chemical Corporation announced the CD-RW format. The design
was largely led by Ricoh, and they were the first manufacturer to introduce a CD-RW drive in May of
1996. It was the MP6200S, which was a 2/2/6 (2x record, 2x rewrite, 6x read) rated unit. At the same
time, the Orange Book Part III was published, which officially defined the CD-RW standard.
Since that time, CD-RW drives have pretty much replaced CD-R–only drives in the market today,
mainly because CD-RW drives are fully backward compatible with CD-R drives and can read and write
the same CD-R media with the same capabilities. So, a CD-RW drive can also function as a CD-R
drive. CD-RW discs can be burned or written to just like CD-Rs; the main difference is that they can
be erased and reburned again and again. They are very useful for prototyping a disc that will then be
duplicated in less expensive CD-R or even stamped CDs for distribution. They can be rewritten at least
1,000 times or more. Additionally, with packet-writing software, they can even be treated like a giant
floppy disk, where you can simply drag and drop or copy and delete files at will. Although CD-RW
discs are about twice as expensive as CD-R media, CD-RWs are still far cheaper than optical cartridges
and other removable formats. This makes CD-RW a viable technology for system backups, file archiving,
and virtually any other data storage task.
Note
The CD-RW format originally was referred to as CD-Erasable, or CD-E.
Four main differences exist between CD-RW and CD-R media. In a nutshell, CD-RW discs are
■ Rewritable
■ More expensive
■ Slower when writing
■ Less reflective
Besides the CD-RW media being rewritable and costing a bit more, they also are writable at about half
(or less) the speed of CD-R discs. This is because the laser needs more time to operate on a particular
spot on the disk when writing. They also have a lower reflectivity, which limits readability in older
drives. Many standard CD-ROM and CD-R drives can’t read CD-RWs. However, MultiRead capability is
now found in virtually all CD-ROM drives of 24x speed or above, enabling them to read CD-RWs
without problems. In general, CD-DA drives—especially the car audio players—seem to have the most
difficulty reading CD-RWs. So, for music recording or compatibility with older drives, you should
probably stick to CD-R media. Look for the MultiRead logo on a CD-ROM drive, indicates the capability
to read CD-RW.
CD-RW drives and media use a phase change process to create the illusion of pits on the disc. As with
CD-R media, the disc starts out with the same polycarbonate base with a wobbled pre-groove molded
in, which contains ATIP information. Then, on top of the base a special dielectric (insulating) layer is
spin-coated, followed by the phase change recording layer, another dielectric layer, an aluminum
reflective layer, and finally a UV-cured lacquer protective layer (and optional screen printing). The
dielectric layers above and below the recording layer are designed to insulate the polycarbonate and
reflective layers from the intense heat used during the phase-change process.
Figure 13.12 shows the CD-RW media layers, along with the pre-groove (raised ridge from the laser
perspective) with burned pits in the phase change layer.