CHAPTER 13

Figure 13.12 CD-RW media layers.
Writable CDs Chapter 13
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Instead of burning an organic dye as with CD-R, the recording layer in a CD-RW disc is made up of a
phase-change alloy consisting of silver, indium, antimony, and tellurium (Ag-In-Sb-Te). The reflective
part of the recording layer is an aluminum alloy, the same as used in normal stamped discs. The
read/write laser works from the underside of the disk, where the groove again appears like a ridge, and
the recording is made in the phase-change layer on top of this ridge.
The recording layer of Ag-In-Sb-Te alloy normally has a polycrystalline structure that is about 20%
reflective. When data is written to a CD-RW disc, the laser in the drive alternates between two power
settings, called P-write and P-erase. The higher power setting (P-write) is used to heat the material in
the recording layer to a temperature between 500° and 700°C (932°–1292°F), causing it to melt. In a
liquid state the molecules of the material flow freely, losing their polycrystalline structure and taking
what is called an amorphous (random) state. When the material then solidifies in this amorphous
state, it is only about 5% reflective. When being read, these areas lower in reflectivity simulate the
pits on a stamped CD-ROM disc.
That would be all to the story if CD-RW discs were read-only, but because they can be rewritten, there
must be a way to bring the material back to a polycrystalline state. This is done by setting the laser to
the lower-power P-erase mode. This heats the active material to approximately 200°C (392°F), which
is well below the liquid melting point but high enough to soften the material. When the material is
softened and allowed to cool more slowly, the molecules realign from a 5% reflective amorphous state
back to a 20% reflective polycrystalline state. These higher reflective areas simulate the lands on a
stamped CD-ROM disc.
Note that despite the name of the P-erase laser power setting, the disc is not ever explicitly “erased.”
Instead, CD-RW uses a recording technique called direct overwrite, in which a spot doesn’t have to be
erased to be rewritten; it is simply rewritten. In other words, when data is recorded the laser remains
on and pulses between the P-write and P-erase power levels to create amorphous and polycrystalline
areas of low and high reflectivity, regardless of which state the areas were in prior. It is similar in
many ways to writing data on a magnetic disk that also uses direct overwrite. Every sector already has
data patterns, so when you write data, all you are really doing is writing new patterns. Sectors are
never really erased; they are merely overwritten. The media in CD-RW discs is designed to be written
and rewritten up to 1,000 times.
CD-RW Speeds
The original Orange Book Part III Volume 1 (CD-RW specification) allowed for CD-RW writing at up
to 4x speeds. New developments in the media and drives were required to support speeds higher than
that, so in May 2000, Part III Volume 2 was published, defining CD-RW recording at speeds from 4x
to 10x. This new revision of the CD-RW standard is called High-Speed Rewritable, and both the discs
and drives capable of CD-RW speeds higher than 4x will indicate this via the logos printed on them.