Handbook for Sound Engineers

Optical Disc Formats for Audio Reproduction and Recording 1143

contains data describing the recorded tracks, a tempo-
rary table of contents, and track skip information. When
the disc is finalized, this data is transferred to the TOC.
On the innermost radius, the PCA (power calibration
area) is used by the recording laser to make an optimal
power calibration test recording to determine proper
laser recording power. A recording is complete when a
lead-in area (with TOC), user data, and lead-out area are
written. A maximum of 99 tracks can be recorded on a
disc. Because the PMA and PCA areas are inside the
normal lead-in radius, conventional CD players do not
read them.
The CD-R standard defines both single-session and
multisession recording (a session is a recording with
lead-in, data, and lead-out areas). In single-session
recording, sometimes called disc-at-once recording, an
entire disc program is recorded without interruption.
Track-at-once recording allows single or multiple tracks
to be written in a session. Recorders using track-at-once
can also write a single-session CD-R. In multisession
recording, sessions can be recorded one or a few at a
time. Tracks can be written singly and recording can be
stopped after each track. Separate recording sessions are
allowed, each with its own lead-in TOC, data, and
lead-out areas. Track-at-once recorders allow both
multisession and single-session recording. In
track-at-once recording, multiple tracks can be written
to a session, adding data one track at a time; no lead-in
or lead-out is written until the session is closed. CD
audio players can read only the first session on a multi-
session disc. A partially recorded disc can be played on
the CD-R recorder but cannot be played on a CD audio
player until the session ends when the final TOC and
lead-out areas are recorded. Using the CD portion of the
universal disk format (CD-UDF), CD-R recorders can
perform packet writing; this allows small amounts of
data to be written efficiently without high overhead.
Data in a file can be appended and updated without
rewriting the entire file.

30.4.3 CD-RW

The CD Rewritable (CD-RW) format allows data to be
written and read, and erased and rewritten. The format
is technically named CD-E and is described in the
Orange Book Part III standard. A CD-RW drive can
read, write, and erase CD-RW media, read and write
CD-R media, and read CD-ROM and CD audio media.
Thousands of rewrite cycles are possible. Any data can
be written, including computer programs, text, pictures,
video, audio, or other files. A CD-RW disc has five lay-
ers built on a polycarbonate substrate: a dielectric layer,

a recording layer, another dielectric layer, a reflective

aluminum layer, and a top acrylic protective layer, as

shown in Fig. 30-12. As in CD-R, the writing and read-

ing laser follows a pregroove spiral track. However, the

CD-RW format employs a phase-change recording

method, using materials that exhibit a reversible crystal-

line/amorphous phase change when recorded at one

temperature and erased at another. In most cases, a

high-reflectivity (crystalline) to low-reflectivity (amor-

phous) phase change is used to record data, and the

reverse to erase. Data is recorded by heating an area of

the crystalline layer to a temperature slightly above its

melting point and cooled rapidly. The area is amorphous

when it solidifies, and the decreased reflectivity is

detected by a low power reading laser. Because the

crystalline form is more stable, the material will tend to

change back to this form. Thus when the area is heated

to just below its melting temperature and cooled slowly,

it returns to a crystalline state, erasing the data. In some

cases, the recording layer comprises gallium antimonide

and indium antimonide; other systems use tellurium

alloyed with elements such as germanium and indium.

The dielectric layers comprise silicon, oxygen, zinc, and

sulfur; they control the optical response of the media

and increase the efficiency of the laser by containing the

heat in the recording layer. The dielectric layers also

thermally insulate and protect the pregroove, substrate,

and reflective layer.

The reflectivity of CD-RW discs is only about 15%

(amorphous state) and 25% (crystalline). Discs will not

play in most CD audio players or CD-ROM drives;

however, many DVD players do play CD-RW discs.

Multiread drives are capable of reading lower reflec-

tivity CD-RW discs. They use an AGC (automatic gain

control) circuit to boost the gain of the signal output

from the photodiodes and compensate for the lower

reflectivity and decreased signal modulation. CD-RW

discs carry a code that identifies them as CD-RW discs

to the player. CD-RW drives are commonly found as

Figure 30-12. CD-RW disc construction showing

embedded recording and dielectric layers.

Protective layer Rewritable

Reflective layer

Recording layer

Polycarbonate

disc substrate

Pregroove

Dielectric layers