1142 Chapter 30
defines two types of blocks: Form 1 for computer data
and Form 2 for compressed audio/video data. The
former provides a 2048-byte user area, and the latter
provides 2324 bytes. An XA track can interleave Form
1 and 2 blocks, but Red Book data cannot be placed in
an XA track. Some products are dedicated to specific
types of CD-ROM/XA discs; the Video CD is an
example of this.
Hybrid audio/data CD formats such as CD Extra and
Mixed Mode CD combine different format types (such
as CD audio and CD-ROM/XA) on one disc. A CD
Extra disc contains CD audio data in the first session,
and CD-ROM-XA mode 2 data in the second session. In
Mixed Mode CDs, ROM data is placed in track 1, and
CD audio data is placed in subsequent tracks. To make
sure an audio player does not access the ROM track, a
pregap may be used so that ROM data is placed after
the disc table of contents (TOC), but before the first
music track. CD-ROM data is placed between Index 0
and Index 1 of Track 1, while the music starts at Track
1, Index 1. An audio player thus skips the data, starting
playback at the first music track. However, the pregap
area is not accessible to all drive software.
Unlike the CD audio standard, the CD-ROM stan-
dard does not stipulate how content is defined. Subse-
quently the ISO/DIS 9660 standard was devised; it
specifies how computer data is placed on a CD-ROM;
to read the data, the computer operating system must
read the ISO 9660 file structure. Content on CD-ROM
discs can be authored for multiple platforms; however,
executable files can only run on the appropriate plat-
form. For example, hybrid CD-ROM titles can be
played on IBM and Macintosh platforms. The different
data types are physically partitioned on the disc surface.
30.4.2 CD-R
The CD recordable (CD-R) format allows users to per-
manently record audio or other data to a CD. The format
is technically named CD-WO (Write Once), as codified
in the Orange Book Part II. CD-R discs that carry audio
and nonaudio data prior to CD replication can be writ-
ten with the PMCD (premastered CD) format; the disc
contains index and other information. CD-R discs with
up to 80 minutes of playing time (about 700 Mbytes)
are available.
CD-R discs are physically different from Red Book
CDs. CD-R discs are manufactured with a pregrooved
spiral track with 0.6μm width and 1.6μm pitch; it
guides the recording laser along the track. The
pregroove is physically modulated with a ±0.03μm
sinusoidal wobble with a frequency of 22.05 kHz.
Recorders use the wobble to control the disc CLV rota-
tion speed. The 22.05 kHz groove wobble is also
frequency modulated with a ±1 kHz signal; this creates
an ATIP (absolute time in pregroove) clocking signal.
CD-R discs are manufactured on a polycarbonate
substrate, and contain a metal (e.g., gold or silver)
reflective layer, an organic dye recording layer, and a
top protective layer. The recording layer is placed
between the substrate and reflective layer as shown in
Fig. 30-11. Together with the reflective layer it provides
a reflectivity of about 73%. A writing laser with wave-
length of 775 nm to 795 nm passes through the polycar-
bonate substrate and heats the recording layer to
approximately 250°C, causing it to melt and/or chemi-
cally decompose to form a depression or mark in the
recording layer. Simultaneously, the reflective layer is
deformed. These depressions or marks have a decreased
reflectivity. During readout, the same laser, reduced in
power, is reflected from the data surface and its
changing intensity is monitored.Either cyanine or phthalocyanine organic dye poly-
mers are often used for the recording layer. They are
designed to absorb light at about 780 nm. Cyanine dye
has a relatively broad range of sensitivity to light and is
generally reliable in a wide range of recorders and laser
powers and writing speeds. Phthalocyanine-based
media are generally said to have greater longevity
because it is less sensitive to ordinary light and stable.
However, this lower sensitivity may result in a small
power margin for the writing laser. Thus the writing
speed and laser power must be more carefully
controlled. In some cases, metallized azo dye is used as
the recording layer in CD-R media. Organic dye layers
are affected by aging. The dye layer will deteriorate
over time because of oxidation, material impurities, or
exposure to ultraviolet light. CD-R discs will play back
on most CD audio players, but the reduced data layer
reflectivity can cause playback incompatibility.
Two areas are written to the inner radius
(22.35–23 mm) of CD-R discs, both inside the Red
Book lead-in radius. The PMA (program memory area)Figure 30-11. CD-R disc construction showing embedded
recording layer.Protective layer Write - OnceReflective layer
Recording layerPolycarbonate
disc substratePregroove