CHAPTER 13

Photo detector
Laser diode
Lenses
Beam splitter
Servo motor
Figure 13.3 Typical components inside a CD-ROM drive.
What Is a CD-ROM? Chapter 13 693
When first introduced, CD-ROM drives were too expensive for widespread adoption. In addition,
drive manufacturers were slow in adopting standards, causing a lag time for the production of CD-
ROM titles. Without a wide base of software to drive the industry, acceptance was slow.
After the production costs of both drives and discs began to drop, however, CD-ROMs were rapidly
assimilated into the PC world. This was particularly due to the ever-expanding size of PC applications.
Virtually all software is now supplied on CD-ROM, even if the disc doesn’t contain data representing a
tenth of its potential capacity. As the industry stands now, if a software product requires more than
one or two floppy disks, it is more economical to put it on a CD-ROM.
For large programs, the advantage is obvious. The Windows 98SE operating system would require
more than 75 floppy disks, an amount certainly nobody would want to deal with.
Track and Sectors
The pits are stamped into a single spiral track with a spacing of 1.6 microns between turns, corresponding
to a track density of 625 turns per millimeter, or 15,875 turns per inch. This equates to a
total of 22,188 turns for a normal 74-minute (650MiB) disc. The disc is divided into six main areas
(discussed here and shown in Figure 13.4):
■ Hub clamping area. The Hub clamp area is just that: a part of the disc where the hub mechanism
in the drive can grip the disc. No data or information is stored in that area.
■ Power calibration area (PCA). This is found only on writable (CD-R/RW) discs and is used only by
recordable drives to determine the laser power necessary to perform an optimum burn. A single
CD-R or CD-RW disc can be tested this way up to 99 times.
■ Program memory area (PCA). This is found only on writable (CD-R/RW) discs and is the area
where the TOC (table of contents) is temporarily written until a recording session is closed.
After the session is closed, the TOC information is written to the lLead-in area.
■ Lead-in. The lead-in area contains the disc (or session) TOC in the Q subcode channel. The TOC
contains the start addresses and lengths of all tracks (songs or data), the total length of the program
(data) area, and information about the individual recorded sessions. A single lead-in area
exists on a disc recorded all at once (Disc At Once or DAO mode), or a lead-in area starts each
session on a multisession disc. The lead-in takes up 4,500 sectors on the disc (1 minute if measured
in time, or about 9.2MB worth of data). The lead-in also indicates whether the disc is
multisession and what the next writable address on the disc is (if the disc isn’t closed).