Drive Mechanical Operation
An optical drive operates by using a laser to reflect light off the bottom of the disc. A photo detector then reads the reflected light. The overall operation of an optical drive is as follows
- The laser diode emits a low-energy infrared beam toward a reflecting mirror.
- The servo motor, on command from the microprocessor, positions the beam onto the correct track on the disc by moving the reflecting mirror.
- When the beam hits the disc, its refracted light is gathered and focused through the first lens
beneath the platter, bounced off the mirror, and sent toward the beam splitter.
- The beam splitter directs the returning laser light toward another focusing lens.
- The last lens directs the light beam to a photo detector that converts the light into electric
- These incoming impulses are decoded by the microprocessor and sent along to the host computer as data.
When introduced, CD-ROM drives were too expensive for widespread adoption. After the production costs of both drives and discs began to drop, however, CDs 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 optical media, even if the disc doesn’t contain data representing a tenth of its potential
Tracks and Sectors
On the traditional 74-minute CD, 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 typical 74-minute (650MiB) disc. Current 80-
minute CDs gain their extra capacity by decreasing the spacing between turns.
The disc is divided into six main areas
¦ 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 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 (PMA)—This is found only on writable 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 lead-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).
¦ Program (data) area—This area of the disc starts at a radius of 25mm from the center.
¦ Lead-out—The lead-out marks the end of the program (data) area or the end of the recording
session on a multisession disc. No actual data is written in the lead-out; it is simply a marker. The first lead-out on a disc (or the only one if it is a single session or Disk At Once recording) is
6,750 sectors long (1.5 minutes if measured in time, or about 13.8MB worth of data). If the disc
is a multisession disc, any subsequent lead-outs are 2,250 sectors long (0.5 minutes in time, or
about 4.6MB worth of data).
The hub clamp, lead-in, program, and lead-out areas are found on all CDs, whereas only recordable CDs (such as CD-Rs and CD-RWs) have the additional power calibration area and program memory
area at the start of the disc.
Officially, the spiral track of a standard CD starts with the lead-in area and ends at the finish of the
lead-out area, which is 58.5mm from the center of the disc, or 1.5mm from the outer edge. This single
spiral track is about 5.77 kilometers, or 3.59 miles, long. An interesting fact is that in a 56x CAV (constant angular velocity) drive, when the outer part of the track is being read, the data moves at an
actual speed of 162.8 miles per hour (262km/h) past the laser. What is more amazing is that even
when the data is traveling at that speed, the laser pickup can accurately read bits (pit/land transitions)
spaced as little as only 0.9 microns (or 35.4 millionths of an inch) apart!