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The most common CD
is an audio CD, or more precisely a CD-DA
(Compact Disk-Digital Audio). This type of CD is a machine pressed
aluminum disk (which reflects the laser beam that reads the disk).
The aluminum disk is coated on the playing surface (bottom) with
polycarbonate plastic. It has a top surfaced with lacquer. It
consists of up to 99 stereo tracks using 16-bit pulse-code modulation
(PCM) coding at a sampling rate of 44.1 kHz. It is about 4.7 inches
(120mm) in diameter and stores digital information. A CD is played
on a player that uses a laser beam to read the imprinted information.
The disk can hold up to 90 minutes of music or 800 megabytes of
computer data.
Most commercially sold music CDs have about 60 minutes (12
compositions) of music on each disk. The sounds recorded on a
CD playback to near perfection with no surface noise and with
full dynamic range (See: SOUND--A CLOSER LOOK).
The disk does not deteriorate with use as
did the old vinyl type record.
In the record industry CD masters are cut by laser onto a glass
blank from which metal stampers are made. These stampers are then
used to machine press the production copies. The pressing cost
of a CD is about 7 percent of its retail price. Packaging costs
are an additional 7 percent. See: RECORD,
CONTRACT-RECORDING CONTRACT clause #33, COMPUTER under "Disk,"
and "Magnetic Disk."
A second common type of CD is the CD-ROM (Compact Disc-Read-Only
Memory). CD-ROMs are played via a CD-ROM drive with a computer.
These types of CDs are called "Multimedia" since they
are capable of reproducing sound, video, text, and graphics. See:
MULTIMEDIA, RECORD.
A third type of CD is the CD-R (Compact Disc-Recordable). This
type of CD is used, via a CD-R drive, to record multimedia data.
A CD-R disk is constructed differently than a regular CD. In place
of the aluminum, as in a CD-DA and CD-ROM, the CD-R has a layer
of organic dye. This dye is the recording medium. A thin layer
of gold is used to protect the dye. Gold is used because it does
not corrode. The gold reflects the laser beam in the same way
as the aluminum layer does in the CD-DA and CD-ROM.
See: MULTIMEDIA, RECORD.
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