1
About Compact Disc
Digital Audio…
To most people, a Compact Disc (CD) looks like a small, silver
phonograph record. In fact, there are some important differences
between records and CDs.
The information on a phonograph record is contained within its
grooves. This
analog
information is
read by a stylus riding along in
the grooves and transmitted
via a cartridge. The
digital
information on a CD,
though arranged in
similar grooves, is read
by a laser mechanism
(called a
pickup
). A
phonograph’s stylus
must be in contact
with the record, but a
CD player’s pickup
never touches the CD.
The information on the
CD is transmitted via
complex decoder circuitry.
The CD rotates
counterclockwise, and the laser
pickup reads the underside of the CD
(the side opposite the label) beginning at the center and travelling
toward the outer edge. As the pickup moves away from the center,
the speed of rotation gradually decreases from 500 to 200 revolutions
per minute.
Programs on CDs are organized into
tracks
, usually corresponding to
songs or movements. This allows you to select a part of a program
easily and precisely,
with virtually no wear
and tear on the CD.
There may be as many
as 99 tracks on a CD.
On some CDs, tracks
are further broken down
into
indexes
. These
"tracks within tracks"
allow even more precise selection and programming. A single track
may have up to 99 indexes.
Direction the disc rotates
T
r
a
c
k
3
T
r
a
c
k
2
T
r
a
c
k
1
Direction the laser
pickup travels
Lindsay's First Symphony
01 02 03 04 05 06 94 95 96 97 98 99
Index Numbers from Track 2
(Track 1) (Track 2) (Track 3)
First Movement Second Movement Third Movement
Phonograph records
vs. compact discs
Tracks and indexes
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