Exercise 3: Compact Disk (Segmented)
Description Case 3: Compact Disk
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A compact disk (CD), like a
phonograph record, stores
information physically for
electronic replay, and, like a
phonograph record, it stores the
information along a single spiral
groove on the side of a plastic
circle.
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In contrast to a phonograph
record, however, a CD
* stores the information in
digital (on/off) rather than
analog (variable shape) form,
and
* is read by reflected (laser)
light, rather than by vibrating
a needle that travels along the
groove.
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A compact disk is a circle of
clear plastic (polycarbonate)
about 12 cm in diameter and
1 mm thick, with a 1.5-cm
diameter hole in the center.
CDs are stamped from a mold that
leaves a spiral track lined with
pits (little dents) on the CD's
bottom side (details below),
while the top side is smooth.
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The surface area of a CD is just
less than twice (1.77 times) as
large as the surface area of a
3.5-inch magnetic "floppy" disk.
But because the pits store
information much more densely
than the iron oxide particles on
a floppy disk, a CD holds at
least 350 times more data (at
least 500 Mbyte on a CD, only
1.44 Mbyte a magnetic disk).
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CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the
stamped, grooved plastic.
(b) Above that lies a very thin
film of aluminum (or chrome-
aluminum alloy). This
metallic film reflects any
light entering the disk from
below, except where the pits
fall in the plastic layer.
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(c) Above the reflective metal
layer is a coat of acrylic
lacquer that protects the
metal from scratches and
oxidation. It also allows
printing descriptive labels
safely on the top side of
the disk.
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The groove on the bottom surface
of a CD is a single channel that
spirals outward from the center
to the edge. This track is
thinner than a human hair and
several kilometers long.
(The spiral differs from the
many concentric rings of iron
oxide (see Fig. 2) that store
information on a magnetic disk.)
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The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed
motor (unlike the constant-speed
motor on a phonograph turntable).
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The disk turns (clockwise) about
500 revolutions/minute (e) when
the reading laser beam is at the
center, but only about 200
revolutions/minute when the beam
reaches the outer edge.
This causes the track to pass
over the read-out system (f),
which gradually moves from the
center to the edge, at a
constant linear speed, to help
reliably detect the pits.
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The moving spiral track is lined
with pits (dents) and flat spots
("lands"). These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally
encode text, images, computer
programs, or the left- and right-
hand audio signals of a stereo
sound recording.
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Additional pits
* give location and timing
information (for player
display), and
* control the motor speed
so that the reading rate
remains constant.
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Two lenses and a semi-transparent
(partially silvered) mirror
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.
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If the laser beam strikes a pit
on the track (g), it is not
reflected. The light-sensitive
photodiode (detector) below the
mirror sees no beam and produces
no signal.
If the laser beam strikes a land
between pits on the track (h),
it reflects back straight through
the mirror to the photodiode
below. This detector then
produces an electric signal.
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For compatibility with other
electronic equipment, a special
reversing circuit (a "not gate")
then turns these pit
interruptions into ON signals
(binary 1s) and turns the land
reflections into OFF signals
(binary 0s).
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