Exercise 3: Compact Disk (Scaffolded)
Description Case 3: Compact Disk
Description Analysis
Overview
A compact disk (CD), like a FEATURE:
phonograph record, stores WHY:
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.
In contrast to a phonograph FEATURE:
record, however, a CD WHY:
* 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.
General Shape
Size
A compact disk is a circle of FEATURE:
clear plastic (polycarbonate) WHY:
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.
Capacity
The surface area of a CD is just FEATURE:
less than twice (1.77 times) as WHY:
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).
The Layers FEATURE:
WHY:
CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the FEATURE:
stamped, grooved plastic. WHY:
(b) Above that lies a very thin
film of aluminum (or chrome-
aluminum alloy). This
metallic film reflects any FEATURE:
light entering the disk from WHY:
below, except where the pits
fall in the plastic layer.
(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.
The Groove
One-track Design
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 FEATURE:
thinner than a human hair and WHY:
several kilometers long.
(The spiral differs from the FEATURE:
many concentric rings of iron WHY:
oxide (see Fig. 2) that store
information on a magnetic disk.)
Moving the Groove
The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed FEATURE:
motor (unlike the constant-speed WHY:
motor on a phonograph turntable).
The disk turns (clockwise) about
500 revolutions/minute (e) when FEATURE:
the reading laser beam is at the WHY:
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.
The Pits
The moving spiral track is lined FEATURE:
with pits (dents) and flat spots WHY:
("lands"). These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally FEATURE:
encode text, images, computer WHY:
programs, or the left- and right-
hand audio signals of a stereo
sound recording.
Additional pits
* give location and timing
information (for player
display), and
* control the motor speed
so that the reading rate
remains constant.
The Optical Read-Out System
Two lenses and a semi-transparent FEATURE:
(partially silvered) mirror WHY:
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.
If the laser beam strikes a pit FEATURE:
on the track (g), it is not WHY:
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.
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).