How does a CD work?
Like gramophone records, the information on optical
discs is recorded on a spiral track. However, with
a CD the laser starts reading the disc from the inside
ring (table of contents) and ends up on the outside.
When play back starts, a laser beam shines on the ridges
and lands on the data membrane layer. If you look at
the image on the right you can see the data layer moving
During playback, the number of revolutions of the
disc decreases from 500 to 200 rpm (revolutions per
to maintain a constant scanning speed. The disc data
is converted into electrical pulses (the bit stream)
by reflections of the laser beam from a photoelectric
When the laser beam strikes "land", the
beam is reflected onto a photoelectric cell. When it
strikes a "ridge", the photocell will receive
only a weak reflection. Thus the photoelectrical cell
receives series of light pulses corresponding to the
ridges and lands in the disc. These light pulses are
the foundation of binary 'digital' data. A simple substitution
for the weak signal "0" and the in-focus
signal "1" results in a pure digital playback
without alteration, every time, without failure or
In music playback, a D/A-Converter (digital to analogue
converter; DAC) converts the series of pulses (binary
coding) from a decimal code to a waveform, which can
then be processed for amplification. The longer the
decimal code, the better the sound. Current standard
CD audio is 44,100 pulses per second and 16 bit (decimal
places) in digital word length. Thus a 24 bit system
sounds all that much better, in fact DVD audio is set
to allow 24 bit AND pulse at 97,000 times per second!
The Compact Disc player mechanism
The laser pickup
reads the disc from below.
Thanks to this optical scanning system, there is
no friction between the laser beam and the disc.
result, the discs do not wear, no matter how often
they are played. However, they must be treated carefully,
as scratches, grease stains and dust might intercept
or diffract the light, causing whole series of pulses
to be skipped or distorted. This problem can be solved,
as during the recording the Cross Interleaved Reed
Solomon Code (CIRC) is added, which is an error correction
system that automatically inserts any lost or damaged
information by making a number of mathematical calculations.
Without this error correction system optical disc
players would not have existed, as even the slightest
of the floor would cause sound and image distortions.
When the laser beam hits land, all of its light is
reflected and the cell gives off current. When the
laser beam shines on a ridge, half of the light hits
the upper surface and the other half hits the lower
down service. The difference in height between the
two places is exactly a quarter of a wavelength of
the laser beam light, so the original beam is totally
eliminated by the interference between the beam reflected
from the surface of the disc and the beam reflected
from the ridge. The photocell does not produce current.
It should be noted that the ends of the ridges seen
by the laser are "ones" and all lands and
ridges are "zeros"; thus turning on and off
the reflection is one, steady state is a string of
zeroes. As it is not possible to have two ones next
to each other, Eight to Fourteen Modulation (EFM) is
used to convert 8-bit data bytes to 14 bit units that
always have a minimum of 2 and a maximum of 10 zeros
between ones. This makes the pits/ridges and lands
separating them 3 to 11 bits long, no less, no more.
This conversion is done in hardware using a ROM lookup
table. To connect these 14 bit units 3 merge bits are
used to make sure that there are no "ones" too
close to each other. In audio, the third merge bit
is used to make sure that the cumulative lengths of
the lands and ridges stay equal in the long run, otherwise
a low frequency component is created that the processing
amplifiers can not handle. Thus 8 data bits are actually
17 channel bits on the disc, but called 16 bit for
The scanning must be very accurate because the track
of ridges is 30 times narrower than a single human
hair. You can see the "ridge" in the illustration
above -it is the DARK ROUND CIRCLE. When the laser
light is over top of it, the light 'splits' in two,
causing a weak signal. There are 20,000 tracks on one
audio compact disc. The lens which focuses the laser
beam on the disc has a depth of field of about 1 micron
(micron = micrometer = one-millionth of a meter).
It is quite normal for the (compact) disc to move back
and forth 1mm during playback. A flexible regulator
keeps the lens at a distance of +/- 2 micron from the
rotating disc. For the same reason, a perfect tracking
system is required. The complex task of following the
track is controlled by an electronic servo system.
The servo system ensures the track is followed accurately
by measuring the signal output. If the output decreases,
the system recognizes this as being "off track" and
returns the tracking system to its optimum state.
Many CD players use three-beam scanning for correct
tracking. The three beams come from one laser. A polarized
prism projects three spots of light on the track. It
shines the middle one exactly on the track, and the
two other "control" beams generate a signal
to correct the laser beam immediately, should it deflect
from the middle track.
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The CD is a plastic disc 1.2mm thick and 12cm in diameter,
with a silver-colored surface that reflects laser light.
The maximum playing time for music recorded on compact
disc is 74 minutes. The CD has several layers. First,
to protect the 8 trillion microscopically small pits
against dirt and damage, the CD has a plastic protective
layer. On the top of this layer the label is printed.
Then there is the reflecting aluminum coating, which
contains the ridges. Finally, the disc has a transparent
carrier through which the actual reading of the disc
takes place. This plastic forms a part of the optical
system. Mechanically, the CD is less vulnerable than
the analogue record, but that does not mean that it must
not be treated with care.
The protective layer on the label side is very thin:
only 0.002mm. Careless treatment or granular dust can
cause small scratches or hair cracks, enabling the air
to penetrate the evaporated aluminum coating. This coating
then starts oxidizing immediately at that spot. If the
CD is played extensively, it may be advisable to protect
the label side with a special protective foil, which
is commonly available in shops.
A CD must never be bent, so care should be taken when
removing it from the jewel case. Even slight bending
causes stress fractures. The aluminum then becomes deformed,
causing some ridges to be blocked. As a consequence,
error correction always has to be applied in that area,
affecting the final sound.
The reflecting side of the CD is the side that is read.
People tend to set the CD down with the reflecting side
up. But the more vulnerable side is not the reflecting
side but the label side. On the label side, the reflecting
layer with its ridges has been evaporated. The sensitive
layer on the reflecting side has been protected better
than the one on the label side. It is therefore better
to store CDs with the reflecting side down. It is best
to store the CD back in the jewel case, where it is safely
held by its inside edge.
Never write on the label side, even with a felt-tipped
pen. The ink may penetrate the thin protective coating
and affect the aluminum layer.
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CDs are easily scratched, and should never
be cleaned with just any cloth. CDs should be cleaned
not along the grooves, but at right angles to the direction
of the grooves. If a smear, however small, should remain
on the CD, running along the direction of the grooves,
much information would be lost. It is advisable to use
special CD cleaner that operates with a rotating brush
at right angles to the direction of the grooves.
Many people think that the digital CD is produced completely
digitally, but this is not always the case. Many CDs
have an analogue master tape as their source tapes still
kept in the library of the record company, used in the
past to make records. The quality of a CD made from analogue
tape can be surprisingly high. A CD recorded, processed
and dubbed digitally does not always sound better than
a CD produced with one or two analogue processing stages.
To indicate what stages have been treated in what ways,
a useful three-letter code is used on recordings. The
letters represent: the recording, the editing/mixing
process, and dubbing, respectively. They are printed
on the CD and/or on the insert label in a rectangular
box. There are three possibilities: DDD (completely digital
CD); ADD (analogue recording, digital processing and
dubbing); and AAD (analogue recording and processing,
digital dubbing). Many CDs carry the ADD or AAD indication.
This does not mean that they are inferior to the DDD
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