- Inside the CD
A CD has a single spiral track of data, circling from the inside of the disc to the outside.
The spiral track starts at the center ensures that the CD can be smaller than 4.8 inches (12 cm),
and you can see nowaday, there are plastic business cards that you can put in a CD player. The figure
in the right shows the side of a CD which has spiral track facing the read laser.
Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about
1.2 millimeters thick. During manufacturing, this plastic is impressed with the microscopic bumps that
make up the long, spiral track.
The bumps appear as pits on the aluminum side (see the second figure), but on the side the laser reads from,
they are the bumps.
A thin, reflective aluminum layer is then coated on the top of the disc,
covering the bumps. Then a thin protective acrylic layer is sprayed over the aluminum. The CD label is
printed on this layer of arylic.
The CD surface is a mirror covered with billions of tiny bumps that are arranged in a long, tightly wound
spiral. The CD player reads the bumps with a very precise laser and interprets the information as bits of data.
CD tracks are so small that they have to be measured in millionths of a meter, called microns. The
CD track is approximately 0.5 microns ( 0.5 × 10 -6m) wide, with 1.6 microns separating
one track from the next.
The elongated bumps are each 0.5 microns wide, a minimum of 0.83 microns long, and 125 nanometers
(125 × 10 -9m) high. The length of CD track is about 5 km.
The bumps and tracks on a CD need to be very small so that the CD can store lots of data. A CD holds up to
74 minutes of music. There are 44,100 samples per second and 2 bytes per sample, and a stereo recording requires 2 channels.
A detailed calculation is given as follows.
44,100 samples/channel/second × 2 bytes/sample × 2 channels × 74 minutes ×
60 seconds/minute = 783,216,000 bytes
In terms of bits, there are 783,216,000 bytes × 8 bits/byte = 6,265,728,000 bits stored in a compact disc.
The area nearest the center of disc, known as the lead-in, contains information on how the disk is read.
- The CD Players
The fascinating work of CD technology is to read all the tiny bumps correctly, in the right order, at the
right speed. In fact it is not a trivial job since the movement of the laser is at micron resolutions.
To archieve this, the CD player has to be exceptionally precise when it focuses the infrared laser on the
track of bumps. When you play a CD, the laser beam passes through the CD's polycarbonate layer, reflects off
the aluminum layer, and hits an optoelectronic device that detects changes in light. The bumps reflect
light differently than the
flat parts of the aluminum layer, which are called lands. The optoelectronic sensor detects this change in
reflectivity, and the electronics in the CD player's driver interpret the changes as data bits.
The most difficult job of the CD player is to keep the laser beam centered on the data track.
This centering is the job of the tracking system.
An elegant system which ensures a correct tracking works in the following ways.
If focus and tracking are correct, the reflected light forms
a circular spot spread equally over the four sensors. If focus or tracking are 'out' the spot formed is
either skewed (case ) or uneven in intensity (case ). The right figure illustrates the two cases.
These irregularities in the distribution of light over the four
sensors are translated
into movements of two tiny motors that move and refocus the beam.
The tracking system, as it plays the CD, has to continually
move the laser outward. When the laser moves outward from the center of the disc, the bumps
move past the laser faster if the angular speed of the disc if fixed. It is absolutely not acceptable,
since the sensor reads data at a non-uniform rate.
Therefore, as the laser moves outward, the spindle motor must slow the speed
of the CD. For your reference, the revolutions per minute (RPM) of the disc reaches its highest as 500 RPM
when the laser is near the disc center, but it slows down when the laser has a larger distance from the disc center.
That way, the bumps travel past the laser at a constant speed, and the data coming off
the disc has a constant rate. The animation on the right demonstrates the varying speed of the disc when the laser
is at different radial distance from disc center. (Credit: Marshall Brain's How Stuff Works).
Each pit is so finely spaced on the disc, a small particle of dust could block large amounts of data and cause
major problems. Error correction - ways of eliminating or hiding errors -enables a player to give a
smooth and accurate output. In the recording, extra information is added to the samples written on a disc. These parity
bits can be used by the microprocessor inside the player to check if any data is missing and fill in small errors.
Large errors are avoided by splitting up and interleaving. In fact, a scratch only obliterates small pieces of
several samples rather than one in its entirely. With interleaving, data is stored nonsequentially around
the disc's circumference. The drive reads data one revolution at a time, and it uninterleaves the data in
order to play it.