Digital Data on a Compact Disc

Binary data (0's and 1's) are encoded on a compact disc in the form of pits in the plastic substrate which are then coated with an aluminum film to make them reflective.

The data is detected by a laser beam which tracks the concentric circular lines of pits. The pits are 0.8 to 3 micrometers long and the rows are separated by 1.6 micrometers.

Compact Disc Drive Details

In a compact disc player, a laser beam must track a spiral row of pits which are 0.5 micrometers wide with track spacing 1.6 micrometers. Tracking is aided by a three-beam laser arrangement. In addition to staying on the track, which is much narrower than the 100 micrometer groove separation on a vinyl record, the rotation speed must be adjusted as the beam tracks inward or outward. A linear speed of 1.25 m/s is maintained by increasing the rotation speed from 3.5 to 8 revolutions per second as the beam tracks inward toward the center of the disc.

Laser Tracking on the CD

The laser beam in the compact disc player must precisely track a row of pits which encode the binary data on the disc. In the "three-beam" system, a grating is used to produce the first order diffraction maximum to each side of the main beam. Those diffracted beams overlap the track, and the reflected light from the two side beams should be equal, on the average, if the main beam is centered on the track. If they are unequal, then their difference can be used to generate an error voltage to correct the tracking. The illustration of the side beam positions is not to scale; they deviate about 20 micrometers from the main beam.

Scaled Views of a Compact Disc

Data on a compact disc is stored in the form of pits in the plastic substrate.

A reflective layer of aluminium is applied to reflect the laser beam. A protective coating is then applied to the top. The laser system reads the data from below.

Detection of CD Pits

The tracking laser beam sees the pits as raised areas which are about a quarter-wavelength high for the laser light.

The reflected light from the pit is then 180° out of phase with the reflection from the flat area, so the reflected light intensity drops as the beam moves over a pit. The threshold of the photodiode detector can be adjusted to switch on this light level change.