The compact disc is a fairly recent invention. It was jointly developed by Philips and Sony. Standardisation was agreed on in 1980. The disc would be 120 millimeters in diameter and 1.2 millimeters thick. The storage capacity for audio would be 74 minutes.
The dimensions of a compact disc track are very impressive. The total length of the track is 5.7 kilometers. The width of these ridges is a mere 6 micrometers, with their depth being 0.16 micrometers. To give some idea of just how small these ridges are, imagine the width of a human hair. In the same width you could draw 30 tracks!
The material used for the base of the compact disc is a mixture of polycarbonate plastics, which has constant optical and mechanical properties. The plastic disc is manufactured by means of injection moulding. After the disc has been pressed, it is given a layer of aluminium to give high reflection and then a protective layer of lacquer. The compact disc is read through its base, so the optical properties of the lacquer are relatively unimportant.
To play back a compact disc, a laser is directed at the surface of the disc, the audio on the disc is made up of `pits and flats' which reflect the laser in different directions and therefore regenerate the digital audio stored on the disc. These signals are then slightly amplified and error corrected before being made available via the audio connectors at the rear of the player.
