The music information (audio data) is stored on the CD in digital form (value 0 and 1). The data is scanned by the reflection of beams of light. The pick-up sends a strongly bundled beam of infrared light to the CD surface. This beam of light is reflected by the silver or gold vapour-coated surface and taken up and evaluated by an optical sensor. On a purely physical basis, the system detects elevations and depressions which, due to their length, change the reflection in certain patterns such that a differentiation can be made between the values 0 and 1.
The audio data are stored in concentric circles and not in a spiral as on a conventional vinyl disc. In addition to the music, the CD tracks therefore contain further information which serves the purpose of pick-up tracking. Among other things, this makes it possible to jump arbitrarily from one title to another during playback. At this point, it should be borne in mind that 30 of these information tracks are about as wide as one human hair. We are therefore dealing with microscopic dimensions.
In addition to tracking, irregularities on the CD surface must also be balanced out. For this reason, the focus of the reflected infrared beam is permanently monitored and corrected by raising and lowering the pick-up lens. This procedure is known as focusing.
If data are lost during the pick-up procedure (due to scratches, impact etc.), to a certain extent, these data can be reconstructed. For this purpose, the last value read is compared with the next readable value. From both values, the error correction facility calculates a kind of mean value. This procedure cannot be perceived by the human ear.