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The compact laser disc (CD) brought the world of digital audio to the consumer. The CD was a revolution in music with sound quality far exceeding that of records and audio tapes. Information is stored on the reflective surface of a CD through a series of dark pits burned with a laser. The information is arranged in a single track that spirals from the inside to the outside of the disk. The rotation of the CD is changed from 480 to 210 rpm as the information is read by an optical sensor from the inside to the outside of the spiral resulting in a constant scanning velocity of 1.2 meters per second. The sensor detects the presences or absense of pits if the surface is reflective or nonreflective. The data is encoded with an eight-to-forteen modulation (EFM) code which results in pits no shorter than 0.3 µm and no longer than 3.5 µm, required for low error detection. (A human hair has diameter of 75 µm.)
Microscopic view of CD surface: 
In addition to the low level EFM code, the data is encoded with an error detection/correcting code combining the left and right stereo channels called two-level Reed-Solomon coding. Digital errors detected during playback are either corrected using redundant data, concealed by interpolating between adjacent samples, or muted by setting the sample value to zero.
The effective sampling for a CD is 16 bit samples at 44.1 kHz but the digital to analog curcuit uses a clever multirate technique to convert the digital data to a higher sampling rate of 176.4 kHz before the DAC to allow use of a lower cost 14 bit DAC.
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