CD's Generate "Real" Computer Data
The pits which are formed during the mastering and molding processes are read by a small optical system which includes a laser and an optical detector.
The disc and corresponding pit structures rotate under the laser beam during playback. The pits cause the amount of reflected light traveling from the disc to the detector to fluctuate, creating a high frequency signal. The drive determines points in time where this signal crosses a certain threshold value. These crossings, signifying a logical "1", occur at each pit edge.
Due to physical limitations of pit size, it is necessary to maintain at least two "0"s between each "1". An encoding scheme called Eight-to-Fourteen Modulation (EFM) translates every 8 bit combination to a 14 bit code to be written to the disc. Three merging bits are used between each byte to further ensure the separation of "1"s.