The Light Fantastic

Before compact disks came along, the method of capturing and replaying music had changed little since Thomas Edison invented the phonograph in 1877. Conventional records store sound in the form of tiny waves cut into vinyl grooves. When a diamond or sapphire stylus passes over them, its vibrations create a tiny electrical current that is converted back into sound. Tape players work in a similar way, reading sound from magnetized particles on plastic ribbon. Both methods involve a process known as analog recording, in which the music is represented as a physical replica, or analog, of the original sound. The chief drawback in each case is that the phonograph stylus or tape head rides constantly on the playing surface. This causes wear and distortion that come across as hissing and crackling sounds.

Compact disks replace the old technology with a digital system based on computers and laser light. On a CD, sound is broken down into binary digits (zeros and ones). Those numbers are stored on an aluminum disk in some 15 billion microscopic pits. When the CD plays, rotating at up to 500 r.p.m., a laser silently scans the pits and then beams their information to a microcomputer that converts the digits back into sound. Because no mechanical part touches the disk's surface, the resulting tone is virtually free of distortion.

The laser can even pass noiselessly over deep scratches that would cause a stylus to make a clicking sound and perhaps get stuck. When the light encounters a blemish, the microcomputer instantly uses the material stored just before and after the scratch to cover up the missing part.