SOUND RECORDING

transcription of vibrations in air that are perceptible as sound onto a storage medium, such as a phonograph disc. In sound reproduction the process is reversed so that the variations stored on the medium are converted back into sound waves. The three principal media that have been developed for sound recording and reproduction are the mechanical (phonographic disc), magnetic (audiotape), and optical (digital compact disc) systems. Richard E. Berg transcription of vibrations in air that are perceptible as sound onto a storage medium, such as a phonograph disc. In sound reproduction the process is reversed so that the variations stored on the medium are converted back into sound waves. The three principal media that have been developed for sound recording and reproduction are the mechanical (phonograph disc), magnetic (recorded tape), and optical (motion-picture sound tracks and digital compact discs) systems. The American inventor Thomas A. Edison developed the talking machine, which could both record and reproduce sound, in 1877. The original Edison cylinder recordings used indentations embossed into a sheet of tinfoil by a vibrating stylus attached to a diaphragm. Emil Berliner, the German-born American inventor of the Gramophone, introduced the flat disc, as well as the practice of using electroforming to make a negative of the master, which then could serve to mold copies. Early sound recording and reproduction relied entirely on acoustical means. In the early 1920s the vacuum-tube amplifier, invented by the American Lee De Forest, came into use, marking the transition from acoustical to electrical recording. Microphones replaced acoustical horns, and soon there was developed the modern electric phonographconsisting of an amplifier, a motor-driven turntable (that often incorporated a record-changing device), a cartridge, and loudspeakers. The 78-rpm (revolutions per minute) record, originally made of shellac and later of synthetic thermoplastic resins and with a playing time of about 4 1/2 minutes, became standard. The long-playing (LP) record, first introduced in 1948, was designed to be played at a speed of 33 1/3 rpm; it used microgrooves, allowing up to 30 minutes of playing time per side. The 45-rpm disc, playing up to eight minutes per side, was introduced in 1949. Stereophonic recordings, with two separate channels of sound recorded in the same groove, were introduced in 1958. Quadraphonic discs, with the two additional channels often used to reproduce ambient sound, became available in the early 1970s but were not successful commercially. Magnetic systems of recording sound date to the 1890s, when the Danish engineer Valdemar Poulsen invented a device that stored electrical information by magnetizing particles on a steel wire. American and German scientists had developed magnetic tape by the 1920s, and during the following decade recorders were further refined, but not until after World War II did the tape recorder begin to have widespread application in recording music. In tape recording, the electrical signal to be recorded is usually applied to the tape by means of a head, consisting of a coil wound around a core of magnetic iron, that has a gap at the point where the tape moves across its surface. The current in the coil produces a magnetomotive force across the gap, magnetizing the particles on the tape. In reproduction the tape is passed over the playback head, and the magnetized portions of the tape cause the magnetic flux in the core to change, generating a voltage. Recording tapes consist of a plastic-base film coated with a magnetic material, usually iron oxide, although chromium dioxide and pure metal particles also are used. The principal tape-recording formats are the open-reel and the cassette. Open-reel recorders, which were the first to be developed, are now used most commonly for professional recording. They operate at several speeds and have great flexibility, including the ability to record up to 24 separate tracks. A cassette consists of a feed reel of tape and a take-up hub enclosed in a sealed rectangular package. Although the cassette is less flexible and generally has lower fidelity than the open-reel format, it became more popular, largely because of its ease of operation. The first optical system was invented by De Forest, who by 1923 had developed techniques for transcribing sound waves into impulses of light that could be photographed on a strip of film. When the developed film was then passed between a light source and a photoelectric cell in the motion-picture projector, the images were transformed back into electrical voltages that could be converted into sound through a loudspeaker system. A second type of optical recording is the digital compact disc (CD). Unlike all other methods of recording and reproduction, which create analogies of the original sound and thus are called analogue methods, digital recording samples the sound at specified intervals and converts the samples into binary (base-2) numbers that are then recorded on tape as a series of pulses. Digitally mastered tapes converted into conventional phonograph discs first appeared in the 1970s, and fully digital compact discs, which are read, or played, by a laser, became available in the early 1980s and became the most popular mode by the beginning of the 1990s. Other digital modes being introduced were the digital audio tape (DAT) and the digital compact cassette (DCC). Additional reading Harry F. Olson, Acoustical Engineering (1957), and Modern Sound Reproduction (1972), are the advanced classics in the field, including detailed discussions of loudspeaker design. An excellent introduction to audio equipment is provided in Institute of High Fidelity, Official Guide to High Fidelity, 2nd ed. (1978). A later introduction to audio reproduction is offered in Kenneth W. Johnson, Willard C. Walker, and John D. Cutnell, The Science of Hi-Fidelity, 2nd ed. (1981). Advanced treatment of electromechanical transducers is found in Josef Merhaut, Theory of Electroacoustics, trans. from Czech (1981); John Borwick, Microphones: Technology and Technique (1990); and Martin Colloms, High Performance Loudspeakers, 4th ed. (1991). John Eargle, Sound Recording, 2nd ed. (1980), is of interest for its treatment of older audio reproduction technology, including LP disc recordings and quadraphonic sound. Eargle's later work, Handbook of Recording Engineering, 2nd ed. (1992), also covers digital sound recording. Both audio and video magnetic tape technology is discussed in John C. Mallinson, The Foundations of Magnetic Recording (1987); and C. Denis Mee and Eric D. Daniel, Magnetic Recording, 3 vol. (198788). For digital techniques, see Ken C. Pohlmann, Principles of Digital Audio, 2nd ed. (1989), on the compact disc; and John Watkinson, RDAT (1991), on the rotary head digital audiotape. Richard E. Berg