equipment for sound recording and reproduction that utilizes two or more independent channels of information. Separate microphones are used in recording and separate speakers in reproduction; they are arranged to produce a sense of recording-hall acoustics and of the location of instruments within an orchestra. The effectiveness of stereophonic reproduction was demonstrated as early as 1933. Two-track stereophonic tape for the home became common in the 1950s and the stereophonic phonograph record, with two separate channels of information recorded in a single groove, in 1958. In the early 1970s, quadraphonic sound systems, employing four independent channels of information for even greater realism, became commercially available and later led to “surround-sound” systems.