Stereophonic Sound Recording

a method of sound recording in which the recording contains information about the spatial arrangement of the sound sources. The term “stereophonic sound recording” denotes both the recording process and the information recorded.

When a stereophonic recording is reproduced, the listener hears a more natural sound that seems to emanate from many separate sources and to be arranged in the same way as during the recording. The listener has the impression that the sound is “three-dimensional” and possessed of an added “depth.” This effect is achieved through the separate recording of electrical signals from different microphones on individual channels and through the separate reproduction of the sound on each channel by loudspeakers (Figure 1). The arrangement of the loudspeakers must be similar to that of the microphones; that is, the right and left channels must coincide. The gain in the separate reproduction channels must be equal, and the phases of the signals in these channels must bear a definite relationship to one another. The fidelity of reproduction can be checked with the aid of special recordings, such as check disks. A striking effect is created by listening to a two-channel stereophonic recording through headphones, with the right and left earphones connected to separate channels.

Figure 1. Diagram of a system for three-channel stereophonic recording and reproduction: (1) microphone, (2) recording channel, (3) recording medium, (4) direction of the medium’s movement, (5) recording path, (6) recording, (7) reproduction channel, and (8) loudspeaker

The quality of stereophonic sound reproduction improves with the number of channels used. However, the number of channels is usually kept within certain limits to avoid undue complexity and excessive cost. The highest number of channels (five) is used in sound recording in panoramic and wide-screen motion-picture systems. Home stereophonic units, such as magnetic tape recorders and electrophones, usually have only two channels; even so, the quality of their sound is clearly superior to that of monaural (single-channel) equipment. In the early 1970’s, four-channel (quadrophonic) recordings became available. In reproducing these recordings, two speakers are placed in front of the listener and two behind him.

Stereophonic sound recording for tape recorders and motion pictures is accomplished by applying the signals of the right and left channels to two separate sound tracks. With stereophonic disks, the signals are inscribed on the two faces of a groove. In quadrophonic disk recording, the four signals are first converted to two combined signals and then inscribed on the faces of the groove. Any method of stereophonic sound recording must be compatible with monaural systems so that a stereophonic recording can be played, but without a three-dimensional effect, on monaural reproducing equipment.