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Stereo or stereophony generally refers to dual-channel sound recording and sound reproduction – sound that contains data for more than one speaker simultaneously. Compact disc audio and some radio broadcasts are stereo. The purpose of stereo recording is to recreate a more natural listening experience where the spatial location of the source of a sound is, at least in part, reproduced.
Stereo comes from the Greek word for solid, and the term can be applied to any system using more than one channel, such as such as the multichannel audio 5.1- and 6.1-channel systems used on high-end film and television productions. However it is more commonly used to refer exclusively to two-channel systems.
Although stereo can have two independent mono channels, usually the signal on one channel is related to the signal on the other channel. For example, if the same signal is recorded on both channels, then it will appear as a central phantom image when played on loudspeakers. That is, the sound appears to be coming from the center between the loudspeakers.
Many methods for encoding stereo information are backwards compatible, meaning that they can be understood by monaural systems as well. This is most commonly done by including, for example, the sum of the right and left (R+L) signal on the main channel, and the difference (R-L) on a second channel. A monaural system can thus use only the signal on the main channel and to produce a reasonable monaural "translation" of the stereo information, while a stereo system can add and difference the two channels to recover the right and left speaker information. This practice is used in, for example, FM radio; an additional benefit in this situation is that if the signal is too weak for both channels to be received clearly, a stereo receiver can use only the main channel.
Clément Ader designed the first stereo system in 1881 for theaters.
Stereo recording was introduced in the music business in the fall of 1957, superseding monaural (single-channel) recording.
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