Stereos

The first stereo transmission was made telephonically by Clement Ader in 1881. The BBC made radio's first stereo broadcast in December 1925. In the 1930s, Harvey Fletcher of Bell Laboratories investigated techniques for stereophonic recording and reproduction. The first commercial motion picture to be exhibited with stereophonic sound was Walt Disney's Fantasia (1940). By the mid-1950s, multichannel sound was common for big-budget Hollywood motion pictures. In 1953, Remington Records began taping some of its sessions in stereo, with the first stereophonic phonograph discs available to the general public in 1958. The US Federal Communications Commission announced stereophonic FM technical standards in April 1961, and licensed regular stereophonic FM radio broadcasting to begin in the United States in 1961. In 1984, Multichannel television sound was adopted by the FCC as the U.S. standard for stereo television transmission.

During two-channel stereo recording, two microphones are placed in strategically chosen locations relative to the sound source, with both recording simultaneously. The two recorded channels will be similar, but each will have distinct time-of-arrival and sound-pressure-level information. During playback, the listener's brain uses those subtle differences in timing and sound-level to triangulate the positions of the recorded objects. Stereo recordings often cannot be played on monaural systems without a significant loss of fidelity. Since each microphone records each wavefront at a slightly different time, the wavefronts are out of phase; as a result, constructive and destructive interference can occur, if both tracks are played back on the same speaker. This phenomenon is known as phase cancellation.

Engineers make a technical distinction between "binaural" and "stereophonic" recording. Of these, binaural recording is more like stereoscopic photography. In binaural recording, a pair of microphones is put inside a model of a human head which includes external ears and ear canals. Each microphone is where the eardrum would be. The recording is then played back through headphones, so that each channel is presented independently, without mixing or crosstalk. Thus, each of the listener's eardrums is driven with a replica of the auditory signal it would have experienced at the recording location. The result is an accurate duplication of the auditory spatiality that would have been heard by the listener placed where the microphones were. Because of the nuisance of wearing headphones, true binaural recordings have remained laboratory and audiophile curiosities.

Descriptions of stereophonic sound tend to stress the ability to localize the position of each instrument in space, but in reality many people listen on playback systems that do a poor job of re-creating a stereo "image". Many listeners assume that "stereo" sound is "richer" or "fuller-sounding" than monophonic sound. This is inaccurate — stereo and mono can have equally detailed abilities to play recorded notes. The spatial illusion is what sets stereo recordings apart from mono recordings. When playing back stereo recordings, best results are obtained by using two speakers, in front of and equidistant from the listener, with the listener located on the center line between the two speakers.