Stereophonic technology began in 1931, when English inventor Alan D. Blumlein patented a system for producing stereo discs by combining Edison's vertical-cut groove (one cut with only up-and-down agitation) with lateral-cut grooves (those cut with side-to-side agitation) like those of monaural recordings. The 1940 Walt Disney film Fantasia was presented in stereo, marking the first public appearance of the new technology.
Stereo was introduced into home listening in two ways. Stereo tape recordings were available as early as 1954, when equipment to play them appeared. The cost was very high, and few people owned magnetic tape equipment, so this was impractical. In order for stereo to catch on, it would have to be integrated with disc technology.
A revival of Blumlein's stereo system occurred in 1957. Contrary to Blumlein's system, which involved cutting two separate bands of grooving (and two styli to pick them up), this system used both sides of the "V" groove to correspond to each channel of recording. By 1970 the entire disc industry had changed over to stereo, and scarcely a mono disc could be purchased. The stereo technology was also extended magnetically through four-track, eight-track, and the revolutionary cassette tapes. A new form of stereo, quadraphonic technology, was developed in 1969.
Monaural technology makes use of a single sound channel, or track. Thus, regardless of the number of speakers attached to a unit, a mono sound source will send the same sound signal to each speaker. The listener has only one dimension of the sound, forward-rear. Stereophonic technology, on the other hand, makes use of two distinct sound channels. For a musical composition with voice, for example, one channel may contain the music, the other the voice. Alternately, one channel may provide percussion instruments and voice, and the other channel the tonal instruments. This allows for distinct separation of the tracked sounds, and the listener appreciates a much clearer presentation of the sound. Stereo is an aesthetic illusion that provides impact on the listener. Studio trickery can make a sound "flow" from one channel to the other. The listener, in effect, feels as if the sound is passing from left to right. Stereo can have one channel louder than the other, or sound can arrive at a different time in each channel, creating an echo effect. The possibilities are endless.
In order for stereo technology to be enjoyed and appreciated
by the listener, the sound source for each channel must be separated.
Ideally, each ear is only allowed to hear one channel, as with headphones.
This produces the strongest stereo effect possible, and the listener feels
as if the music is being performed inside their head. If speakers
are used, they should be far enough apart to allow proper separation of
the channels, for example, in opposite corners of a room. This will
produce a moderate to strong stereo effect, depending on the quality of
the equipment. If the speakers are placed together, no separation
of channels will occur and the stereo effect is lost.