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An electronic device for reproducing stereo sound.

About Stero Sound

Sign for stereo
Sign for stereo

Stereophonic sound, commonly called stereo, is the reproduction of sound, using two independent audio channels, through a pair of widely separated speaker systems, in such a way as to create a pleasant and natural impression of sound heard from various directions as in natural hearing.

The word "stereophonic", from Greek stereos = "solid" and phōnē = "sound", was coined by Western Electric, by analogy with the word "stereoscopic". Western Electric first demonstrated it at an SMPTE meeting in 1937, then to the general public at Carnegie Hall in 1940.

In popular usage, stereo usually means 2-channel sound recording and sound reproduction using data for more than one speaker simultaneously.

In technical usage, stereo or stereophony means sound recording and sound reproduction that uses stereographic projection to encode the relative positions of objects and events recorded. A stereo system can include any number of channels, such as the multichannel audio 5.1- and 6.1-channel systems used on high-end film and television productions. However, it more commonly means only two-channel systems.

The electronic device for playing back stereo sound is often called "a stereo".

Stereo recording

Stereo is derived from the term stereographic projection, which here generates a stereo image during playback. During two-channel stereo recording, two microphones are placed in strategic locations in relation to the source, both record at once. Each channel will be similar, but each will have distinct time-of-arrival difference and sound pressure level difference information. On playback, the listener's brain uses the subtle differences in timing and level to triangulate the positions of the recorded objects.

Stereo recordings often cannot be played by monaural systems without a significant loss of fidelity. Since each microphone records each wavefront at a slightly different time, constructive and destructive interference can occur, if both tracks are played on the same speaker. This phenomenon is known as comb filtering.

Various methods of stereo recording

X-Y technique is intensity sterophony

Here there are two directional microphones at the same place, and typically pointing at an angle 90° or more to each other. A stereo effect is achieved simply through sound pressure level differences of the sound entering each microphone. Due to the lack of time-of-arrival stereo information, the stereo effect in X-Y recordings is less realistic than a stereo recording.
Intensity stereo is an unfortunate linguistic misnomer which has come to mean the recording of stereophonic signals that are distinguished only by level differences. These "level differences" have been called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor would it be valuable in music recording if it could. Our ear drums are only sensitive to the sound pressure, like microphones.

A-B technique is time-of-arrival sterophony

This uses two omnidirectional microphones some distance apart, so capturing time-of-arrival stereo information as well as some level (amplitude) difference information.

Near-coincident technique is mixed stereophony

e.g. the ORTF stereo technique of the Office de Radiodiffusion Télévision Française = Radio France, calls for a pair of cardioid microphones placed 17 cm apart at an angle of 110 degrees. In the NOS stereo technique of the Nederlandse Omroep Stichting = Holland Radio, the angle is 90 degrees and the distance is 30 cm, so capturing time-of-arrival stereo information as well as level information. Even the spacing of 17 cm has nothing to do with human ear distance. The developed signals are made for stereo loudspeakers and not for ear phones.

Binaural recording

Using a model of a human head

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.

Playing back stereo recordings

Stereophonic sound attempts to create an illusion of location for various instruments within the original recording. The recording engineer's goal is usually to create a stereo "image" with localization information. When a stereophonic recording is heard through loudspeaker systems rather than headphones, each ear of course hears sound from both speakers. The audio engineer may and often does use more than two microphones, sometimes many more, and may mix them down to two tracks in ways that exaggerate the separation of the instruments to compensate for the mixture that occurs when listening via speakers.

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.



Clément Ader introduced the first two-channel audio system in 1881.


In the 1930s, Harvey Fletcher of Bell Laboratories investigated techniques for stereophonic recording and reproduction. One of the techniques investigated was the 'Wall of Sound,' which used an enormous array of microphones hung in a line across the front of an orchestra. Up to eighty microphones were used, and each fed a corresponding loudspeaker, placed in an identical position, in a separate listening room.

Modern stereo, using two channels and coincident microphone techniques, was developed by Alan Blumlein at EMI in the 1931 and patented the same year. The first stereo disc, using the two walls of the groove at right angles to carry the two channels, was cut at EMI in 1933.

Stereo recording was introduced in the music business in the fall of 1957, superceding monaural (single-channel) recording.

1949 to 1970

From 1940 to 1970, the progress of stereophonic sound was paced by the technical difficulties of recording and reproducing two (or more) channels in synchronization, and by the economic and marketing issues of introducing new audio media and equipment. To a rough approximation, a stereo system cost twice as much as a monophonic system. Actually, in the 1950s that was an accurate approximation, since a stereo system had to be assembled by buying two preamplifiers, two amplifiers, and two speaker system. It was not clear whether consumers would think the sound was so much better as to be worth twice the price.

The 1940 Carnegie Hall demonstration

The 1940 Carnegie Hall demonstration used three huge speaker systems. Synchronization was achieved by making the recordings in the form of three motion-picture soundtracks recorded on a single piece of film. Because of dynamic range limitations, volume compression was used, with a fourth track being used to regulate volume expansion. The Dolby noise-reduction system of the 1970s was a far more sophisticated version of a basically similar technique. The volume compression and expansion were not fully automatic, but were designed to allow manual studio "enhancement", i.e. the artistic adjustment of overall volume and the relative volume of each track.

The recordings had been made by the Philadelphia Orchestra, conducted by Leopold Stokowski, who was always interested in sound reproduction technology. Stokowski personally participated in the "enhancement" of the sound.

The speakers used generated 1500 watts of acoustic power, producing sound levels of up to 100 decibels, and the demonstration held the audience "spellbound and a little terrified", according to reporters. Sergei Rachmaninoff, who was present at the demonstration, commented that it was "marvellous" but "somehow unmusical because of the loudness". "Take that 'Pictures at an Exhibition'", he said. "I didn't know what it was until they got well into the piece. Too much 'enhancing', too much Stokowski."

The motion picture era

The advent of magnetic tape recording made high-fidelity synchronized multichannel recording technically straightforward, though costly. Motion picture theatres could afford the cost, and that is where the real introduction of stereophonic sound to the public occurred, first with Cinerama in 1952. Cinerama was a spectacular wide-screen process fully comparable to today's IMAX. Cinerama practically required a specially built theatre for its presentation. It used six magnetic sound tracks. The system was developed by Hazard Reeves, a pioneer in magnetic recording technology. By all accounts, including accounts by those who have experienced the process in rare recent showings, the sound was as spectacular as the picture and excellent even by modern standards. The movie industry moved quickly to create simpler and cheaper wide-screen systems, such as CinemaScope, which were capable of being retrofitted into existing theatres. Cole Porter memorialized the era in a 1954 song:-

If Zanuck's latest picture were the good old-fashioned kind,
There'd be no one in front to look at Marilyn's behind.
If you want to hear applauding hands resound
You've gotta have glorious Technicolor,
Breathtaking Cinemascope and
Stereophonic sound.

In the mid-1950s, companies such as Concertapes began releasing stereophonic recordings on two-track prerecorded reel-to-reel magnetic tape. Serious audiophiles, the sort of people who would later be called "early adopters", bought them, and stereophonic sound came to at least some living rooms.

Common usage

In common usage, a "stereo" is a two-channel sound reproduction system, and a "stereo recording" is a two-channel recording. This is a cause for much confusion, since five (or more) channel home theater systems are not popularly described as "stereo". It is worth noting that most film soundtracks are not recorded using stereo techniques, so while capable of stereo playback, most home theater systems rarely do.

Most two-channel recordings are stereo recordings only in this weaker sense. Pop music, in particular, is usually recorded using close miking techniques, which artificially separates signals into several tracks. The separate tracks are then mixed into a two-channel recording which often bears little or no resemblance to the actual physical and spatial relationship of the musicians at the time of the original performance. Indeed, it is not uncommon for different tracks of the same song to be recorded at different times, and even in different studios, and then mixed into a final two-channel recording for commercial release. Classical music recordings are a notable exception.


Balance can mean to the amount of signal from each channel reproduced in a stereo audio recording.

Other uses of the word

Because of this usage, "stereo" or "in stereo" is sometimes used colloquially for when two, as distinct from one, of something are present.

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