Stereophonic sound



Stereophonic sound, commonly called stereo, is the reproduction of sound, using two or more independent audio channels, through a symmetrical configuration of loudspeakers, in such a way as to create a pleasant and natural impression of sound heard from various directions, as in natural hearing. It is often contrasted with monophonic (or "monaural", or just mono) sound, where audio is in the form of one channel, often centered in the sound field (analogous to a visual field).

The word "stereophonic" &mdash; derived from Greek stereos = "solid" and phōnē = "sound" &mdash; was coined by Western Electric, by analogy with the word "stereoscopic".

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 surround sound 5.1- and 6.1-channel systems used on high-end film and television productions. However, in common use it refers to systems with only two channels.

The electronic device for playing back stereo sound is often referred to as "a stereo".

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.

X-Y technique: intensity stereophony
Here there are two directional microphones at the same place, and typically pointing at an angle 90° or more to each other - see also "The Stereophonic Zoom" by Michael Williams. A stereo effect is achieved through differences in sound pressure level between two microphones. Due to the lack of differences in time-of-arrival / phase-ambiguities, the sonic characteristic of X-Y recordings is generally less spacy and has less depth compared to recordings employing an AB-setup.

When the microphones are bidirectional and placed facing +-45° with respect to the sound source the X-Y-setup is called a Blumlein Pair. The sonic image produced by this configuration is considered by many authorities to create a most realistic, almost holographic soundstage.

See also Acoustic intensity.

A-B technique: time-of-arrival stereophony
This uses two parallel omnidirectional microphones some distance apart, so capturing time-of-arrival stereo information as well as some level (amplitude) difference information, especially if employed in close proximity to the sound source(s). At a distance of about 50 cm (0.5 m) the time delay for a signal reaching first one and then the other microphone from the side is approximately 1.5 msec (1 to 2 msec). According to Eberhard Sengpiel this is enough to locate the sound source exactly at the speaker on the respective side, resulting in a stereophonic pickup angle of 180°. If you increase the distance between the microphones you effectively decrease the pickup angle. At 70 cm distance it is about equivalent to the pickup angle of the near-coincident ORTF-setup.

M/S technique: Mid/Side stereophony
This coincident technique employs a bidirectional microphone facing sideways and a cardioid(of any characteristic; Alan Blumlein described the usage of an omnidirectional transducer in his original patent) at an angle of 90° facing the sound source. The left and right channels are produced through a simple matrix: Left = Mid + Side, Right = Mid - Side (the polarity-reversed side-signal). This configuration produces a completely mono-compatible signal, the width of which can be manipulated after the recording has taken place, which makes it especially useful for the usage on film-based projects.

Near-coincident technique: mixed stereophony
For example, the ORTF stereo technique of the Office de Radiodiffusion Télévision Française (i.e., Radio France), calls for a pair of cardioid microphones placed 17 cm apart at a total angle between microphones of 110 degrees which, according to Eberhard Sengpiel, results in a stereophonic pickup-angle of 96°. In the NOS stereo technique of the Nederlandse Omroep Stichting (i.e., Holland Radio), the total angle between microphones is 90 degrees and the distance is 30 cm, so capturing time-of-arrival stereo information as well as level information. It is noteworthy that the spacing of 17 cm has nothing to do with human ear distance. The recorded signals are generally intended for playback over stereo loudspeakers and not for ear phones.

Binaural recording
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.

Stereo in vinyl records
In 1958 the first stereo two-channel records were issued – by Audio Fidelity in the USA and Pye in Britain, using the Westrex "45/45" single-groove system. While the stylus moves horizontally when reproducing a monophonic disk recording, on stereo records the stylus moves vertically as well as horizontally.

One could envision a system in which the left channel was recorded laterally, as on a monophonic recording, with the right channel information recorded with a "hill-and-dale" vertical motion; such systems were proposed but not adopted, due to their incompatibility with existing phono pickup designs (see below). In the Westrex system, each channel drives the cutting head at a 45 degree angle to the vertical. During playback the combined signal is sensed by a left channel coil mounted diagonally opposite the inner side of the groove, and a right channel coil mounted diagonally opposite the outer side of the groove.

It is helpful to think of the combined stylus motion in terms of the vector sum and difference of the two stereo channels. Effectively, all horizontal stylus motion conveys the L+R sum signal, and vertical stylus motion carries the L-R difference signal. The advantages of the 45/45 system are:
 * greater compatibility with monophonic recording and playback systems. A monophonic cartridge will reproduce an equal blend of the left and right channels instead of reproducing only one channel. Conversely, a stereo cartridge reproduces the lateral grooves of monophonic recording equally through both channels, rather than one channel.
 * a more balanced sound, because the two channels have equal fidelity (rather than providing one higher-fidelity laterally recorded channel and one lower-fidelity vertically recorded channel);
 * higher fidelity in general, because the "difference" signal is usually of low power and thus less affected by the intrinsic distortion of hill-and-dale recording.

This system was invented by Alan Blumlein of EMI in 1931 and patented the same year. EMI cut the first stereo test discs using the system in 1933. It was not used commercially until a quarter of a century later.

Stereo sound provides a more natural listening experience where the spatial location of the source of a sound is, at least in part, reproduced.

The development of quadraphonic records was announced in 1971. These recorded four separate sound signals. This was achieved on the two stereo channels by electronic matrixing, where the additional channels were combined into the main signal. When the records were played, phase-detection circuits in the amplifiers were able to decode the signals into four separate channels. There were two main systems of matrixed quadrophonic records produced, confusingly named SQ (by CBS) and QS (by Sansui). They proved commercially unsuccessful, but were an important precursor to later 'surround sound' systems, as seen in SACD and home cinema today. A different format, CD-4 (not to be confused with compact disc), by RCA, encoded rear channel information on an ultrasonic carrier, which required a special wideband cartridge to capture it on carefully-calibrated pickup arm/turntable combinations. Typically the high frequency information inscribed onto these LPs wore off after only a few playings, and CD-4 was even less successful than the two matrixed formats.

FM
In FM broadcasting, the Zenith-GE pilot-tone stereo system is used throughout the world.

AM
Because of the limited audio quality of the majority of AM receivers, and because of the relative scarcity of AM stereo receivers, relatively few stations employ stereo. Various modulation schemes are used for AM stereo, of which the best-known is Motorola's C-QUAM which is the official method for most countries in the world which decide to use AM Stereo.

More AM stations are adopting digital HD Radio which allows the transmission of stereo sound on AM stations.

DAB
MP2 audio streams are used.

DAB is one of the Digital Radio format which is used to broadcast Digital Audio over terrestrial broadcast network or Satellite network. DAB is extended to Video and called new format as DMB.

Analog TV (PAL and NTSC)
Various modulation schemes are used in different parts of the world to broadcast more than one sound channel. These are sometimes used to provide two mono sound channels in different languages rather than stereo.
 * Multichannel television sound is used mainly in the Americas.
 * NICAM is widely used in Europe.
 * Zweikanalton is used in Germany.
 * EIAJ FM/FM subcarrier system is used in Japan.

Digital TV
MP2 audio streams are widely used within MPEG-2 program streams.

1881
Clément Ader demonstrated the first two-channel audio system in Paris in 1881, with a series of telephone transmitters connected from the stage of the Paris Opera to a suite of rooms at the Paris Electrical Exhibition, where listeners could hear a live transmission of performances through receivers for each ear. Scientific American reported,
 * Every one who has been fortunate enough to hear the telephones at the Palais de l'Industrie has remarked that, in listening with both ears at the two telephones, the sound takes a special character of relief and localization which a single receiver cannot produce. . . . This phenomenon is very curious, it approximates to the theory of binauriclar auduition, and has never been applied, we believe, before to produce this remarkable illusion to which may almost be given the name of auditive perspective.

This two-channel telephonic process was commercialized in France from 1890 to 1932 as the Théâtrophone, and in England from 1895 to 1925 as the Electrophone. Both were services available by coin-operated receivers at hotels and cafés, or by subscription to private homes.

1930s
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.

Several stereophonic test recordings, using two microphones connected to two styli cutting two separate grooves on the same wax disc, were made with Leopold Stokowski and the Philadelphia Orchestra at Philadelphia's Academy of Music in March 1932. The first, made on March 12, 1932 of Scriabin's Prometheus: Poem of Fire, is the earliest surviving stereo recording.

Duke Ellington and His Orchestra made some accidental stereo recordings on February 3, 1932 for RCA Victor. It was a fairly standard practice in that era to record using more than one microphone and disc cutter. The various versions could be compared, to see which had the best microphone positioning. It also allowed for safety masters in case something happened to the original. Even through the records are fairly rare, a collector had both versions and noticed that while they appeared to be the same performance, the sound mix was different on each. When the two recordings were synchronized, it became stereo.

Bell Laboratories gave a demonstration of three-channel stereophonic sound on April 27, 1933 with a live transmission of the Philadelphia Orchestra from Philadelphia to Constitution Hall in Washington, D.C. Leopold Stokowski, normally the orchestra's conductor, was present in Constitution Hall to control the sound mix. Bell Labs also demonstrated binaural sound, using a dummy with microphones instead of ears, at the Chicago World's Fair in 1933.

Two stereophonic recording methods, using two channels and coincident microphone techniques (X-Y with bidirectional transducers / Blumlein-setup & M/S-stereophony), were developed by Alan Blumlein at EMI in 1931 and patented in 1933. A stereo disc, using the two walls of the groove at right angles to carry the two channels, was cut at EMI in 1933, twenty-five years before that method became the standard for stereo phonograph discs.

1940 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.

In the early 1950s, companies such as Concertapes and Victor 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. Stereo recording became widespread in the music business by the fall of 1957, superseding monaural (single-channel) recording.

In 1952 Emory Cook (1913–2002), who already made fame by designing new feedback disk cutter heads to improve sound from tape to vinyl, developed a 'binaural' record. This record consisted of two separate channels cut into two separate grooves running next to each other. Each groove needed a needle and each needle was connected to a separate amplifier and speaker. The set-up was intended to give a demonstration at a New York audio fair of Cook's cutter heads rather than to sell the record. But soon afterwards the demand for such recordings and the equipment to play it grew, and Cook Records began to produce such records commercially. He recorded a vast array of sounds, ranging from railroad sounds to thunderstorms. (The term 'binaural' that Cook used should not be confused with the modern use of the word, where 'binaural' is an inner ear recording using small microphones placed in the ear. Cook used conventional microphones but gave his stereo record the name 'binaural' record.)

The small record company Audio Fidelity released the first commercial stereophonic disc in November 1957. Sidney Frey, founder and president, had Westrex cut a disk for release before any of the major record labels. Side 1 was the Dukes of Dixieland, Side 2 was railroad sound effects. On December 16, Frey advertised in the trade magazine Billboard that he would send a free copy to anyone in the industry who wrote to him on company letterhead.

That move generated a great deal of publicity. Frey promptly released four additional stereo disks. The equipment dealers had no choice but to demonstrate on Audio Fidelity Records.

The 1940 Carnegie Hall demonstration
The Carnegie Hall demonstration by Bell Laboratories on April 9 and 10, 1940, 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 1,500 watts of acoustic power, producing sound levels of up to 100 decibels, and the demonstration held the audience "spellbound, and at times not a little terrified," according to one report. 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
Bell Laboratories in New York City gave a demonstration in 1937 of two-channel stereophonic motion pictures, developed by Bell Labs and Electrical Research Products, Inc.

In 1938, MGM started using three tracks to record movie soundtracks instead of one and very quickly upgraded to four tracks. One track was used for dialogue, two for music and one for sound effects. These optical soundtrack recordings could easily be "mixed" down to a mono track for film release. The very first true stereo recording MGM made (although released in mono) was "It Never Rains But What It Pours" by Judy Garland, recorded on June 21, 1938 for the movie Love Finds Andy Hardy.

The first commercial motion picture to be exhibited with stereophonic sound was Walt Disney's Fantasia, released in November 1940, for which a specialized sound process, Fantasound, was developed. Fantasound used a separate film containing four optical sound tracks. Three of the tracks were audible, and the fourth track controlled the volume level of the theater's amplifiers. The film was not a financial success, however, and after two months of road-show exhibition in selected cities, its soundtrack was remixed into mono sound for general release.

In the early 1940s, the forward thinking Alfred Newman directed the construction of a sound stage equipped for multi channel recording for 20th Century Fox studios. Several soundtracks from this era still exist in their multichannel elements, some of which have been released on DVD including How Green Was My Valley, Anna and the King of Siam, Sun Valley Serenade, and The Day the Earth Stood Still.

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. Stereo sound first became a wide success with the release of This Is 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 seven magnetic sound tracks, six of them audible plus a seventh track that controlled the volume level of the amplifiers. 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.

In April 1953, while This Is Cinerama was still playing only in New York City, most moviegoing audiences heard stereophonic sound for the first time with the Warner Bros. 3-D film production of House of Wax, starring Vincent Price. The sound system, WarnerPhonic, was a combination of a 35mm magnetic full-coat that contained Left-Center-Right, in synchronization with the two, dual-strip Polaroid system projectors, one of which carried an optical surround track, and one which carried a mono backup track should anything go wrong. Only one other film carried WarnerPhonic sound, the 3-D production of The Charge at Feather River. Both magnetic tracks to these films are considered lost.

Many 3-D films carried variations on 3-track magnetic sound. Other instances include It Came From Outer Space, I, The Jury, The Stranger Wore a Gun, Inferno, Kiss Me Kate, and many others.

By the summer of 1953, the movie industry moved quickly to create simpler and cheaper wide-screen systems, such as CinemaScope, which used up to four magnetic sound tracks, and 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.

Early broadcasting in stereo
Radio: The BBC's experimental transmitting station 5XX in Daventry, Northamptonshire, made radio's first stereo broadcast in December 1925, of a concert conducted by Sir Hamilton Harty from Manchester, with 5XX broadcasting the right channel nationally by long wave, and local BBC stations broadcasting the left channel by medium wave. The BBC repeated the experiment in 1926, using 2LO in London and 5XX at Daventry. Following experimental FM stereo transmissions in the London area in 1958, the first regular BBC transmissions using an FM stereo signal began on the BBC's Third Programme network on August 28, 1962.

Chicago AM radio station WGN and its sister FM station WGNB collaborated on an hour-long stereophonic demonstration broadcast on May 22, 1952, with one audio channel broadcast by the AM station and the other audio channel by the FM station. New York City's WQXR initiated its first stereophonic broadcasts in October 1952, and by 1954 was broadcasting all of its live musical programs in stereophonic sound, using its AM and FM stations for the two audio channels.

After several years of experimental stereo broadcasts, and six competing systems, the Federal Communications Commission announced stereophonic FM technical standards in April 1961, and licensed regular stereophonic FM radio broadcasting to begin in the United States on June 1, 1961. WEFM in the Chicago area and WGFM in Schenectady, New York reported as the first stereo stations.



Television: A closed-circuit television performance of Carmen from the Metropolitan Opera House in New York City to thirty-one theaters across the United States on December 11, 1952 included a stereophonic sound system developed by RCA. The 1958-1959 season of The Plymouth Show (i.e., The Lawrence Welk Show) on the ABC network was broadcast with stereophonic sound in some cities, with one audio channel broadcast via television and the other over the ABC radio network. By the same method, NBC television and the NBC radio network offered stereo sound for The George Gobel Show on October 21, 1958. ABC's Walt Disney Presents made a stereo broadcast of The Peter Tchaikovsky Story, including scenes from Disney's latest animated feature Sleeping Beauty, on January 30, 1959 by using ABC-affiliated AM and FM stations for the left and right audio channels.

With the advent of FM Stereo in the 1960's, a small number of music oriented shows were broadcast with stereo sound using a process called simulcasting in which the audio portion of the show was carried over a local FM stereo station. In the 1960's and 1970's, these shows were usually manually synchronized with a mail delivered reel-to-reel tape to the FM station (unless the concert or music was locally originated). In the 1980's, satellite delivery of both television and radio programs made this fairly hard process of synchronization moot. One of the last of these simulcast programs was Friday Night Videos on NBC, just before MTS stereo was approved by the FCC.

Cable TV systems delivered many stereo programs utilizing this method for many years until prices for MTS stereo modulators dropped. One of the first stereo cable stations was The Movie Channel, though the most popular cable TV station that drove up usage of stereo simulcasting was MTV.

MTS: Stereo for television

Multichannel television sound, better known as MTS (often still as BTSC, for the Broadcast Television Systems Committee that created it), is the method of encoding three additional channels of audio into an NTSC-format audio carrier. It was adopted by the FCC as the U.S. standard for stereo television transmission in 1984. Regular network transmission of stereo audio began on NBC that year.

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 thus worth noting that most film soundtracks are not recorded using stereo techniques, so while they are capable of stereo reproduction, most home theater systems rarely are called upon to do this.

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
Balance can mean the amount of signal from each channel reproduced in a stereo audio recording. Typically, a balance control will have 0 dB of gain in the center position for both channels, and attenuate one channel as the control is turned, leaving the other channel at 0 dB.

See also Panning

Other uses of the word
The phrase "rechanneled stereo" refers to the artificial rechanneling of mono recordings to simulate stereo that was common in the 1950s-1960s (see Duophonic). Because of this usage, "stereo" or "in stereo" is sometimes used colloquially for when two, as distinct from one, of something are present.