Stereophony  or Stereo sound (from other Greek στερεός “stereo” - “solid, spatial” and φωνή - “sound”) - a method of reproducing sound , which creates the illusion of “sound perspective” with preserving directions to different sound sources . This is achieved through the use of the binaural effect and the simultaneous transmission of sound information through two or more independent channels , in contrast to monophonic sound transmission, when sound is transmitted through a single channel.
In stereophonic usage, it is customary to call two-channel sound recording , however this concept covers a wider sphere, denoting surround sound with any number of channels. The term “stereo” equally refers to concepts such as “quadrophony”and surround sound . When a stereo recording is played through a single speaker, the sound quality is reduced due to the interference of the signals of different channels that differ in phase .
- 1 Varieties of stereo
- 1.1 In the cinema
- 1.2 On the radio
- 2 Stereo format development
- 3 Quadrophony
- 4 See also
- 5 notes
- 6 Sources
- 7 Literature
Varieties of stereo
There are two fundamentally different ways to get stereo recordings. At the first, two or more microphones spaced a certain distance from each other are used to fix the sound field. The pulsating sound frequency current generated by each of the microphones is amplified by separate amplifiers independent of each other, and recorded on separate channels of the medium . During playback, the sound received from each of the microphones is amplified independently and is transmitted to individual speakers or speakers located in accordance with the placement of the microphones. In this way, the sound picture that existed at the time of recording is recreated. An ideal way to reproduce a sound field is a conventional plane, consisting of infinitely small speakers that transmit sound from microphones located in the same way. In the sound recording of musical works to obtain a stereo effect, the presence of two independent channels is considered sufficient, and in the cinema there should not be less than three to eliminate the “failure” in the center of the sound picture corresponding to the middle of the screen. So, in the widescreen cinema systems “ CinemaScope ” and “ Wide Screen ”, three frontal channels were used, and in the widescreen systems “ Todd-AO ” and “ NIKFI ” - five   .
The second method is called pseudo stereo , and unlike the first, it does not require several microphones to record the sound field. A stereo phonogram is created by distributing several different monophonic recordings ( see Multitrack Recording ) through different channels using a “panoramic mixer”  . When creating pseudo-stereo phonograms, different sound sources are recorded separately or separate microphones are installed for them. Then, in the process of mixing (mixing), the received signals are distributed over the channels of the finished phonogram. In this case, the effect of localization of sound sources is created both by adjusting the recording level in different channels and by correcting the frequency response , since it is known that high frequencies affect the illusion of direction to the greatest extent. In addition, the effect is additionally achieved by adjusting the intensity of the reflected sound and the degree of delay using the delay lines .
In the 1960s, the United States often practiced the release of so-called “duophonic” phonograph records , in which initially monophonic recordings were given a stereo effect. Mono recording was distributed over two channels with simultaneous correction of the frequency response. In the channel corresponding to the partial localization of sound, high frequencies were emphasized, while in the opposite, they were muffled while maintaining the level of low-frequency signals. After that, the sound in one of the channels was delayed for a split second, thus creating a false echo effect. Record companies did this with single-channel recordings made before the introduction of stereo technology ( Elvis Presley , Frank Sinatra records), or for any technical reasons recorded on a single-channel system, despite the possibility of stereo sound ( The Beatles [* 1] , The Beach Boys ; the so-called fake stereo ).
In the cinema
In cinema, pseudo stereophony replaced the multi-channel source recording in the early 1960s, due to the technological complexity of synchronous filming . In Hollywood, rulers of several microphones were abandoned in 1958, and in the USSR this technology was declared obsolete in 1968; one of the last Soviet large-format films, the soundtrack of which was recorded with five-channel stereo microphones, was the picture " War and Peace "  . Subsequently, the initial phonogram was recorded with single-channel tape recorders , followed by distribution over the channels using a panoramic mixer; at the Mosfilm film studio, a stereo dubbing console with a panoramic control was designed for this. 
The Perspekt's American pseudo stereophonic sound reproduction system was based on the automatic switching of a single sound track to different speakers. For this, additional control signals with frequencies of 30, 35 and 40 Hertz not reproduced by the amplifier were recorded in the phonogram. The signals of each frequency were connected by the corresponding loudspeakers located behind the screen of the cinemas of the Vistavision system, creating the effect of the sound following the image. 
On the radio
In FM broadcasting, the so-called pilot tone at a frequency of 19 kHz shows that there is stereo information at a frequency of 38 kHz (19 × 2, second harmonic). The radio doubles the frequency of the pilot tone and uses it as a reference phase to demodulate stereo information.
Stereo Format Development
For the first time the principle of stereo in practice was implemented in 1881 in Paris by Clement Ader . On the stage of the Paris Opera, Ader arranged a series of telephone translators, the signal from which went through wires to the cabins of the Paris Electric Exhibition . Visitors to the booths could hear the concert live through two handsets (for each ear).
Subsequently, the commercial production of dual-channel phones was adjusted: in 1890-1932. in France under the name "Théâtrophone", in 1895-1925. in the UK under the name "Electrophone"; they did not have mass distribution.
In sound recordings, the first tests in stereo were carried out in the early 1930s. A great contribution to the development of stereo was made by the invention of the English engineer Alan Blumlein . His stereo pair of microphones (“Blumlein pair”) still finds application in the practice of sound recording. It consists of two closely spaced bi-directional microphones oriented at right angles to each other. In 1931, he and his colleagues created a test movie called “Walking And Talking” using stereo sound, and in 1935, with his participation, the short film “Trains at Hayes” was created, which is the first ever film with stereo sound  .
In March 1932, a recording was made at the Philadelphia Academy of Music using two microphones , the signal from which went to two needles that cut two separate grooves on the same wax disk. The first of these recordings, made on March 12, 1932 (during the performance of Prometheus by Alexander Skryabin ), is the earliest stereo recording that has survived to this day. The first stereo record in history was released in 1933 by EMI : two separate channels, respectively, were placed on two sides of the groove. In 1940, the first full-length animated film " Fantasy " was created with a pseudo-stereo three-channel phonogram. The recording was reproduced according to the Fantasound system from a separate film. However, due to the extreme complexity of the system and the weak stereo effect, it did not receive further development  .
In February 1954, RCA Victor made stereo recordings using multichannel equipment: the performance of the Boston Symphony Orchestra was recorded simultaneously on both mono and two-channel film. Several more similar recordings followed, but they were not intended to be released on records, since there was no market for mass stereo audio playback equipment yet. However, a year later, in 1955 , the first stereo tape recorders appeared and the production of stereo bobbins started. Since 1957, the stereo recording process has become prevalent in the American music industry.
In 1958, Western Electric manufactured the Westrex apparatus, which made it possible to cut stereo discs, from which, finally, it was possible to replicate ordinary phonograph records (it was Western Electric that introduced the words “ stereo ”, “ stereo ” in the recording field). In the same year, the first commercial stereo record was released. However, until the end of the 1960s. a significant number of phonograph records came out in mono sounding, despite studio stereo recording.
Quadrophony (from lat. Quattuor - four and others. Greek. Φωνή - sound) - a kind of stereo, in which 4 independent channels are used. In contrast to the two-channel stereo sound that forms a panorama in front of the listener, the quad sound is intended to surround the listener from 4 sides and create a feeling of his presence “inside the sound”. Quadrodynamics, like microphones, are located in the corners of the room, but other arrangements are possible, for example, in the center of the walls. Quadro had limited success in the 60-80s of the last century, but did not commercially take place primarily due to the cost of the equipment, and also, as a consequence, the low demand for multichannel sound carriers. Today, interest in quadro is reviving due to the prevalence of multi-channel audio formats (5.1).
- Dolby digital
- Binaural record
- Sound recording
- Stereophony in dictionaries
- Konoplev, 1975 , p. 29.
- Handbook of the cameraman, 1979 , p. thirty.
- Cinema and stereo sound systems, 1972 , p. 122.
- Cinema and stereo sound systems, 1972 , p. 171.
- Cinema and stereo sound systems, 1972 , p. 164.
- Fundamentals of Film Engineering, 1965 , p. 498.
- About Alan Dower Blumlein, the godfather of sound recording and stereo sound
- Cinema and stereo sound systems, 1972 , p. 126.
- M.Z. Vysotsky. Cinema systems and stereo sound / Eisymont L. O .. - M .: "Art", 1972. - S. 122-124. - 336 p. - 3500 copies.
- E.M. Goldovsky . Fundamentals of film technology / L.O. Eisymont. - M.,: “Art”, 1965. - 636 p.
- I. B. Gordiychuk, V. G. Pell. Section I. Cinema systems // Directory of the cameraman / N. N. Zherdetskaya. - M.,: “Art”, 1979. - S. 25-34. - 440 s.
- B. N. Konoplev . Fundamentals of film production / V. S. Bogatova. - 2nd ed .. - M .: "Art", 1975. - 448 p. - 5,000 copies.