It all started in 1881 as one of the attractions World Exhibition of Electricity in Paris when Clément Ader presented the "Teatrófono". A technical device intended to make a stereo broadcast two channels (headphones) through a system of telephone lines between the Palace of Industry and the Paris Opera. On the edge of the stage microphones placed 10 to 2 carbon microphones correspond to "auditor". These microphone pairs were targeted to corresponding pairs of headphones. The result of this experience was the approximate representation of the movements of the actors on the stage space.
In the early years 30, research focused on two major laboratories: Hervey Fletcher of the Bell Telephone in USA and Alan Blumlein EMI in England. Fletcher stood, with little success, a line of omnidirectional microphones facing the stage connected to an equal number of speakers in your listening room. Meanwhile, Blumlein proposed using only two channels and thus defined stereophonic hearing with the auditor opposite two speakers forming with it an equilateral triangle with angles corresponding 60 °. This positioning scheme is preserved to this day as an international standard in stereo. Result of their investigations, Blumlein presented in 1931 a patent which describes what "Stereo Intensity" be called and proposes 3 collection systems intensity matched pairs of microphones: ESTEREOSONIC, XY and MS, which explain later.
In 1940, Kees de Boer takes experience 1927 developed by Barlett Jones in Chicago (the OSCAR doll with 2 microphones in place of ears), and analyzes the location of a virtual sound source based on the differences in level and arrival time of the signals. In the first film we stereophonic experience with Fantasound FANTASIA film set in 1941. Then came the CINERAMA to 6 1952 channels and Dolby Stereo in 1975. The first stereo TV broadcasts were made in Japan and only until 1978 1984 were conducted in Europe.
Stereophonic uptake is based on the principles of binauricular perception. This means that we have two ears that "collect" the sounds and send them to the auditory cortex of the brain to be processed and analyzed by correlating the information from them. The "location" of a sound source, both the horizontal and vertical planes, demand a differential analysis of the intensities (ΔI) and gaps (DT) in the arrival times of sound at each ears.
The location also performed taking into account the depth or the distance at which the sound source is. That third dimension to consider, together with the vertical and horizontal, allows a three-dimensional spatial analysis as if it were a solid, hence the name stereophonic STEREOS from the Greek and means solid.
If we take a sound source located in front of an auditor (0 °) and move to the right, the source will be little, but significantly closer to the right ear than the left: The difference in intensity (ΔI) perceived to be analyzed accordingly and the inference is that the sound source is located to the right of auditor.
Likewise, the right ear the sound reaches first the left is a little further (some milliseconds apart). The time difference (.DELTA.T) of arrival will be analyzed by the auditory cortex and the result will be the same. The lateral location extreme, that is, when the sound source is 90 ° indicates that the source will be perceived with a time difference (.DELTA.T) of 0,65 milliseconds (ms), which corresponds to an additional path 21 cm to reach the left ear. Contrary maximum intensity difference (ΔI max.) Which is 7 dB (decibels) does not correspond to a sound source located 90 ° but 60 ° and 120 °. A 90 ° is about 6 dB.
It's good to point out that the minimum threshold of angular discrimination in the horizontal plane is 1 2 ° ° against the auditor. We can not say the same in the vertical plane where the location is less accurate (15 20 ° °) for a source located above the head.
For each position of a sound source the difference in perceived intensity for each ear is a function of frequencies containing the perceived sound. Hence talk-ear function Transfer to define the parameters related to the location (ΔI, DT). Man stored in the course of his life a multitude of functions corresponding to different directions Transfer. Even small head movements instinctual need better location giving the auditor several transfer functions for each sound source.
When we listen with headphones talking about a location in the absence of intracranial Transfer Functions: We can "recover" those functions using the stereo capture device called "artificial head" where two microphones are located at the site of the ears of a dummy. (OSCAR; BLUMLEIN, already cited).
Stereophonic hearing is based on sound reproduction with the help of two speakers. As we have already said, the angle stereophonic hearing is 60 °. The distance auditor to each of the speakers must be the same and equal to the distance between the two speakers (speakers auditor and forming an equilateral triangle).
The aim of the estereofonía is to create an illusion of relief acoustic sound acoustic image. Psychoacoustic stereo localization mechanism like hearing hearing aid is based on differences in time and intensity. When the restituted sound source is located in one of the speakers, we are talking about a "Source Real". When we locate a sound source between or beyond the loudspeakers, we are receiving a "virtual source".
To conclude, we can say that it is possible to "compensate" a shift of the sound source restituted DT, with a variation of intensity ΔI to return to its initial position sound source.
Uptake systems stereophonic sound.
This system takes into account only one of the parameters of the location as intensity difference ΔI. There 3 types of stereo systems Intensity: XY, the ESTEREOSONIC and MS. These systems require 2 coincident microphones angulation between them (physical angle), that produces a modified variation of useful pickup angle. When the physical angle between the microphones is reduced, the useful angle is increased uptake and vice versa.
It is 2 identical cardioid directivity, which are arranged such that their capsules match on the vertical axis at an angle between 80 ° and 130 °, which corresponds to a useful pickup angle 180 ° to 130 ° respectively . Among its features we can mention their good monophonic compatibility due to lack of time difference .DELTA.T. This absence results in a lack of spatial depth acoustic image and sound elements but remain well located in the stereophonic space.
Is the location of two bidirectional microphones forming a physical pickup angle 90 ° between the mating capsules: The useful angle resulting feedback is 70 ° forward and backward. You must be careful with their use because the location of the subsequent uptake is reversed and superimposed on the front.
This system associates a cardioid microphone directionality directed toward the scene and a bidirectional microphone oriented perpendicularly relative to the other microphone and therefore the axis of symmetry of the soundstage. The cardioid microphone captures a "monophonic" information (M) and the bi-lateral information (S). The right (R) resulting signals left (L) and obtained by sending the M and S signals to a decoder circuit which leave the left channel (L) = M + S, and the right channel (R) = MS (-S it is the negative lobe of phase bidirectional microphone 180 ° to s).
The great advantage of the MS system and hence its usefulness in the sound recording film and TV is the ability to vary the useful collection angle without mechanically intervene on the microphones, simply by varying the signal S with respect to the signal M. the more increased the level of S, more useful pickup angle decreases and vice versa.
In the film sound application of the "opening" of the stereo basis depending on the other elements (music, dialogues) it is controlled.
In this case the arrangement of the microphone pair does not consider the difference of intensity due to the use of omnidirectional microphones and a source-microphones large distance from the separation between them. When the space between microphones is decreased, the useful collection angle is increased and vice versa. To maintain good homogeneity should be used stereophonic space distances between 25 and 50 cm, corresponding to angles uptake 180 ° to 130 ° respectively.
Stereo time and intensity
As its name implies, this system takes into account both .DELTA.T as ΔI. In addition to the space between the two microphones intervenes angulation. We also call this system stereo AB, slightly spaced. Cardioid microphones are used. the useful collection angle is increased if we decrease the separation or close physical angle between the microphones or the reverse.
There are several types of pairs AB which were proposed by different broadcasters as the ORTF (Office of French Radio and Television - Radio France currently), of which the most widely used system in the world which they called "colette" arises. This pair of cardioid microphones is separated 17 cm and the physical angle is 110 °, which gives us a pickup angle 90 °.
For a better understanding of the concept of stereo in the next installment will be made a more detailed description of the technical resources applied specifically to the control room (Control Rooms).