Stereophony

It all started in 1881 as one of the attractions of the World Electricity Exhibition in Paris when Clément Ader presented the "Therophone". A technical device that aimed to make a stereophonic diffusion to two channels (headphones) through a system of telephone lines between the Palais des L'Ane d'Autofère and the Paris Opera. On the edge of the stage they placed 10 charcoal microphones that would correspond to 2 microphones per "auditor". These microphone pairs were directed to the corresponding pairs of hearing aids. The result of this experience was the approximate representation of the movements of the actors in the scenic space.

At the beginning of the 30s, research was concentrated in two important laboratories: Hervey Fletcher of Bell Telephone in the USA and Alan Blumlein of EMI in England. Fletcher placed, with little success, a row of omnidirectional microphones in front of the stage connected to an equal number of speakers in the audition room. For his part, Blumlein proposed the use of only two channels and thus defined stereophonic hearing with the auditor located in front of two speakers that form with him, an equilateral triangle with corresponding angles of 60°. This positioning scheme is preserved to this day as an international standard in stereophony. As a result of his research, Blumlein filed a patent in 1931 in which he describes what would be called "Intensity Stereophony" and proposes 3 intensity capture systems with pairs of matching microphones: STEREOSONIC, XY and MS, which I will explain later.

In 1940, Kees de Boer takes up an experience of 1927 developed by Barlett Jones in Chicago (the OSCAR doll with 2 microphones in the place of the ears), and analyzes the location of a virtual sound source based on the differences in level and time of arrival of the signals. In cinema we will have the first stereophonic experience with the FANTASOUND system of the film FANTASIA in 1941. Then came the CINERAMA 6 channels in 1952 and the Dolby Stereo in 1975. The first STEREO TV broadcasts were made in Japan in 1978 and only until 1984 were made 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 correlating the information of the same. The "location" of a sound source, both horizontally and vertically, requires a differential analysis of the intensities (∆I) and the gaps (∆T) in the times of arrival of the sound to each of the ears.

The localization is also done taking into account the depth or distance at which the sound source is located. This third dimension to consider, added to the vertical and the horizontal, allows a three-dimensional spatial analysis as if it were a solid, hence the name stereophony that comes from the Greek STEREOS and means solid.

If we take a sound source located in front of an auditor (0°) and move it to the right, the source will be little, but significantly closer to the right ear than to the left: The perceived difference in intensity (∆I) will be analyzed accordingly and the deduction will be that the sound source is to the right of the auditor.

Likewise, to the right ear the sound arrives first than to the left one that is a little further away (a few milliseconds of difference). The difference in arrival times (∆T) will be analyzed by the auditory cortex and the result will be the same. The extreme lateral location, that is, when the sound source is at 90° indicates that the source will be perceived with a time difference (∆T) of 0.65 milliseconds (ms), which corresponds to an additional path of 21 cm to reach the left ear. Conversely, the maximum difference in intensity (∆I Max.) which is 7 dB (decibels) does not correspond to a sound source located at 90° but at 60° and 120°. At 90° it is on the order of 6 dB.

It is good to specify that the minimum threshold of angular discrimination in the horizontal plane is 1° to 2° in front of the auditor. We cannot say the same in the vertical plane where the location is less precise (15° to 20°) for a source located above the head.

For each position of a sound source the difference in the intensity perceived by each ear is a function of the frequencies contained in the perceived sound. Hence, we talk about Intraural Transfer functions to define the parameters linked to localization (∆I, ∆T). Man memorizes in the course of his life a multitude of Transfer Functions corresponding to different directions. Even small instinctive head movements better require localization by giving the auditor several Transfer Functions for each sound source.

When we listen with hearing aids we talk about an intracranial location in the absence of Transfer Functions: We can "recover" those functions using the stereophonic capture device called "artificial head" where we place two microphones in the place of the ears of a mannequin. (OSCAR; BLUMLEIN, already cited).

Stereophonic hearing is based on the reproduction of sound with the help of two speakers. As we had already mentioned, the stereophonic listening angle is 60°. The distance from the auditor to each of the speakers must be the same and equal to the distance between the two speakers (auditor and speakers forming an equilateral triangle).

The goal of stereophony is to create an illusion of acoustic relief from a solid acoustic image. The psychoacoustic mechanism of stereophonic localization like hearing with hearing aids is based on differences in time and intensity. When the restored sound source is located in one of the speakers, we are talking about a "Real Source". When we locate a sound source between or beyond the speakers, we are perceiving a "Virtual Source".

To conclude, we can say that there is the possibility of "compensating" a displacement of the sound source restored ∆T, with a variation of intensity ∆I to return the sound source to its initial position.

This system takes into account only one of the parameters of the location such as the difference in Intensity ∆I. There are 3 types of stereophonic intensity systems: the XY, the STEREOSONIC and the MS. These systems to 2 matching microphones require an angulation between them (physical angle), which when modified produces a variation of the useful angle of capture. When the physical angle between the microphones is decreased, the useful angle of capture is increased and vice versa.

It consists of 2 identical microphones with cardioid directionality, which are arranged in such a way that their capsules coincide on the vertical axis forming an angle between 80° and 130°, which corresponds to a useful angle of capture of 180° to 130° respectively. Among its characteristics we can mention its good monophonic compatibility due to the lack of time difference ∆T. This absence produces an acoustic image lacking spatiality and depth but the sound elements remain well located in the stereophonic space.

It consists of the location of two bidirectional microphones forming a physical angle of capture of 90° between the matching capsules: The resulting useful angle of capture is 70° to the front and back. You have to be careful with its use because the location of the rear capture is inverted and superimposed on the front.

This system associates a microphone with cardiode directionality directed towards the scene and a bidirectional microphone oriented perpendicular to the other microphone and, therefore, to the axis of symmetry of the sound scene. The cardiode microphone captures a "monophonic" information (M) and the bidirectional one a lateral information (S). The resulting left (L) and right (R) signals are obtained by sending the M and S signals to a decoder circuit from which the left channel (L) = M+S, and the right channel (R) = M-S (-S, is the negative lobe of the bidirectional microphone out of phase 180° with respect to S).

The great advantage of the MS system and hence its great utility in sound recording for Film and TV is to be able to vary the useful angle of capture without having to intervene mechanically on the microphones, simply by varying the S signal with respect to the M signal. the more we increase the S level, more the useful angle of capture is decreased and vice versa.

In the cinematographic sound application, the "opening" of the stereophonic base is controlled according to the other elements (music, dialogues).

In this case the arrangement of the microphone pair does not take into account the difference in Intensity due to the use of omnidirectional microphones and a large source-microphone distance with respect to the separation between them. When the space between the microphones is decreased, the useful angle of capture is increased and vice versa. To maintain a good homogeneity of the stereophonic space, distances between 25 and 50 cm must be used, which corresponds to catchment angles of 180° to 130° respectively.

As the name implies, this system takes into account both ∆T and ∆I. In addition to the space between the two microphones, an angulation is involved. We also call this system AB stereophony, slightly spaced. The microphones used are cardiodes. The useful angle of capture is increased if we decrease the separation or if we close the physical angle between the microphones or vice versa.

There are several types of AB pairs that were proposed by different broadcasting entities such as the ORTF (French Radio and Television Bureau – Radio France today), from them arises the most used system in the world which they called "colette". This pair of cardiodes microphones is separated 17 cm and the physical angle is 110°, which gives us a capture angle of 90°.

For a better understanding of the concept of stereophony, a more detailed description of the technical resources applied specifically to Control Rooms will be made in the next installment.