Runtime stereophony

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Run-time stereophony or AB stereophony is a recording method of loudspeaker stereophony in which two parallel (parallel is common here with a microphone base smaller than 1 m) pointing forward individual microphones at a given distance from each other - the microphone base - and at a suitable distance from the sound body as the main microphone system to be ordered.


The in microphones used for this purpose microphones are mainly those having the characteristic bullet when everyone else without restrictions directional characteristics can be used. A distinction is made between “small A / B” up to a microphone base width of less than 35 cm and “large A / B” with a correspondingly larger base. This value is not generally determined.

Procedure for AB microphone recording

In the case of AB stereophonic microphones, transit time differences ∆ t arise and thus also phase differences between the two identical microphones, which enable stereo imaging when reproducing via loudspeakers. The perception effect is used that the human ear can localize the direction of a sound source due to the time difference, which is given by the distance between the eardrums , here (in contrast to so-called intensity stereophony ) solely through the transit time difference instead of through a phase angle (> 0 °) of the microphone membranes.

The choice of the stereo microphone base , i.e. the distance between the left-right microphones, has a considerable influence on the perceived 'breadth' of the stereophonic impression. In practice, this can be between a few cm and several meters, depending on the width of the sound source being microphone (solo instrument - symphony orchestra) and the distance between the pair of microphones and the sound source.

In addition, depending on the directional characteristics of the microphones used, the microphone base (distance between the microphone diaphragms) also determines the recording area (detection cone, spheres, cardioids, etc.).

With A / B miking, the membrane axes are aligned parallel (axis angle = 0 °), as long as there is no technical reason for the axis rotation (> 0 °), because otherwise, in addition to the transit time differences, there are also frequency-weighted level differences that increase in the high frequency range, i.e. unwanted partial differences (differences in multiple wavelengths, see partials ) that lead to discoloration and unclear localization in stereo loudspeaker reproduction.

If the distance between the two microphone diaphragms is set too large in relation to the recorded sound source, the effect of the so-called “hole in the middle” occurs (absence of the so-called phantom sound source ). A similar effect occurs when the AB microphone pair is positioned proportionally too close to the sound source. This leads to a magnifying glass effect in the loudspeaker reproduction . On the other hand, a proportionally too small microphone base impairs a clear localization in the stereo sound stage.

The basic width of 20 cm between the microphones, which is sometimes chosen for a main microphone system , which is based on an average interaural distance, leads to a not fully utilized maximum stereo width of the loudspeaker base .

A left-right time difference of 1 - 1.5 milliseconds is required for maximum precise localization of the so-called phantom sound sources via a pair of speakers. - This running time results (at normal air pressure) with a microphone distance of approx. 73 cm and an axial angle of the membranes of approx. 45–90 °.


There is no such thing as pure run-time stereophony, because the different distances between the microphones and the sound source always result in a certain level difference, which also has an effect on the directional localization. A transit time difference between one and two milliseconds ( ∆ t = 1 to 2 ms) leads to a hearing event direction of 100%, i.e. fully from the direction of a loudspeaker (calculated value ∆ t = 1.5 ms). Any greater difference in transit time causes the phantom sound source to emit from only one loudspeaker direction. The precedence effect ("law of the 1st wavefront", Haas effect ) also works here. Percussive signals, crackling and high frequencies require a small transit time difference in the vicinity of 1 ms for full deflection of the auditory event direction in the direction of a loudspeaker. Softly oscillating signals and low frequencies, on the other hand, need a significantly larger delay time up to a little more than 2 ms for full deflection. The respective deflection of the phantom sound source on the loudspeaker base is called the direction of the auditory event .

In the past, this type of recording was not welcomed because of the incompatibility with mono, especially in broadcasting. A sound recording with a main microphone system is often supplemented by support microphones .

AB microphone systems have different tasks that should be carefully distinguished:

The AB main microphone system
When recording in stereo, it is necessary to strive for the closest possible “direct signals”, which are in the foreground as phantom sound sources in the sound image, as evenly as possible on the loudspeaker base (→ direction of the audio event ). If this is not the case, then the particular runtime stereo system cannot be used as the main microphone. The microphone base is a maximum of one meter.
The AB mixer microphone system
This mixer microphone system with a base width of about 1.20 m AB (spheres) is at a certain distance behind the main microphone system, which is not too close to the sound sources. This brings "life" to the recorded sound and "air" - as well as a feeling of envelopment for low frequencies, the depth gradation also increases and the sound gains in size in acoustically good rooms. Due to the dissimilarity of the signals to the main microphone, there are no audible comb filter effects . The microphone base, which is usually too large for an even distribution of sound sources, is less critical because the signals generated in this function do not necessarily have to be directional.
The AB room microphone system
When setting up an "AB room microphone system" you don't have to worry about an even distribution of the room signals, because you live with the added left-right-flank space, which must not exceed a certain mixing level, so as not to be considered Errors become noticeably audible. In extreme cases, a boundary microphone is attached to the left and right of the side wall of the concert hall. Due to the very large microphone base, there is one room from the left speaker and one from the right speaker. This is the typical "flank" space, which can still be easily integrated into the sound. The microphone base is 2 m and more.

Some theorists propose an ORTF stereo system as a room microphone pointing away from the sound source.

In particular, the transit time delay when mixing a sound recording is often used for the purpose of artistic design (manipulation) with different effects.


  • Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr: Handbuch der Tonstudiotechnik , 7th completely revised and expanded edition, published by the ARD.ZDF medienakademie, Nuremberg, 2008, 2 volumes, publisher: KG Saur, Munich, ISBN 3- 59811-765-5 or ISBN 978-3-598-11765-7
  • Hubert Henle: The recording studio manual. Practical introduction to professional recording technology. 5th, completely revised edition. Carstensen, Munich 2001, ISBN 3-910098-19-3 .
  • Peter M. Pfleiderer: HIFI in a nutshell, playback technology for unadulterated hearing. 1st edition, Richard Pflaum Verlag, Munich, 1990, ISBN 3-7905-0571-4
  • Thomas Görne: Sound engineering. Fachbuchverlag Leipzig in Carl Hanser Verlag, Munich et al. 2006, ISBN 3-446-40198-9 .

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