ORTF stereo system

The ORTF stereo system is an arrangement of two microphones for recording sound in equivalent stereophony to loudspeaker stereophony .

history

The ORTF stereo system was developed around 1960 by sound engineers from French radio (Radio France), which was reorganized in 1964 into the Office de Radiodiffusion Télévision Française = ORTF .

Through a series of practical experiments and listening comparisons, they found a stereo main microphone system, which consists of two small diaphragm microphones with cardioid characteristics and results in a fairly even distribution of the phantom sound sources on the stereo loudspeaker base as the direction of the audio event . There was an axis angle between the two microphones of α = ± 55 ° = 110 °, as well as a microphone base (distance between the microphone capsules) of a = 17.0 cm.

The effective recording area

Microphones in ORTF arrangement - equivalence stereophony

The effective recording range of the microphone system is ± 48 ° = 96 °. In the formation of the respective frequency Hörereignisrichtung neutral act Inter Channel - level differences Δ L and the same direction Interchannel- time differences Δ t as loudspeaker signals together. This recording technique is called mixed stereophony or equivalent stereophony . Thus the intensity stereophony and the transit time stereophony are used at the same time . The use of these signal differences is called equivalence .

Usually this special microphone system (microphone arrangement) has to be put together by the sound engineer from two individual small diaphragm microphones. Large diaphragm microphones or those with a double diaphragm should not be used for this because of the frequently unbalanced directional characteristics and the larger phase response.
Even if it seems advisable to experiment with the two parameters axis angle α for Δ L and microphone base a for Δ t , for which there are practical microphone holders, a permanently mounted ORTF microphone system is available. Another microphone setup in equivalence stereophony is z. B. the NOS stereo system .

Modifications

It is not uncommon to find the distance between the microphones to be 17.5 cm. This deviation from the original is practically completely insignificant, but it does not correspond to the authentic value.

Under the name ORTF arrangement there are also other similar angle and distance relationships in microphone systems. An arrangement with a microphone base of 30 cm and an axis angle of ± 45 ° = 90 ° is very common in practice, see NOS stereo system .

Not to be forgotten is the EBS stereo system according to Eberhard Sengpiel , in which the set angle of the main axes between the microphones, i.e. the axis angle α with a total of 90 °, is just as large as the resulting recording area of the microphone system. This means that this microphone system must be set up at a distance in front of the ensemble so that the main microphone axes point exactly to the edge of the sound box. Then the sound source is reproduced evenly on the speaker base from speaker to speaker. This condition is always fulfilled when the microphone system is at a distance from the sound body that is half the length of the sound body.

The ORTF system is therefore the only one with the purely empirically determined values of 17 cm and ± 55 ° = 110 °. The ear spacing in the human head, which is often quoted at this point because of the apparent clarity , was not of importance here. This distance is also generally of no direct significance when generating signals for loudspeaker reproduction.

use

A sound recording with such an ORTF system as the main microphone is often supplemented by support microphones and reverb microphones .

The ORTF system is rejected by some sound officers because the signal transit time, which is decisive for the spatial quality of a recording, is relatively low. In many cases, preference is given to the NOS or EBS system.

literature

Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr (eds.), "Handbuch der Tonstudiotechnik", 8th, revised and expanded edition, 2 volumes, publisher: Walter de Gruyter, Berlin / Boston, 2014, ISBN 978-3- 11-028978-7 or e- ISBN 978-3-11-031650-6
Thomas Görne: Microphones in theory and practice. 8th edition, Elektor-Verlag, Aachen, 2007, ISBN 978-3-89576-189-8

Web links