ORTF stereo technique
As ORTF stereo system refers to a set of two microphones for recording sound in the loudspeaker stereophony Äquivalenzstereofonie.
History
The ORTF stereo system was developed around the year 1960 by engineers of the French radio ( Radio France ), which was reorganized in 1964 to the Office de Radio Télévision Française = diffusion ORTF.
Through a series of practical experiments and listening tests they found a main stereo microphone system, which consists of two small diaphragm microphones, cardioid and a fairly uniform distribution of the phantom sound sources on the stereo - speaker basis results as Hörereignisrichtung. There was this one axis angle between the two microphones of α = ± 55 ° = 110 °, and a microphone base (distance between the microphone capsules ) of a = 17.0 cm.
The effective receiving area
The resulting effective receiving area of the microphone system is ± 48 ° = 96 °. In the formation of the respective frequency Hörereignisrichtung neutral interchannel level differences Δ L and in the same direction interchannel delay time differences Δ t together act as a loudspeaker signals. This recording technique is called mixed stereophonic stereophonic or equivalent. Thus, at the same time the intensity stereophony and Laufzeitstereofonie is applied. Using this signal differences is denoted by equivalence.
Usually this special microphone system ( microphone array ) must be assembled by the sound engineer of two single small diaphragm microphones. Large diaphragm microphones or those with double diaphragm should not be used because of the frequentially unbalanced directional characteristics and the larger phase transitions it. Even if it seems advisable, even with the two parameters axis angle α for Δ L and base a microphone for Δ t to experiment, to which there are practical microphone mountings, a permanently mounted ORTF microphone system is available. Another microphone placement in equivalence Stereophonic sound is, for example, the NOS stereo system.
The recording
A sound recording with a main microphone system is supplemented frequently still by spot microphones. Not rare to find as the distance of the microphone from one another indication of 17.5 cm. This deviation is indeed almost completely insignificant, but it is not the authentic value. Under the name ORTF arrangement also circulating about it even more similar angles and distance ratios at microphone systems. So an arrangement with a microphone base of 30 cm and an axial angle of ± 45 ° = 90 ° quite common (NOS stereo system ).
Not to forget the EBS stereo system in which the set angle of the major axis between the microphones, so the axis angle with 90 ° α is quite as great as the resultant recording range of the microphone system. That means this microphone system must be placed in front of the ensemble that the microphone - major axes point exactly to the edge of the sound in the distance. Then the sound source is uniformly distributed on the speaker base of speakers played to speaker. This condition is always satisfied when the microphone system is at a distance from the sound that is half of the body of sound expansion.
The ORTF system is therefore only one with the purely empirically determined values of 17 cm and ± 55 ° = 110 °. The account of the apparent clarity often quoted at this point the human ear distance head here was also not a concern. This distance has also generally in the recovery of signals for the loudspeaker reproduction no meaning.
The ORTF system is rejected by some Tonverantwortlichen because the key for the space of a recording duration proportion of the signals is relatively low. In many cases, the NOS or EBS system is given preference. This does not diminish the work of the ORTF to perfect stereo recordings.