Office de Radiodiffusion Télévision Françaiseis (ORTF) a stereo microphone technique based on taking the average diameter of a human head (7”) and placing two cardoid microphones at this distance splayed at an angle of 110 degrees. The technique was designed to represent what we hear naturally with our ears. ORTF is a near-coincident microphone technique.
Discussion
The width of the microphones creates slight frequency inconsistencies due to the timing mismatches, which may or may not be beneficial to a recording. When using ORTF a sound that enters the technique off center axis will have a time delay. (Maximum off axis sound incidence angle example) v = (wavelength) x (frequency) and when solving for frequency, f = 1948Hz; therefore frequencies around 2kHz would be polarity inverted relative to the other microphone. When working with near-coincident microphone techniques there is an infinite amount of incident angles creating time delay variations which may or may not be best for a particular recording application. Using critical listening and experimentation to find optimum results are common when working with near-coincident microphone techniques.
Additional Thoughts
ORTF technique is made to mimic ears, which are spaced the distance of the heads diameter. If each microphone is panned hard left and right in a stereo playback system. There would be twice the distance (14”) of variation between the left and right ear to the listener; 7” from the recorded ORTF technique and an additional 7” of distance from diameter of the listeners head.