ORTF stereo technique
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The ORTF stereo microphone system is a microphone technique used to record stereo sound.
It was devised around 1960 at the Office de Radiodiffusion Télévision Française (ORTF) at Radio France.
ORTF combines both the volume difference provided as sound arrives on- and off-axis at two cardioid microphones spread to a 110º angle, as well as the timing difference as sound arrives at the two microphones spaced 17 cm apart.
The microphones should be as similar as possible, preferably a frequency-matched pair of an identical type and model.
The result is a realistic stereo field that has reasonable compatibility with mono playback. Since the cardioid polar pattern rejects off-axis sound, less of the ambient room characteristics are picked up. This means that the mics can be set back further from the sound sources, resulting in a blend that may be more appealing. Further, ORTF is easy to achieve, as purpose-built microphone mounts are available.
As with all microphone arrangements the distance and angle can be manually adjusted slightly by ear for the best sound, which may vary depending on room acoustics, source characteristics, and so on. But this arrangement is defined as it is because it was the result of considerable research and experimentation, and its results are predictable and repeatable.
These interchannel differences have nothing to do with interaural differences, which come only from artificial head recordings. Even the spacing of 17 cm has nothing to do with interaural ear spacing. The recording angle for this microphone system is ±48° = 96°.
References for the recording angle:
- The stereophonic zoom - pdf
- Microphone Techniques for Stereo Recording
- The ORTF-Stereo-Microphone System - pdf
