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An Introduction to Compression: Basic Compression - A free download from Audio Masterclass

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Coincident crossed pair

Description and application of a coincident crossed pair of microphones for stereo recording.


The coincident crossed pair technique traditionally uses two figure-of-eight microphones angled at 90 degrees pointing to the left and right of the sound stage (and, due to the rear pickup of the figure-of-eight mic, to the left and right of the area where the audience would be also). More practically, two cardioid microphones can be used. They would be angled at 120 degrees were it not for the drop off in high frequency response at this angle in most mics. A 110-degree angle of separation is a reasonable compromise. This system was originally proposed in the 1930s and mathematically inclined audio engineers will claim that this gives perfect reproduction of the original sound field from a standard pair of stereo loudspeakers. However perfect the mathematics look on paper, the results do not bear out the theory. The sound can be good, and you can with effort tell where the instruments are supposed to be in the sound image. The problem is that you just don’t feel like you are in the concert hall, or wherever the recording was made. The fact that human beings do not have coincident ears might have something to do with it. Separating the mics by around 10 cm tears the theory into shreds, but it sounds a whole lot better. The ORTF system, named for the Office de Radiodiffusion Television Francaise, uses two cardioid microphones spaced at 17 cm angled outwards at 110 degrees, and is simply an extended near-coincident crossed pair.

The redeeming feature of the coincident crossed pair is that you can mix the left and right signals into mono and it still sounds fine. Mono, but fine. We call this mono compatibility and it is important in many situations – the majority of radio and television listeners still only have one speaker. The further apart the microphones are spaced, the worse the mono compatibility, although near-coincident and ORTF systems are still usable.

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By David Mellor Saturday April 12, 2003
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