There are a number of basic stereo miking techniques. The classic one is the coincident crossed pair where you take two identical cardioid, hypercardioid or figure-of-eight mics, site them as close together as possible and point one to the left of the sound stage, the other to the right. This will nearly always result in a basically good recording.
However there is one situation where the coincident crossed pair just doesn't seem quite right somehow. That is when you are recording a solo instrument in stereo. I remember a music magazine writer once asking what would be the point of recording a single instrument like a violin in stereo, because the whole point of stereo is to produce a 'sound stage' possessing apparent width, and clearly the violin is virtually a point source of sound when heard from a normal listening position.
The answer to this conundrum is of course that the violin is not the only source of sound in the sound stage; there is also the reverberation of the room, which definitely benefits from being recorded in stereo.
However, going back to the violin itself, if you use a coincident crossed pair then you will find yourself pointing one microphone to the left of the instrument and the other to the right. Neither microphone will point actually at the violin.
There seems something fundamentally wrong with this, particularly since all microphones are optimized for their on-axis response.
The solution is to use MS (mid-side) stereo microphone technique. Here, a cardioid (typically) microphone is pointed directly at the instrument, A figure-of-eight microphone is directed with its side towards the instrument. Since a figure-of-eight microphone is insensitive to the side, it will not pick up the instrument directly but only through the reverberation from the room.
The amazing thing is that although the M signal and S signal in themselves do not make a proper stereo sound stage, they can easily be decoded into conventional L and R. So you get all the advantages of the coincident crossed pair, plus you have a microphone actually pointing at the instrument.
Sometimes you will come across a microphone that embodies two capsules in MS orientation in the same body, such as the Sanken mic picture. Where's the fun in that though when you can set up a cardioid and figure-of-eight microphone yourself?
Of course, MS-format signals need to be decoded properly to work in stereo. Another time...
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