What many folks don’t realize is that a Cardioid microphone pickup pattern can actually be created by the sum (or difference) from the output signals of a closely spaced or coincident pair of microphones: an omnidirectional and figure-8.
What’s even more intriguing is a number of these techniques were pointed out in Alan Blumlein’s seminal 1931 British patent 394,325 on Improvements to Sound Systems.
Further, by specifically pointing the omni microphone at the sound source, and then coincidently placing the figure-8 microphone such that the “8” pattern is normal or at 90-degrees to the sound source (the top of the “8” points 90-degrees to the left, the bottom of the “8” points 90-degrees to the right of the sound source); that summing these two patterns provides a left facing cardioid, taking the electronic difference between these two patterns provides an inverted phase output for a right facing cardioid. Correcting for the phase inversion of one of the mixed pairs, automatically creates a back to back cardioid stereo pickup pattern, if the two signals are equivalent in output level. For more info. on this topic, see the late John Eargle’s excellent work: The Microphone Book.
As noted above, varying the level of either the Omni or Figure-8 microphone changes the “stereo width”, as the omni pattern alone becomes the “center” monophonic microphone, and the Figure-8 mixes provide the necessary information, when added or subtracted electronically from the omni pickup, to create a stereo image of varying “width” depending on how much of this microphone’s signals are mixed with the omni. Other pattern microphones may be substituted for the front facing omnidirectional, but this is the basic form for this Mid-Side, or M-S technique. Additionally, summing the two “back to back” extracted cardioid patterns together, simply creates an omnidirectional pattern with higher output level than the originals.
A Mid-Side decoder simply accomplishes all of this phase inversion and sum-difference “jiggery-pokery” with a couple of simple controls for the 2 microphone channels. A similar approach has been implemented by Schoeps with their KFM-360 system, invented by Jerry Bruck in 1997, to create a powerful, small, 4 channel microphone array for surround-sound pickup that is quite impressive! Check out the Schoeps website for further information on this technique: http://www.schoeps.de/
In both instances of these microphone techniques, recording the mics straight through to a multi-channel recorder during a session, permits extensive post-production balance and mixing flexibility, well beyond what is normally achieved with a spaced pair, two or more microphone recordings. The difficult part for live recording is achieving an acceptable distance from the performers so one performer isn’t out of balance with another, and the overall balance for the performers and ambient acoustic is acceptable; as even with the flexibility of post-production mixes, improper mic placement too close or too far away from the performers can severely color the sound, creating too “close” an acoustic, or a “wallowy” reverberant one on playback.
Hope this proves helpful, and if you really dig into it, the above references, including the astonishing Blumlein patent (Which explains in detail how to both record and extract 2 channels of audio information from the media of the time: a black, 78 rpm disk; pre-dating the actual implementation of black-disk stereo records by about 25 years!) are very informative!