OK, here we go again. What are commonly (inaccurately) referred to as "Matrix" recordings are in fact "Blends" of independently recorded SBD and AUD sources of an event. In the audio world, a matrix recording is in fact:
Mid-side Matrix
One cardioid mic (M) points at subject (0°), and a figure-8 mic (S) picks up ambient sound by pointing at sides(90° and 270°).
The signals are then run through a decoder matrix (software or hardware) which creates two stereo channels:
n M+S = channel one
n M - S = channel two
One channel is a mix of M and S and the other be a mix of M and Sų (which means S is 180° out of phase)
n The more S, the stronger the separation.
MS Matrix recording is mostly used in acoustic concert settings to capture the true image of the hall.
One technique that Dan Healy perfected when mixing and recording the Grateful Dead, was to place a MS Matrixed microphone array at the mix position, and record it on a source that was locked to the same source as the SBD recording (assuring that the recordings were speed accurate to each other) What Dan called an "Ultra Matrix" was (as I understand it) actually a BLEND of a Matrix recording and a SBD recording. There are also time coincidence issues that are addressed when combining these sources.
Many "Matrix" recordings in circulation are simply blends of 2 different recordings and present a significant challenge in Post Production in terms of correctly time and pitch aligning the sources. The results of these efforts often sound really good, but are difficult to produce.
The cow has left the barn in terms of correcting the proper use of the term "Matrix", but in fact, it's nice to know the proper terms to apply to these very different methods.
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