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MS Mastering of Stereo Microphone Signals

MS (mid-side) technique is a classic tool for recording and mastering based on coincident microphone setups. For these, localization theory tells us that on reproduction via loudspeakers, the interaural level differences (ILDs) will yield stable and fully mono-compatible results. Yet, when a mastering engineer obtains a mix, he will rarely be told how the specific sources were recorded. Was it as mono signals, level panned to a certain reproduction angle, or with coincident, spaced, or mixed stereo microphone setups. While the first two techniques only involve ILDs, spaced microphone techniques also imply interaural time differences (ITDs). MS matrixing techniques can be applied to change the apparent width of a full stereo mix in postproduction. The engineer should be aware that he then cross-feeds signals between the channels. The signal paths from a signal sound source, via the microphones, the matrixing, and the loudspeakers, to finally reach the ears, can now be seen as eight, instead of four for coincident techniques. An increased sensation of spaciousness or envelopment may arise, but one should be aware of the physics behind the process. Some of the arising comb filters are shown here, for spaced and ORTF setups.

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