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MS RECORDING
Purpose If a stereo soundcue is played back over a typical Left/Right playback system in a theatre, only listeners sitting in the middle of the theatre will hear properly centered localization's. Any listener sitting off to the right or the left will hear mostly just the speaker nearest to them. The other speaker of the stereo pair will not be heard, resting in an acoustical shadow. In our theatre spaces at Kansas City Repertory theatre and the UMKC Department of theatre, we have been using a playback system that is an imitation of the Middle/Side stereo microphone technique. This M/S Stereo Playback system has allowed us to maintain a centered image in relationship to the stage that everyone in the theatre will hear as coming from the stage. Using two side speakers allows us to place the audience "within" the sound image, having the spatial quality that makes stereo sound so much preferred to mono. The best speaker available functions as the Middle or Mono playback (M) and is centered with the stage performing area. This is the most critical loudspeaker and provides the most important playback information. A pair of loudspeakers, called the Side or Surround (S) are located in one of several ways:
The choice of S loudspeakers (now an S+ and S-) should be identical to each other, maybe even match the M speaker. These two S speakers must oppose each other, i.e., they must be out of polarity with each other. If not they will sum the S signal and place it as a phantom image in the middle of the soundfield, kind of where the M signal is coming from. You won't hear anything special if that happens. When listening to this M/S Stereo Playback in a theatre, it is important that the M speaker dominate and that the S speakers support. All listeners in the theatre, no matter how far off center, will localize music and sound cues to where ever you have placed the M. They will also have a great sense of depth and diffusion in the audience area that is provided by the S speakers. Other benefits of an M/S system are the rock-solid quality of the center playback and the ability of the soundfield to expand and collapse, to move toward and away from the listener. L/R systems can move sound left or right, but do not expand and collapse in the way we have encountered with M/S. A center phantom image is also "softer" in L/R because two speakers are trying to play it from different locations, a function of inter-aural crosstalk. M/S does not suffer from inter-aural crosstalk problems. Setup Setting up a playback system is very easy. Choose 3 loudspeakers, 3 speaker cables and 2 amplifier channels. Both S speakers can take their signal feed from the same amp channel. Both S speakers must be wired out of polarity. We prefer to listen in a home or control room where an S speaker faces an ear and the M speaker aims at a face. The 3 speakers should be equidistant from the head. The M output can rock, but the S signal rarely gets loud. Creating M/S program material is trickier. The easiest thing to do to experience this is to make an M/S recording but do not matrix the tracks. In other words, record an M mic on one track and the figure-eight S mic on the other. Feed those into the playback system, balance and enjoy. Since one frequently has to break down a L/R signal for M/S playback consider first this M/S algebra, then we'll describe a couple of methods. Converting M/S to L/R: M+S=L M- S =R Converting L/R to M/S: L+R=M L - R= S Once someone gets the conversion concept down, M/S to L/R and vice versa becomes like an open window one can jump through at any time, and either way. Ways we make these conversions are:
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M/S STEREO PLAYBACK by
