SPL Gemini - M/S Mastering Processor (Red)Gemini is the first mid side processor in 120V technology. In mastering, M S processing is often the best way to get targeted access to individual elements of a mix. What Is M S? The decomposition of the stereo signal in mid (voice, snare, bass) and side (guitars, pads, cymbals) allows their separate processing. The center signal (Mid) is the sum of the left and right channel of the stereo signal (L+R), i. e. the "in phase" part of the stereo signal.