A method and corresponding equipment by which a synthesizer/MIDI (musical instrument digital interface) device (10) is able to optimally perform a MIDI file (11) taking into account not the polyphony required by the MIDI file (11) as in SP-MIDI (scalable polyphony MIDI), but taking into account instead extended scalable polyphony (XSP) data 12b including the maximum number of instantaneous voices required by the MIDI file and the categories in which they occur for different channel masking, and also taking into account the architecture of the synthesizer/MIDI device (10) in terms of a voice complexity coefficient table (12b) indicating the relative complexity (corresponding to a resource requirement) for voices in each category. The result is a total voice requirement table 12c-1 indicating typically less masking than would be required for the same synthesizer/MIDI device to play the MIDI file according to SP-MIDI.