Scalable Polyphony MIDI. SP-MIDI is a specification approved by the MIDI Manufacturers Association to handle MIDI data for “3rd Generation” mobile applications such as cellular telephones and handheld games, which have limited and variable polyphony. SP-MIDI allows composers to ensure that a song can be played comprehensibly on devices of varying polyphony by choosing the priority of MIDI channels and their available notes. It offers an alternative to the note stealing that would normally occur with a limited-polyphony device.
Recently manufactured cell phones incorporate the capability to play polyphonic ringtones but different models support different degrees of polyphony – generally ranging from 4 notes on low-priced phones up to 32 notes on advanced models.
SP-MIDI handles these variations with a few new concepts. First the composer defines which MIDI channels receive priority when played back by a limited-polyphony device, a process called Channel Masking. Second, the composer establishes the Maximum Instantaneous Polyphony (MIP) for each channel. SP-MIDI supports a limited number of control changes and recognizes the General MIDI 2 instrument set.
Here’s an example to illustrate how SP-MIDI works: A composer has written a song that contains piano, bass, drums, synth, and saxophone, with a maximum polyphony of 16 simultaneous voices. On a device capable of producing 16 voices of polyphony, all the instruments will play. If the piano and drums each use up to four simultaneous voices and the composer has given those instruments’ channels the highest priority, they will both simultaneously play on an eight-voice device. If the piano channel was given the highest priority, a four-voice device will only play the piano part. While no composer relishes the thought of eliminating musical parts from an arrangement, Scalable Polyphony MIDI at least allows the writer to choose which parts play back. The MMA considers this approach to be preferable to note stealing, which the composer can’t control completely.
SP-MIDI compatible songs must be converted to an instruction set the cell phone or other device understands. Different manufacturers use different formats. However, several available software applications make creating these files relatively uncomplicated.