The DR-550 is an economical drum computer with 48 PCM internal sounds. Features include 64 preset rhythm patterns plus 64 programmable patterns, 8 song memory capacity, and for the first time in a Boss drum unit - midi (midi in only). Other features include stereo audio output, headphone output, and tape save/load jacks, and 12 note polyphony. Midi sync input is supported.

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This little pad can send midi trigger from it's own touch sensitive pad, and in addition, can accept 3 additional triggers, allowing the MPD-4 ($199 list) to control up to 4 midi sound sources. Midi channel can be assigned to any one of sixteen channels, and gate times are selectable. Four separate midi note numbers can be assigned.