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Reactive-balsa

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Reactive-balsa is a live MIDI event processor based on the Reactive-banana framework and ALSA. (Thus it is currently bound to Linux.) It is the successor of the package streamed, that uses a kind of arrow types.

The MIDI event processor can be used for accompaniment, gags, games and more. It is usually inserted between a MIDI input device such as a USB piano keyboard and a MIDI controlled synthesizer, say an internal software synthesizer or an external hardware synthesizer.

Some of the included examples:

  • arpeggiator: play a pattern according to pressed keys
  • generate a beat with controllable tempo
  • instrument cycling: every note is played with a different instrument
  • LFO-controlled change of MIDI controllers
  • simulate playing chords on a guitar
  • interval trainer
  • keyboard with mirrord order of keys: perfect support for lefties :-)

The reactive programming paradigma has precisely the right abstractions that we need for MIDI event processing:

  • Events are discrete MIDI events for key press and release, knob turning and fader move, instrument change,
  • Behaviours are things like current position of a knob or fader or the set of currently pressed keys.

This becomes especially useful for the arpeggiator. There are several ways to obtain a set of keys:

  • The actual set of pressed keys.
  • Latched keys: Every key press toggles between press and release
  • Group latched keys: A chord is hold until a new chord is played.
  • Serial latch: A queue of fixed length holds pressed keys. Newly pressed keys release old keys.

And there are many ways to generate MIDI events from a set of keys:

  • cycle them up or down, or play them in ping-pong mode,
  • play them randomly,
  • play algorithmic patterns, like de-Bruijn sequences or k-nary cross sums, Gray codes or Johnson-Trotter-permutations,
  • play the set of keys with random inversions.


Package reactive-balsa
Hackage http://hackage.haskell.org/package/reactive-balsa
Repository darcs get http://code.haskell.org/~thielema/reactive-balsa/