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Diamond is a cross-platform MIDI arpeggiator that I've created in Ruby.
This is a screencast of playing around with Diamond in IRB, feeding MIDI to a two voice modular synth, a kick drum module and a reverb. I adjust some of the arpeggiator controls, create two more arpeggiators and sync them to the first.
Here's a blog post which explains what's happening in further depth: bit.ly/lh4YFh
For more info about Diamond, check it out on github