output.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

dir = File.dirname(File.expand_path(__FILE__))
$LOAD_PATH.unshift "#{dir}/../lib"

require 'alsa-rawmidi'

# Selects the first MIDI output and sends some arpeggiated chords to it

notes = [36, 40, 43] # C E G
octaves = 5
duration = 0.1

# AlsaRawMIDI::Device.all.to_s will list your midi devices
# or amidi -l from the command line

puts 'Press Control-C to exit...'

AlsaRawMIDI::Output.first.open do |output|
  loop do
    (0..((octaves - 1) * 12)).step(12) do |oct|
      notes.each do |note|
        output.puts(0x90, note + oct, 100) # NOTE: on
        sleep(duration) # wait
        output.puts(0x80, note + oct, 100) # NOTE: off
        sleep(duration)
      end
    end
  end
end