Sonic Pi script to create four-part harmony from incoming notes using the “rotating chords” effect pioneered by Robby Kilgore in his work with Michael Brecker. Video here: https://vimeo.com/214204872

spi_mbe.rb

# Thirty years ago, Robby Kilgore invented a neat harmonization effect for Michael Brecker: http://robbykilgore.com/?p=19
# The basic idea is that incoming notes are doubled by rotating intervals. So you get diverse chords from the same melodic notes.
# This script is an attempt to reproduce the same effect.

use_real_time # Prevents latency
use_synth :fm # Other interesting synth sounds for this: [:hoover, :prophet, :blade, :fm]

middle=[-8, -5, -7, -1].ring # Rotating intervals for the middle note of each chord.
low=[-12, -17, 0].ring # Rotating intervals for the “bass” note of each chord.

# Set up the different voices, as silent and long…
melody = play 0, release: 1000, amp: 0, cutoff_slide: 0.02
fifth = play 0, release: 1000, amp: 0, cutoff_slide: 0.02
middle_voice = play 0, release: 1000, amp: 0, cutoff_slide: 0.02
low_voice = play 0, release: 1000, amp: 0, cutoff_slide: 0.02

live_loop :notes do
  note_on, velocity = sync "/midi/USB_Midi_Cable/4/1/note_on" # Get incoming notes from a MIDI device.
  if velocity > 0 # Only act on actual note_on messages.
    control melody, note: note_on, amp: velocity / 127.0
    control fifth, note: note_on + 7, amp: velocity / 150.0
    control middle_voice, note: note_on + middle.tick, amp: velocity / 180.0
    control low_voice, note: note_on + low.look, amp: velocity / 127.0
  end
  
end

live_loop :windy do # Use breath control (CC 2) to modulate filter cutoff.
  control_change, breath = sync "/midi/USB_Midi_Cable/4/1/control_change"
  if control_change==2
    control melody, cutoff: breath
    control fifth, cutoff: breath
    control middle_voice, cutoff: breath
    control low_voice, cutoff: breath
  end
end

You could make this a little succinct by changing line 19 to:

on velocity do
...
end