Chord organ is very simple.

Chord = choose the chord shape; minor, major, seventh etc. Shapes can be defined by editing a text file on the SD Card (see below). Chords can have up to 8 notes.

Root = choose the root note, in semitones from C-2 to C+2

Waveform = push to select one of four waveforms: Sine, Square, Sawtooth and weird-gnarly-pulse-an-octave-below.

Chord + Root CV inputs. The Root CV input is NOT volt/octave, but the output notes are perfectly in standard tuning (A = 440hz).

Trig = Trigger output, which pulses every time the chord changes

Out = Audio output

Editing the SD Card

Chord Organ needs an SD card (any size) to function — you can use the SD card from a Radio Music. It creates a tiny text file called CHORDORG.TXT in the root of the SD card, which can be edited to change the chords.

When you open the file it looks like this.

Each chord is described in numbers. 0 is the root. 12 is the octave above. -12 is the octave below. The text outside the square brackets is ignored, so the ‘Major 7th’ text is just there as a reminder.

The Wikipedia List of Chords page has 70+ chord shapes. The column marked “p.c. #s” shows the numbers you need here, in Integer Notation, (with “10” and “11” replaced by “t” and “e”). For example, the Dominant Ninth is listed as 0 4 7 t 2, which you should enter as 0 4 7 10 2. You probably want to raise the last 2 up an octave, so it becomes 0 4 7 10 14.

If you have different numbers of notes in each chord, you may get clicks as you switch between them; you can experiment to see if you mind.

links 

https://www.muffwiggler.com/forum/viewtopic.php?t=177939&highlight=

https://medium.com/music-thing-modular-notes/chord-organ-the-easiest-way-to-get-simple-chords-from-a-modular-synth-2f48684fdb9a#.bnmbbggts

build : https://github.com/TomWhitwell/RadioMusic/wiki/Preparing-the-Teensy

https://github.com/TomWhitwell/Chord-Organ