18 Chord Organ-ish

Chord Organ-ish for the Music Thing Modular Workshop Computer. Inspired by the behaviour of the standalone Chord Organ module: 16 chords, up to 8 voices per chord, 1V/oct root control, and glide.

Controls

Input	Function
Main knob	Chord selection (0–15). Summed with CV 1.
CV 1	Chord CV (0V to +5V): added to Main knob for voltage-controlled chord selection. Negative voltages ignored.
X knob	Root transpose (semitones).
Y knob	Progression pattern selection (0–8).
CV 2	Root note (1V/oct).
Audio 1	VCA control: 0V to +5V controls output volume (0% to 100%). Full volume when disconnected.
Pulse 1	Trigger: advances progression step and retriggers chord/reset pulse.
Pulse 2	Cycle waveform (sine → triangle → square → saw) on each trigger.
Switch Up	Enable glide (portamento).
Switch Middle	Disable glide.
Switch Down	Cycle waveform (sine → triangle → square → saw).

Outputs

Output	Function
Audio 1 & 2	Mixed chord (same on both).
CV 1	Highest note in current chord (1V/oct, calibrated).
CV 2	Sequenced root note from selected progression (1V/oct, calibrated).
Pulse 1	Brief pulse on chord or root change (or Pulse 1 in rising edge).
LEDs 0–3	Chord index in binary (0–15).
LED 4	Reset indicator (on while reset pulse is high).
LED 5	Waveform index (brightness).

Chord list (built-in)

Major
Minor
Major 7th
Minor 7th
Major 9th
Minor 9th
Suspended 4th
Power 5th
Power 4th
Major 6th
Minor 6th
Diminished
Augmented
Root
Sub octave
2 up 1 down octaves

Progression Patterns

The chord progression sequencer outputs root notes via CV Out 2. Select patterns with Knob Y, advance steps with Pulse In 1.

I-IV-V-I - Classic pop/rock progression (C-F-G-C)
I-V-vi-IV - Popular progression (C-G-Am-F)
ii-V-I - Jazz turnaround (Dm-G-C)
I-vi-IV-V - 50s progression (C-Am-F-G)
I-IV-I-V - Blues progression (C-F-C-G)
vi-IV-I-V - Sensitive progression (Am-F-C-G)
I-bVII-IV-I - Mixolydian (C-Bb-F-C)
Chromatic - Ascending chromatic scale (C-C#-D-D#-E-F-F#-G)
Circle of 5ths - Circle of fifths sequence (C-G-D-A-E-B-F#-C#)

Note: Root offsets are relative to the current root (CV In 2 + Knob X). Chord type is still controlled manually via Main Knob.

Build

Requires Pico SDK and (optionally) PICO_SDK_PATH set.

Toolchain: Arm GCC 15.x can fail with cannot find -lg when linking the Pico SDK boot stage. Use one of:

Arm GNU Toolchain 13.2 or 14.2 from the main downloads page (older releases) — these usually still ship the expected newlib layout.

Install one of the above, put its bin directory first in your PATH, then build.

cd 18_chord_organ
mkdir build && cd build
cmake ..
make

Copy chord_organ.uf2 from the Build folder to the Workshop Computer when in bootloader mode.

Firmware

Platform: Pico SDK (C++), ComputerCard
Sample rate: 48 kHz
Features: Glide (50 ms), 4 waveforms, 9 progression patterns

Workshop Computer Program Cards

chord organ

README