Chorgan Sequencer

Six-voice morphing chord synthesizer with built-in chord sequencer for the Workshop Computer.

Knob X and CV In 1 set the root pitch.

Knob Y selects an interval above the root in semitone steps.

Four extension voices fill out the chord — the extensions are drawn from the same harmonic world as the chosen interval. (A perfect 5th stays open and powerful, a minor 3rd stays dark, a major 3rd stays bright. The Main knob morphs all six voices through sine, triangle, saw, and narrow pulse).

A built-in chord sequencer stores up to eight chords and steps through them on rising edges at Pulse In 2.

Two modes are available, selected at boot:

• Normal mode (default): detune/chorus across the six voices
• Slew mode (hold Switch Down at power-on): chord changes glide — all six voices portamento independently to their new pitches

In both modes, Pulse Out 2 and CV Out 2 fire a timed envelope on every chord event — a gate and a unipolar downward voltage ramp (+5V to 0V) that can drive a VCA, filter, or act as a trigger.

Inputs

Jack	Function
CV In 1	Root pitch 1V/oct (0V = C4) — summed with Knob X
CV In 2	Timbre offset — bipolar, offsets the Main knob position
Audio In 1	Slew speed offset bipolar CV
Audio In 2	Chord inversion — bipolar; shifts voices up or down through octave inversions
Pulse In 1	Rising edge advances the chord extension preset
Pulse In 2	Rising edge recalls the next stored chord

Outputs

Jack	Function
Audio Out 1	6-voice mix
Audio Out 2	Same 6 voices with per-voice phase offsets — stereo width
Pulse Out 1	Square wave one octave below root
Pulse Out 2	PWM envelope — resets to 50% duty on each chord event, ramps to 20% and holds
CV Out 1	Root pitch + voiced interval (1V/oct) — follows X knob, CV In 1, and chord override
CV Out 2	Unipolar downward ramp — +5V on chord event, decays to 0V and holds; tracks Pulse Out 2 timing exactly

Controls

Knob X + CV In 1 — Root pitch Knob X sweeps from C3 to C6. CV In 1 tracks 1V/oct on top of that. Both are summed and applied uniformly to all six voices.

Knob Y — Interval Sets the interval between voice 1 (root) and voice 2 in audible integer semitone steps, 0 (unison) to 12 (octave). Voices 3–6 are chord extensions above the root, chosen by the current preset.

Main Knob + CV In 2 — Timbre Morphs the waveform of all six voices simultaneously. The curve is a W-shape:

Position	Waveform
Fully CCW	Narrow pulse
9 o'clock	Sine
12 o'clock	Saw (fullest)
3 o'clock	Sine
Fully CW	Narrow pulse

CV In 2 offsets the knob position bipolarly but cannot push the timbre into a different detune/slew zone — that is always determined by the physical knob position.

Audio In 1 — Slew speed Patches a portamento glide onto all six voices. 0V (or unpatched) is instant — identical to the current behaviour. Positive voltage adds glide: at +5V the glide time is approximately one minute. The full range is continuously variable between those two extremes.

In normal (detune) mode, Audio In 1 adds slew on top of the existing detune — voices glide to their detuned targets rather than snapping to them. Detune is still active.

In slew mode, Audio In 1 offsets the zone-controlled rate. Positive voltage extends the glide beyond what the zone alone provides; negative voltage shortens it, bringing the glide back toward instant. (At maximum the slew time ~1min)

Audio In 2 — Chord inversions Shifts the voicing up or down through octave inversions. At 0V the voicing is unchanged. Positive voltage steps the lowest-sounding voice up by one octave per step, cycling the voicing upward. Negative voltage steps the highest-sounding voice down by one octave per step. There are six steps available in each direction.

This only affects the audio outputs — CV Out 1 and Pulse Out 1 continue to track the unmodified root pitch and interval.

Switch + Main Knob position — Detune (normal mode) / Slew rate (slew mode)

The four zones also set the rate of the Pulse Out 2 / CV Out 2 envelope in both modes.

In normal mode, four detune levels are selected by switch position and whether the Main knob is CCW or CW of centre:

Switch	Knob position	Outer voice detune	Envelope ramp time
Mid	CCW of centre	0 cents (clean)	~250ms
Mid	CW of centre	5 cents	~100ms
Up	CW of centre	10 cents	~500ms
Up	CCW of centre	15 cents	~3s

In slew mode, the same four zones set the portamento rate (detune is disabled):

Switch	Knob position	Slew rate	Envelope ramp time
Mid	CCW of centre	Instant	~250ms
Mid	CW of centre	Fast (~200ms)	~100ms
Up	CW of centre	Slow (~700ms)	~500ms
Up	CCW of centre	Glacial (~3s)	~3s

Tap Switch Down — Cycle preset A short tap (less than one second) advances the chord extension preset for voices 3–6. There are 6 presets per interval, cycling through different harmonic choices. A rising edge on Pulse In 1 does the same thing.

Chord extension presets

Each of the 13 Y positions (0–12 semitones) has 6 extension presets. Extensions are chosen to reinforce the harmonic character of the chosen interval — no minor 3rds if you've picked a major 3rd, no 3rds at all if you've picked a 5th or unison.

Y	Interval	Harmonic world
0	Unison	5ths and octaves only
1	Min 2nd	Clusters — semitone stacks, tritones, m7ths
2	Maj 2nd	Sus2 — 2nds, 4ths, 5ths, 9ths
3	Min 3rd	Minor — m3, P5, m7
4	Maj 3rd	Major — M3, P5, M7, 9th
5	Perf 4th	Sus4/quartal — 4ths, 5ths, octaves
6	Tritone	Dim/aug — tritones, m3, M6
7	Perf 5th	Power chord — 5ths, octaves, 9th
8	Min 6th	Minor/Phrygian — m3, P5, m6, m7
9	Maj 6th	Major/pentatonic — M3, P5, M6
10	Min 7th	Dominant/blues — P5, m7, 9th
11	Maj 7th	Lydian/maj7 — M3, P5, M7, #4
12	Octave	Quartal/suspended — 4ths, 5ths, 2nds

Boot modes

Normal mode — power on with Switch in any position except Down. LEDs sweep 0→5 at startup.

Slew mode — hold Switch Down before powering on, hold until LEDs flash (all on, then all off, ~200ms). Detune is disabled; chord changes glide instead.

Chord sequencer

Hold Switch Down (1 second) — Store chord Hold Switch Down until the LEDs change (one second). Release immediately — the chord is written to the next slot. The LED pattern shows which slot was just stored (LEDs 1, 3, 5 full bright; LEDs 0, 2, 4 encode the slot number in binary).

Up to eight chords can be stored. Storing more than eight overwrites from the beginning.

Recalling chords — Pulse In 2 Each rising edge at Pulse In 2 recalls the next chord in sequence, stepping through slots in the order they were stored and wrapping at the end. When a chord is held, LEDs 0, 2, 4 are full bright and LEDs 1, 3, 5 encode the recalled slot in binary.

Note: Pulse In 2 must be seen low at least once before it will respond (boot guard).

Breaking out of a held chord Move Knob X or CV In 1 by more than one semitone from where they were when the chord was recalled, or move Knob Y to a different step. The override releases immediately and you return to manual control.

Advancing preset while a chord is held Tapping Switch Down or sending a rising edge to Pulse In 1 advances the preset while the chord override is active — it does not break the override.

LEDs

State	LED pattern
Boot (normal mode)	Single LED sweeps 0→5
Boot (slew mode)	All LEDs flash on then off
Storing (hold active)	LEDs 1, 3, 5 full bright; LEDs 0, 2, 4 = slot number in binary
Chord held	LEDs 0, 2, 4 full bright; LEDs 1, 3, 5 = recalled slot in binary
Normal	LEDs 0–4: interval position; LED 5: preset brightness

In normal operation, LEDs 0–4 show where Knob Y is across the 0–12 semitone range. LED 5 brightness indicates the current preset (dim = preset 0, bright = preset 5).

Voice layout

• Voice 1: always root
• Voice 2: root + Y interval (0–12 semitones)
• Voices 3–6: chord extensions above root, selected by current preset

Quick start

Self-contained pad

1. Patch Audio Out 1 to a mixer or effects
2. Knob Y to 7 (perfect 5th), Main Knob to 12 o'clock (saw — fullest sound)
3. Switch Mid, Main Knob slightly CW for gentle 5-cent detune

Pitched chord voice

1. Patch 1V/oct into CV In 1
2. Knob Y to 4 (major 3rd), tap Switch Down to cycle presets
3. Main Knob to taste — 12 o'clock for saw, 9/3 o'clock for sine, fully CCW/CW for pulse

Slow timbre sweep

1. Patch a slow LFO into CV In 2
2. The timbre morphs symmetrically around the current Main knob position

Chord sequence from a clock

1. Set Pitch (X + CV In 1), Interval (Y), and preset (Switch Down taps) for your first chord
2. Hold Switch Down for one second to store it
3. Repeat for up to eight chords
4. Patch a clock or gate sequence into Pulse In 2 — steps through your stored chords on each rising edge
5. Move Knob X or Y by more than a semitone to drop back to manual

Portamento chord changes (slew mode)

1. Boot with Switch Down held
2. Set Switch Mid, Knob CW of centre for fast slew; Switch Up, Knob CCW for glacial
3. Tap Switch Down or trigger Pulse In 1 to change preset — pitches glide to the new chord

Sub-bass layer

1. Patch Pulse Out 1 to a filter or oscillator — clean square one octave below root
2. Patch Pulse Out 2 alongside it for a PWM version whose timbre narrows on each chord change

Chord-triggered VCA

1. Patch CV Out 2 to a VCA CV input — it fires +5V on each chord event then ramps down to 0V
2. Works directly with any VCA that opens on positive voltage; no inversion needed
3. Use Switch Mid CCW for a quick ~250ms decay; Switch Up CCW for a slow ~3s fade

Chord-triggered filter

1. Patch CV Out 2 to a filter cutoff — the downward ramp darkens the filter after each chord change
2. Combine with Audio Out into the filter input for a natural attack/decay envelope on the chord

Chord-triggered envelope (external)

1. Patch Pulse Out 2 into an external envelope generator's gate input
2. The PWM duty resets to 50% on each chord event — use the wide phase as your gate high time
3. CV Out 2 mirrors the same timing as a direct voltage ramp if you don't have an envelope generator

Interval-keyed filter or oscillator

1. Patch CV Out 1 to a filter cutoff or a second oscillator's 1V/oct input
2. The CV tracks the root pitch plus voiced interval in semitones — use it to transpose an external voice in harmony with whatever Chorgan is playing

Technical notes

• 6-phase-accumulator oscillators, integer arithmetic throughout (RP2040 has no FPU)
• Tuning ratio applied uniformly to all voices — no per-voice stepping on X knob or CV In 1
• Detune zone determined by physical Main knob position; CV In 2 cannot cross zone boundaries
• Stereo width via per-voice phase offsets on Audio Out 2 (0°, 15°, 30°, 45°, 60°, 75°)
• Waveform: W-shape — pulse (CCW extreme) → sine (9 o'clock) → saw (12 o'clock) → sine (3 o'clock) → pulse (CW extreme)
• Pulse Out 2: PWM square at root pitch — resets to 50% duty on each chord event, ramps to 20% at zone-determined rate (~250ms / ~100ms / ~500ms / ~3s)
• CV Out 1: (X knob + CV In 1) + voiced interval in 1V/oct — clamped to ±5V; follows chord override
• CV Out 2: unipolar downward ramp — +5V on chord event, decays to 0V at the same rate as Pulse Out 2; retriggers on pitch semitone change, interval change, preset change, or chord recall
• Chord sequencer: stores pitch, interval, and preset — up to 8 chords; override breaks on >1 semitone movement from position at recall time
• Slew mode: per-voice IIR portamento on phase increment; rates: instant / ~200ms / ~700ms / ~3s
• 200ms startup holdoff before audio begins — eliminates power-on click

Credits

By Andy Jenkinson (uglifruit), developed with Claude Code.

Built on the Workshop Computer platform by Tom Whitwell (Music Thing Modular), using the ComputerCard framework.

V/oct lookup table (voct_vals) from Utility Pair by Chris Johnson, used with permission under the original licence.

Waveform morphing concept inspired by Mutable Instruments Braids (Émilie Gillet).

Licensed under Creative Commons Attribution-ShareAlike 4.0.