Secret

A chaotic oscillator for the Alchemy Lab — twelve strange attractors, with V/oct and one knob from free chaos to a locked note.

Overview

Secret is a chaotic oscillator. It runs strange attractors — sets of equations that never quite repeat — fast enough to hear, and plays them from a V/oct input like any other voice.

The problem with a chaotic oscillator is the word oscillator. Left alone, most of these systems have no pitch at all, or drift as you turn the knobs. Secret’s answer is one control, TAME. At zero the attractor runs free. Turn it up and the module pulls the attractor towards the note — gently at first, so it locks to the pitch but keeps its grit, then all the way to a clean, periodic tone. The interesting part is the middle.

It runs on the Hermetic Modular Alchemy Lab, an open DSP platform with a Daisy inside, six knobs with LED rings, three buttons and six CV jacks.

Named — like everything here — after the nursery rhyme: seven for a secret, never to be told. A system that is completely determined, and still can’t be predicted. Not affiliated with, or endorsed by, Hermetic Modular.

Tame — from noise to a note

The twelve systems fall into three families, and each needs taming differently:

Spirals (Rössler and its relatives) already turn at a nearly steady rate. A small drive at the note’s frequency locks them, and the chaos survives the lock.
Driven systems (Duffing, the pendulum) follow their drive. The drive is the note — and where they go chaotic, they fall onto subharmonics: a twelfth, or two octaves and a third, below it.
Wild ones (Lorenz, Chua) ignore any drive. These get pulled back to a stored point on the attractor once every cycle, so the pitch is imposed but everything inside each cycle stays as rough as it was.

B2 chooses how TAME works: Auto uses each model’s own choice, or force Force or Sync on any model to hear the difference.

Twelve models

B1 steps through all twelve. Both its LEDs show the current model’s colour.

	First six		Second six
orange	Rössler	red	Driven pendulum
yellow	Van der Pol	cyan	Lorenz–Lü–Chen
blue	Lorenz	violet	Moore–Spiegel
magenta	Chua	lime	Forced Brusselator
green	Duffing	pink	Chaotic Colpitts
white	Coupled Rössler	teal	Hindmarsh–Rose

The first six came from the Teensy. The second six were picked by ear from nine measured candidates — and none of them, as far as I could find, has been played as a V/oct voice in Eurorack hardware before.

Controls

Control	Job	Control	Job
TUNE	27.5–880 Hz, plus V/oct on J3	CHAOS	the main parameter — how chaotic; plus CV on J5
CHAR	the second parameter — the character within it	TAME	free chaos (0) to a locked note (1); plus CV on J6
AD	envelope attack and decay	SR	envelope sustain and release
B1	model	B2	TAME mode: Auto, Force or Sync
B3	Drone (always open) or Gated by J4

In	Job	Out	Job
J3	V/oct	J7	X CV — the raw attractor, for a scope
J4	gate: in Gated mode it opens the envelope and restarts the attractor	J8	Y CV
J5	CHAOS CV, ±5 V across the knob’s travel	J9	audio L (X)
J6	TAME CV, ±5 V across the knob’s travel	J10	audio R (Y)

The rings show each knob plus its CV.

Seeing it

Patch J7/J8 into an X/Y scope and you see the shape the sound is making: Rössler’s spiral, Lorenz’s butterfly, Chua’s double scroll. It is also the best tuning aid there is — a locked note draws a figure that stands still, and a slipping one turns.

Known limits

Four models top out inside the playing range at mid settings — Hindmarsh–Rose around 200 Hz, Chua 410, Colpitts 570 and Moore–Spiegel 730. Above that the note stops rising rather than the module overloading.
V/oct uses the board’s own calibration, which sets each jack’s zero but not its scale.
CV is read at 1 kHz, so there is no audio-rate FM.
No presets. The button choices reset at power-on.

Provenance

Joranalogue’s Orbit 3 is an analogue double scroll with V/oct and a tame/wild switch, and there is a long line of attractors as slow CV. What I have not found anywhere is TAME as one continuous control that ends with the attractor itself locked to the note.

Built on Hermetic Modular’s MIT-licensed Alchemy SDK and Electrosmith’s libDaisy. Secret itself is MIT. Thanks to Luke Pendergrass at Hermetic Modular for the Alchemy SDK.