legio-drifter

Alternative firmware for the Noise Engineering Legio Eurorack module. Drifter turns Legio into a random Bézier auto-panner, crossfader, or CV panner.

What drifter does

Drifter moves a value along a random Bézier curve over time. The curve wanders between two points. You set the center point, the wander depth, and the wander speed. Drifter uses the wandering value in one of three modes, set by the left switch.

Controls

Set the left switch to choose the mode:

Switch position	Mode	Function
Up	AUDIO PAN	Pans the audio input between the left and right outputs.
Center	XFADE	Crossfades between two audio inputs. The output is mono, on the left jack.
Down	CV PAN	Sends the CV at the V/OCT input jack to a random position between the left and right outputs.

Other controls:

Control	Function
Top knob + CV	Sets the center point of the wander (left → right, or A → B).
Bottom knob + CV	Sets how far the wander moves from the center point.
Right switch	Sets what happens when the wander goes past the edge: CLIP, FOLD, or WRAP. See the diagram below.
Encoder, rotate	Sets the wander rate. Free-running range: 5 minutes to 20 Hz. Clocked range: ÷8 to ×8.
Encoder, tap	Switches to curve-shape edit mode. The LEDs turn yellow.
Encoder, long-press, CV PAN mode only	Turns on an internal +5 V source. The outputs become a complementary pair of slow random voltages. You do not need an external CV source.
Gate input	Clock input. Each valid edge sets a new random target.

The diagram below shows how the right switch maps an out-of-range value back into range:

Edge behavior: CLIP, FOLD and WRAP transfer curves with sample input/output values

Build, test, and flash

Build the firmware:

make -C lib/libDaisy
make

Run the host-side DSP tests. These tests do not need Legio hardware.

make -C test

Flash the firmware to your module:

make program-dfu

See AGENTS.md for the file layout and hardware notes.

Calibrate the V/OCT input for your module

Only CV PAN mode reads the V/OCT jack. In this mode, drifter reads the jack as a plain DC voltage, not as a pitch signal.

Each Patch SM module has a slightly different ADC offset and gain. The constants kCvZero and kCvScale, in src/cv_in.h, were measured on the author's module. Calibrate your own module if a patched 0 V reads as non-zero, or a patched +5 V reads as not-quite-5-V.

Flash the firmware, then open serial telemetry: screen /dev/cu.usbmodem* 115200. Each line shows cv_norm=<f> (the raw ADC reading, 0 to 1) and cv=<f>V (the reading converted with the current constants).
Patch a known 0 V source into the V/OCT jack. Note the cv_norm value. Use this value for kCvZero.
Patch a known +1 V source into the V/OCT jack. Note the cv_norm value. Call this value raw_1v. Calculate kCvScale = 1.0 / (raw_1v - kCvZero).
Copy src/calibration_local.h.example to src/calibration_local.h. Enter your two values. Rebuild and reflash. Git ignores this file, so your values survive git pull. You can also edit the default values in src/cv_in.h directly.

Set kCvScale to 0 to disable the CV path. Every reading then becomes 0 V. Drifter applies a ±0.05 V deadband around 0 V after conversion, so a patched 0 V reads as exactly 0 V.

License

MIT. See LICENSE. The libDaisy submodule is MIT-licensed by Electrosmith.