legio-sawstack

Alternative firmware for the Noise Engineering Legio Eurorack module. This firmware runs a 5-voice supersaw oscillator. The repository also includes a macOS AU/VST3 plugin, built from the same DSP core.

This firmware is part of a family of alternative firmwares for the Noise Engineering Legio platform (Daisy Patch SM, STM32H750):

• legio-sawstack — supersaw oscillator (this firmware).
• legio-stutterer — stutter, tape delay, DJ filter.
• legio-drifter — random Bézier panner, crossfader, and CV source.

You can flash any of these firmwares onto a Legio module through DFU. To restore the stock firmware, use Noise Engineering's customer portal.

Controls

Control	Function
Top knob + CV	Sets the detune amount. At 0, all voices play in unison. At full, the voices spread up to ±2 octaves.
Bottom knob + CV	Sets the morph amount. The effect depends on the mode: sine↔saw blend, hard-sync richness, or sub-octave mix.
Left switch	Sets the mode: STACK, RICH, or SUB.
Right switch	Sets the stereo VCA width: MONO, STEREO, or WIDE.
Encoder, rotate	Sets fine pitch, ±50 cents.
Encoder, press and turn	Sets coarse pitch, ±2 octaves, in semitone steps.
Pitch jack	V/OCT pitch input. See "Calibrate V/OCT for your module" below.
Gate jack	Hard sync input. A rising edge resets the phase of all voices.

Sawstack outputs stereo audio. It has no internal envelope; this firmware is a pure oscillator.

Build and flash the firmware

make -C firmware                 # build
make -C firmware/test            # run host tests
make -C firmware program-dfu     # flash to your module

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

Calibrate V/OCT for your module

Each Patch SM module has a slightly different V/OCT ADC offset and gain. The constants kVoctZero and kVoctScale, in firmware/src/pitch.h, were measured on the author's module. On your module, these values may be off by a few cents to a semitone. Calibrate your module as follows:

1. Flash the firmware, then open serial telemetry: screen /dev/cu.usbmodem* 115200. Every line shows voct_raw=<f>, the raw ADC reading of the pitch jack (0 to 1).
2. Patch a known 0 V source into the pitch jack. Use a precision offset or voltage-source module, or a DC-coupled output set to 0 V and checked with a meter. Note the voct_raw value. Use this value for kVoctZero.
3. Patch a known +1 V source into the pitch jack. Note the voct_raw value. Call this value raw_1v.
4. Calculate kVoctScale = 1.0 / (raw_1v - kVoctZero). To check your calibration, patch a third voltage, such as −2 V or +3 V. The result must land within about 1 LSB of kVoctZero + volts / kVoctScale. The ADC is linear enough that two points are normally sufficient.
5. Copy firmware/src/calibration_local.h.example to firmware/src/calibration_local.h. Enter your two values. Rebuild and reflash. Git ignores this file, so your values survive git pull and never appear in a commit. You can also edit the default values in pitch.h directly.

Set kVoctScale to 0 to disable the V/OCT path entirely. The jack is then ignored. Use this as a safe state while you calibrate. The plugin build is not affected; it takes pitch from MIDI.

Plugin

This repository includes a macOS AU / VST3 / Standalone instrument, built with JUCE. The plugin reuses the firmware DSP core from firmware/src. See plugin/README.md for full details.

brew install cmake ninja                                          # one-time
git submodule update --init plugin/JUCE
cmake -B plugin/build -G Ninja -DCMAKE_BUILD_TYPE=Release plugin
cmake --build plugin/build

The build places the plugin files in plugin/build/Sawstack_artefacts/Release/. The AU and VST3 files install automatically to ~/Library/Audio/Plug-Ins/. Validate the AU with auval -v aumu Saws Tash.

License

The source in this repository is MIT. See LICENSE. The libDaisy and DaisySP submodules are MIT-licensed by Electrosmith.

The plugin links against JUCE, which is dual-licensed AGPLv3 and commercial. If you do not hold a commercial JUCE license, the AGPLv3 license applies to any AU, VST3, or Standalone binary you build from plugin/, in addition to this repository's MIT terms. The firmware does not use JUCE, so the firmware is not affected.