83_Origami

Dual oversampled wavefolder for the Music Thing Workshop System Computer.

A wavefolder is the other classic West-Coast timbre tool (the sibling of the low-pass gate): instead of removing harmonics like a filter, it adds them. Drive a waveform hard enough and it reflects back on itself at the rails; each fold stacks on a new band of metallic, bell-like harmonics that bloom and shift as you push the drive. Origami gives you two independent folders, one per column of jacks, with three fold characters and a bias control for even-harmonic colour.

The fold is computed at 4× oversampling and band-limited back down, so the (very harmonically rich) output aliases far less than a naïve folder would.

Status: 0.1 — DSP verified in offline host simulation (sim/), built cleanly to a flashable .uf2, and tested on the Computer. Please open an issue/PR with fixes. The DSP is plain integer ComputerCard code (see src/origami.cpp).

Quick start — it needs something to fold

A wavefolder is a processor: patch audio into Audio In and you'll get it back, folded, from Audio Out. With the drive low and a quiet input it stays nearly clean — the folding only bites once you push the signal past the rails with the drive knob (X / Y) or with CV In. Turn the drive up and the harmonics pile on.

Best input is a simple, loud waveform — a sine or triangle from an oscillator. Folding a sine is the canonical sound; folding something already bright (a saw) gets dense fast.
Match the column. Left jacks = channel 1, right jacks = channel 2; the two folders are completely independent. Audio, CV and output must be in the same column to interact.

Panel / I/O

Left column of jacks = channel 1, right column = channel 2.

Control	Function
Audio In 1 / 2	Channel input — the signal to fold.
Audio Out 1 / 2	Folded output.
CV In 1 / 2	Adds to that channel's fold drive — CV over fold depth. Bipolar: positive CV folds more, negative CV backs it off.
CV Out 1 / 2	Envelope follower of the folded output (0..+6 V) — handy for self-patching a VCA or gate elsewhere.
Knob X	Channel 1 fold drive (depth). Low ≈ clean, high ≈ many folds.
Knob Y	Channel 2 fold drive.
Main knob	Bias / symmetry for both channels. Centre = symmetric (odd harmonics); off-centre offsets the signal into the folder for an asymmetric, hollow/reedy colour (even harmonics).
Switch	Fold character: Down = triangle (classic reflecting fold), Middle = sine (smoother, Serge-ish), Up = hard clip (saturation/squarewave).
LEDs	Each column glows with that channel's output level.

Patch ideas

Classic timbre sweep — patch an oscillator (sine/triangle) into Audio In, set the switch to Down (triangle), and sweep Knob X. You'll hear the tone go from pure to bright and clangorous as folds pile on. Add bias (Main knob off centre) for a more vocal/reedy colour.
Dynamic / "opening" fold — send an envelope or LFO to CV In so the fold depth tracks it. With a slow envelope the timbre brightens as the note evolves — the West-Coast "the harder you hit it, the brighter it gets" gesture. Tap the follower out of CV Out to drive a downstream VCA.
West-Coast voice across two cards — fold an oscillator here, then send Audio Out to a low-pass gate on a second Computer for the full fold → gate chain.
Stereo motion — feed the two channels slightly detuned copies of the same oscillator and fold both; the independent folders smear into a wide, shifting texture.

How it works (brief)

Each channel scales its input by a drive amount (knob X/Y plus CV In), adds a bias DC offset (Main knob), and passes the result through a periodic folding nonlinearity selected by the switch:

Triangle — integer reflection: the value is wrapped/reflected into ±1.0, a triangle transfer function (the classic Serge/Buchla reflecting fold).
Sine — a single-period sine lookup table; smoother folds, gentler at low drive.
Hard clip — saturates at the rails (not really folding, but a useful squarewave/overdrive contrast).

Folders generate harmonics far above the audio band, which would alias badly at 48 kHz. So the nonlinearity runs at 4× oversampling: the input is linearly upsampled, folded per sub-sample, then decimated with a Hamming-windowed sinc low-pass (firTab, built once in the constructor) before going to the DAC. All hot-path math is integer/fixed-point (no FPU on the RP2040); only the one-time table setup uses floats.

Build

Requires the RPi Pico SDK with PICO_SDK_PATH set (same as the other ComputerCard cards):

cd releases/83_Origami/src
mkdir -p build && cd build
cmake ..
make

This produces origami.uf2. Flash it to the Workshop System Computer in the usual way: hold BOOTSEL, then copy the .uf2 onto the mounted RPI-RP2 drive.

macOS tip: if the RPI-RP2 drive ejects the instant you start copying and the firmware never lands, flash over USB instead:
brew install picotool
# board in BOOTSEL mode:
picotool load -v -x origami.uf2

Hear it without hardware

sim/ compiles the real src/origami.cpp against a mock hardware layer and renders test WAVs (drive sweep, the three fold characters, a bias sweep, and CV-controlled folding) on any machine with g++:

cd releases/83_Origami/sim
./build.sh

See sim/README.md for details and the caveats on what host simulation does and doesn't verify.

Built with Chris Johnson's ComputerCard library.