Crossbill Harmonic Wavefolder

What folding does

A clipper flattens peaks; a wavefolder reflects them. Each time the signal exceeds the fold threshold, it bounces back — adding a new pair of peaks and a new burst of harmonics. Both of Crossbill's fold functions are bounded to full scale with unity slope at the origin, which means two things:

• It's transparent when you back off. Small signals pass through unchanged — folding only kicks in as the level climbs.
• It can't run away. No setting of Drive or Folds pushes the output past full scale. Harder settings add folds (density, brightness), not level.

Watch the hero scope: the faint line is your input, the glowing copper line is the folded output. The teal inset is the transfer curve — the exact input→output map, with a dot showing where your signal currently sits.

Drive & Folds — the fold core

Two controls drive the folding, and they do different jobs:

• Drive — −12…+36 dB, default +3. Input gain into the folder: how hard you hit the fold points. More drive makes the wave cross the threshold more often.
• Folds — 0–100%, default 50. Fold density, mapping to a 1×–8× fold gain. This is the primary "how metallic" control — raise it for a thicker stack of overtones.

Because the folder is bounded, both controls read as brightness and density rather than volume. Push them freely — you're sculpting harmonics against a constant ceiling.

Shape — Sine vs Triangle

The Shape switch picks the fold function, and it's the single biggest tonal decision:

• Sine — wraps the signal through a smooth sinusoid. Rounded folds, gentler harmonic roll-off. The classic west-coast (Buchla) timbre — warmer and more musical for leads and pads.
• Triangle — a reflective period-4 triangle. Sharper corners, harmonics that extend further and buzz harder. The aggressive, metallic reese/growl side.

Set Shape first, then dial Drive and Folds — the same knob positions sound very different between the two shapes.

Symmetry — odd ↔ even harmonics

Symmetry (−100…+100%, default 0) applies a DC bias before the fold so the wave folds off-centre. This is your harmonic-character control:

• Centred (0%) — symmetric folding produces purely odd harmonics: a hollow, square-ish tone.
• Off-centre (±) — introduces even harmonics for a fuller, more vocal or brassy character. Push it further for more even content.

A DC blocker after the fold keeps the output centred no matter how far you bias, so Symmetry never leaves an offset on your signal.

Tone & Smooth — voicing the folder

Two filters shape the sound around the fold core:

• Tone — −100…+100%, default 0. A pre-fold tilt (±6 dB about 700 Hz). Because it's before the folder, it changes which harmonics get generated: tilt bright to feed highs in for fizz, tilt dark to fold mostly low-end for a thicker growl.
• Smooth — 0–100%, default 0. A post-fold lowpass sweeping 20 kHz → 1.5 kHz. Pull it down to tame the fizz and harshness dense folding creates, until the growl sits in your mix.

Mix, Output & presets

Mix (0–100%, default 100) blends the folded signal against the dry — and the dry copy is delay-aligned to the 4× oversampler, so parallel folding never smears phase. This is the key to adding overtone weight to a sub without touching its fundamental (see the Sub Harmonics preset).

Output (±24 dB) is the final trim with a live meter — use it to match levels against bypass. Crossbill ships with 7 factory presets; each resets all parameters to default then applies its overrides, so it's always a clean starting point. Save your own as .crossbillpreset files in your Documents folder.