What clipping is
Every distortion pedal, from the politest boost to the nastiest fuzz, does one job: it refuses to pass a signal bigger than some ceiling. What it does at that ceiling — slam into it or lean on it, treat the top and bottom of the wave the same or differently — is nearly the whole story of how it sounds. This page is that story; the Transfer Curve chart is its picture.
Why a ceiling creates new notes
A clean amplifier changes a wave’s size, never its shape. The moment something flattens the peaks, the shape changes — and a changed shape is new frequencies. That isn’t a metaphor: any periodic wave is exactly a stack of harmonics (multiples of the note’s frequency), and bending the wave redistributes energy into that stack. The pedal isn’t “adding fuzz dust” on top of your note; it is reshaping your note into a chord of overtones it builds from your note’s own frequency. Which overtones, and how loud, is the pedal’s Harmonic Distortion recipe.
Hard versus soft
Picture the transfer curve — output level drawn against input level.
Hard clipping is a sharp corner into a flat ceiling: below the threshold nothing happens, above it nothing gets through. The abrupt corner spreads energy far up the harmonic series, which reads as bright, aggressive, buzzy. Rat-style circuits and anything that slams a signal into diodes to ground live here.
Soft clipping is a gradual bend: the circuit starts giving up smoothly well before the ceiling. Energy concentrates in the low harmonics, which reads as smooth, warm, “tube-like”. Tube-Screamer-style feedback clippers are the textbook case.
The knob you can’t see on most pedals — how sharp that corner is — is the knee. In Simulation mode the knee is a literal slider: at zero you have a hard clipper, and every bit of softness you add audibly tames the upper harmonics.
▶ Start soft (TS-style), then harden the knee and listen
Symmetry, and why warmth is even
A wave has a top half and a bottom half. If the circuit clips both halves identically, only odd harmonics appear (H3, H5, H7 — the gritty, edgy family). If it treats them differently — one diode more than the other, a bias off center — even harmonics appear too (H2, H4 — the octave family, which the ear hears as thickness and warmth, because an octave can’t clash with the note that made it).
That’s the entire trick behind “silicon vs. LED” switches and “symmetry” trims: they move the top ceiling relative to the bottom one. On the transfer curve you see it as a curve that flattens at a different height going up than going down; on the Harmonic Distortion chart you see the even (orange) stems rise out of the floor.
▶ Drag the asymmetry slider and watch H2 climb
What this buys you as a player
A hard, symmetric clipper: focused grit, holds its character, less interaction with pick strength.
A soft knee: dynamics survive — dig in and more of your attack gets through before the bend takes over (see touch dynamics).
Asymmetry: octave-flavored thickness, at the price of a recipe that shifts more as levels move.
None of these is better. They are the axes of the space you’re choosing from — or designing in — when you pick a drive circuit.
For the curious: why symmetry decides even vs. odd
Write the clipper as a function y = f(x). Clipping both halves identically means f is odd-symmetric: f(−x) = −f(x). Feed it a sine and expand the output in harmonics: an odd function of an odd waveform can only contain odd harmonics — every even term would violate the symmetry. Any asymmetry adds an even-symmetric component to f, and that component alone generates the even harmonics. The measured recipe quantifies the split directly, and the transfer curve shows the asymmetry as different flattening heights in the upper and lower halves.