Transfer Curve
How it clips. The transfer curve is the pedal’s clipping shape drawn directly: while a steady low note plays, the app plots the signal coming out against the signal going in, moment by moment.
What it shows
While a low note plays, this draws the output signal against the input signal. A device with no memory draws a single S-shaped curve — its clipping shape. An open loop means the output also depends on where the signal has been. But beware: plain tone filters (a Rat’s input high-pass and output low-pass) rotate phase and open the loop without any real memory. The compensation toggle divides out the pedal’s measured filter response, and the loop number is split in two: the best-fit ellipse a linear filter’s phase would draw, and the nonlinear remainder — the memory figure the app quotes.
Two kinds of “input” in PedalScope. This chart is the odd one out: its axes show the signal’s instantaneous value — the waveform’s voltage at each moment, which swings positive and negative as the string vibrates. Everywhere else (Compression, Gain Map, level sliders), “level” means the signal’s strength averaged over time (RMS, measured in dB), which is never negative — a quiet signal has a low level, not a negative one (the minus signs on those dB axes measure distance below digital full scale, not polarity). One waveform sweeping through this whole curve, corner to corner, corresponds to a single point on the Compression chart’s level axis.
What exactly is plotted
Each point of the trace is one instant of audio: its horizontal position is the input signal’s amplitude at that instant, its vertical position the output’s amplitude at the same instant (after the loop’s measured latency is removed, so the instants really correspond). Amplitudes are plain instantaneous signal values, swinging ± about zero, on a linear scale as fractions of digital full scale at the interface — not dB, not RMS. While the probe note plays, the point sweeps back and forth along the curve once per cycle; the chart you see is one full cycle of that motion, averaged over the whole capture (the first few cycles are skipped while the circuit settles) so noise averages out but the shape doesn’t.
The negative half is the bottom of the waveform. A sine swings symmetrically above and below zero, so the trace extends into negative input and negative output — the lower-left quadrant is the wave’s downswing. That’s what makes the symmetry reading possible: if the curve flattens at a different height in the upper-right than it dips in the lower-left, the circuit treats up-swings and down-swings differently, and that asymmetry between quadrants is precisely what puts even harmonics (the octave-flavored warmth) into the Harmonic Distortion chart.
Traveling around the loop is memory. If the trace were a pure function — every input value giving exactly one output value — the up-swing and down-swing would retrace the same line. When instead the trace encloses area, the output at a given input depends on where the signal has just been: the circuit is remembering — a shifting bias point, charge stored somewhere it shouldn’t be. Musically that’s the difference between a clipper that responds only to where your signal is and one that responds to where it’s been — the compressing, breathing quality of fuzz-face-style circuits under sustained input. Mind the compensation toggle before crediting a pedal with memory, though: ordinary tone filters rotate phase and open the loop without any memory at all (details below).
How to read it
The middle of the curve is your quiet playing: the closer to a straight line, the cleaner the pedal there.
The bends are where clipping starts. A gradual bend is soft clipping (think Tube Screamer); a sharp corner into a flat ceiling is hard clipping (think Rat, at drive).
Top vs. bottom: if the curve flattens at a different height going up than going down, the clipping is asymmetric — that asymmetry is exactly what creates even harmonics (the octave-flavored warmth in the Harmonic Distortion).
The two loop numbers split the open loop into what a linear filter explains and what it can’t. Linear phase loop is the smooth ellipse a tone filter’s phase rotation draws — real, but not memory. Memory (nonlinear) is the area left after that best-fit ellipse is subtracted: the lobes and bands only bias shift or another true memory effect can draw. Near zero means a well-behaved memoryless clipper; a stubbornly high memory number is fuzz-face-style circuits misbehaving under sustained input. Area is counted lobe by lobe, ignoring the direction each lobe is traced: an earlier version summed them signed, so a figure-eight — two lobes wound in opposite directions — cancelled itself and could report a visibly open loop as 0.
Mid-range memory readings (roughly 0.05–0.15) are ambiguous. Tone filtering before the clipper rotates the drive itself, every harmonic inherits a multiple of that rotation, and the compensation toggle cannot undo it (it only knows the chain’s overall response). A pedal with a strong input filter can sit here while having no memory at all — the app’s wording hedges accordingly.
What’s musically meaningful
The curve is the pedal’s touch translated to a picture. Soft knees respond to pick strength gradually — dynamics survive, compressed. Hard ceilings turn everything past a threshold into the same output — sustain, at the cost of dynamics. Asymmetry adds even-order color. None of this is good or bad; it’s what you’re choosing between when you choose or design a drive pedal.
Hear it
The view’s Hear-it panels play the curve. Drive ramp raises a low note from whisper to full over six seconds through the pedal’s measured model — you hear the moment the sound enters the bends, and a marker climbs the curve in sync (the ramp’s top is mapped to the loudest measured excursion, so the marker walks the whole curve). Symmetry morph holds a note at the measured knee and lets you strip the even harmonics away: while it plays, a marker holds still at that operating point on the curve, and the asymmetric warmth disappearing is the top-versus-bottom difference of this very curve, audible. Both comparisons are loudness-matched, so neither side wins by being louder. Use headphones or full-range monitors — laptop speakers hide what these comparisons teach (see Listening conditions on this page).
Listening conditions
Hear-it comparisons are only as honest as the playback chain. Listen on headphones or decent full-range monitors when you can. Built-in laptop speakers do two things that work directly against these demos: they band-limit (almost nothing comes out below ~150 Hz, so a low E’s fundamental — and the deep difference tones the intermodulation demos isolate — simply never reaches your ears), and at high volume they compress, flattening exactly the level and dynamics differences the panels are teaching. Every Hear-it page in this guide assumes a reasonably full-range, uncompressed playback path.
Listen for: The first seconds are nearly clean; from the middle onward the note grows hair as it pushes into the curve's bends — that arrival of grit, not a volume change, is what the ramp teaches.
Common misreadings
An open loop doesn’t automatically mean “memory”. Ordinary filters open the loop by shifting phase. On the project’s reference Rat, 0.52 of a 0.56 total loop was the linear ellipse — nearly all of that loop was just the tone filters. Judge memory by the Memory (nonlinear) number with compensation on (it uses the H1 from the pedal’s own Harmonic Distortion measurement at the same knob settings — a different setting is a different filter, so the app never borrows another setting’s H1; measure Harmonic Distortion at the setting you’re judging).
One curve is one level. The shape can change dramatically with drive level — a curve measured at −40 dBFS says little about behavior at −12 dBFS. Match levels before comparing devices.
The probe is a low note on purpose (default E2, 82 Hz): low frequencies minimize filter phase rotation so the clipper itself is what you see.
Try it on a record: open a Transfer Curve measurement.