Gain Map

How the dirt changes as you play harder. One Harmonic Distortion measurement is one input level — one slice of a pedal that lives on a whole range of them. The gain map repeats the harmonic sweep at stepped input levels (eight by default, quiet to hot) and stacks the results into a map of the pedal’s entire dynamic life.

What “drive” means on this chart: the level of the input signal driving the circuit — which the app sweeps electronically, exactly as your pick strength and your guitar’s volume knob would. The pedal’s own drive knob is never touched while the map is measured; the knobs stay wherever you set them, and that setting is recorded with the measurement. To map what the knob does, measure it as a set of variants — one gain map (or Harmonic Distortion run) per knob position.

What it shows

Every row of the map is one input level (bottom = playing soft or rolling your volume down, top = digging in hard); left to right is the note you play. Brightness is how loud the chosen ingredient is — total distortion, or one harmonic at a time. A pedal that “cleans up” goes dark along the bottom rows; a fuzz stays bright everywhere. “Drive” here is always the signal level driving the circuit — the app never turns the pedal’s knob. Pick single harmonics to watch the recipe change: many circuits trade even warmth for odd grit as the input level rises.

Gain Map view: a THD heatmap over note (x) and drive level (y), palest (most distorted) across the loud upper rows and darkening toward the quiet bottom rows and the treble edge — The bundled Fuzz-ish simulated rig's gain map: hot nearly everywhere — still over 1% THD at the quietest measured level, so it never truly cleans up — with distortion easing at the treble edge, where the output filter takes over and the highest notes' harmonics leave the audible band.from sim-journey.json

On macOS 26 the same data also renders as a rotatable 3D surface — frequency × level × loudness. Same numbers, different grip on them; the 2D map is the reference reading.

The note axis follows the app-wide Range selector (Full / Audio / Guitar) and, on the 2D map, the Hz-vs-note Unit selector — see Harmonic Distortion. The 3D surface keeps its octaves-above-20 Hz labeling (its ticks are the framework’s own), with the caption as decoder ring.

How to read it

Scan bottom to top at your favorite note: that vertical line is what your volume knob and pick strength traverse. A tall dark-to-bright gradient is a touch-sensitive pedal; bright everywhere is a fuzz that’s always itself.
The cleanup contour — where the map crosses ~1 % THD — is the border between “effectively clean” and “audibly driven”. Its height tells you how much rolling off actually helps.
Switch the displayed ingredient. Total THD answers “how dirty”; individual harmonics answer “dirty how”. Watching H2 versus H3 across level shows whether warmth survives at full drive or gives way to grit.
Tilt across frequency is character. The project’s reference Rat shows its famous slew-limit signature here: at high drive, distortion rises toward the treble — a diagonal tilt in the map that reads as that aggressive, torn top end. A tone-filtered soft clipper tilts the other way, darker as you go up.
The treble edge counts fewer harmonics — that’s physics, not the pedal cleaning up. THD cells integrate the audible band (20 Hz–20 kHz), and near the top of the map most of a note’s harmonics sit above it, so the rightmost columns naturally read lower. When you view a single harmonic, columns past that harmonic’s reliable measurement edge draw faded — those cells are analysis artifacts, and hover says so.

What’s musically meaningful

This is the chart for “does it clean up?”, asked precisely. It also separates two pedals that sound alike at noon: one may hold its harmonic recipe steady across levels (predictable, mix-friendly) while another reshapes itself every 6 dB (alive, or unruly — your call). When you’re choosing where to set a drive knob, you’re choosing a row of this map.

Hear it

Two Hear-it panels play the level axis. Decay cleanup sends one long plucked low E through the model: as the note dies away it slides down the map’s rows, and the distortion melts off in real time — a marker descends the level axis with it. Touch dynamics plays the same string picked soft, medium, and hard; the whole phrase is loudness-matched once, not per note, because the ratio of dirt between your soft and hard picking is exactly the thing to hear. Gain maps measured before this release play through a single-level model and wear a “coarse” badge — remeasure to get level-interpolated renders. Headphones or full-range monitors recommended — see Listening conditions.

A long low-E pluck decaying through the bundled simulated rig's model.

Listen for: The attack is fully distorted; as the note fades, the fizz thins out well before the note itself disappears — the pedal is sliding down the dark rows of its own map.

▶ Hear this in PedalScope

▶ See a frequency-tilted gain map: raise the slew limit in Simulation mode

Common misreadings

The level axis is interface drive, not your drive knob. The map varies how hard the signal hits the pedal — the electronic equivalent of pick strength and guitar volume — with the knobs untouched. To map the knob itself, measure a variant per knob position (family runs automate the switch version).
Rows are separate measurements. The pedal isn’t being ridden in real time; each level is its own sweep, measured seconds apart. Circuits that drift or sag between notes can show row-to-row wobble that isn’t “the drive knob doing something”.
Dark isn’t silent. Dark cells mean low added distortion, not low output — the pedal may be loud and clean there. Loudness lives in the Compression chart.

Try it on a record: open a Gain Map.