Setting up your rig

One loop, two cables:

interface line/monitor OUT ──► pedal IN
pedal OUT ──► interface line/instrument IN

That’s the whole rig. PedalScope plays test signals out of the interface, through the pedal, and reads what comes back. Your guitar is not part of the loop — never plug it into this path, and never route the test loop to anything you’d hear at stage volume.

Which jacks

• Cables: plain TS, everywhere. Use ordinary TS (“mono” guitar) cables for the whole loop — including the straight loopback cable you calibrate with. A TRS cable into a balanced input travels a different electrical path that typically arrives about 6 dB hotter, so a TRS-calibrated loop doesn’t describe the path your pedal actually drives — and the compensation built from it quietly poisons every measurement after it.

• Output: any line or monitor output works. Note which one, and use the same one you’ll calibrate with.

• Input: the input you return the pedal into is what the app calls “Input 1”, “Input 2”, … counted from 1. On a Focusrite Scarlett Solo, for example, the XLR mic jack is Input 1 and the ¼″ instrument/line jack is Input 2 — a pedal comes back into Input 2.

• Instrument (Hi-Z) switch: off/line is fine for pedal outputs, which are low impedance. Whatever you choose, calibrate with the same setting.

Levels

Interface line outputs can drive a pedal far harder than any guitar ever would. PedalScope expresses drive levels in dBFS — decibels below the loudest signal your interface can produce — and draws a shaded “guitar range” band on level axes for orientation. Two habits matter:

• Stage the input gain for the pedal first. With the pedal in the loop at the loudest drive you plan to use, set the interface input gain so peaks land in the −12…−6 dBFS window — using the converter’s resolution, with headroom to spare. Calibrate at that same gain (the calibration page walks the full order of operations), then leave every knob where it is; changing input gain or output volume afterwards silently invalidates the calibration. If a knob moves, recalibrate — it’s two minutes.

• Start at the defaults. The default sweep level (−26 dBFS) at minimum interface input gain lands near hot-pickup territory on typical interfaces. Push toward −12 dBFS to hear a pedal’s ceiling, drop toward −50 dBFS to find where it cleans up.

Source impedance (why your fuzz measures “wrong”)

A guitar pickup is a weak, high-impedance source; an interface output is a strong, low-impedance one. Most pedals don’t care. But fuzz faces, treble boosters, and other circuits whose input loads the pickup behave differently when fed from an interface — typically brighter and harder.

Practical fix: put a series resistor (start around 10 kΩ) — or a re-amp box — between interface out and pedal in, and keep it there for every measurement of that pedal. The absolute numbers still won’t be guitar-exact, but comparisons between settings and between pedals stay fair, which is what the charts are for. Note the resistor in the measurement notes so future-you knows.

Hum and ground loops

The loop connects two devices to the same computer, so hum is usually mild, but if a pedal chains through a power supply shared with other gear you can pick up 50/60 Hz garbage that pollutes the low end of a measurement.

• Power the pedal from a battery or an isolated supply if you see unexplained low-frequency content.

• Keep the loop’s cables short and away from power bricks.

• Never lift a safety ground to chase hum.

Oscillation

High-gain pedals at some settings can self-oscillate — often ultrasonically or just below Nyquist, where you can’t hear it but every measurement can. PedalScope checks the stimulus-silent pre-roll of each sweep for strong narrowband tones (mains-hum families excluded) and warns — “Possible oscillation at 27.4 kHz — check gain settings; results may be invalid” — both during the run and as a flag on the saved record. When it fires: back off the gain, try a different power supply, and re-measure. Numbers taken while a pedal squeals to itself describe the squeal, not the pedal.

The microphone permission

The first time you run a calibration or measurement, macOS asks whether PedalScope may access the microphone. That wording is Apple’s, not ours: macOS files all audio input — including your interface’s line and instrument jacks — under the Microphone permission. PedalScope never listens outside a run you started; allow it and the prompt never returns.

If you clicked Don’t Allow (or IT policy blocked it), every run will fail with a clear message and an “Open Microphone Settings…” link — flip PedalScope on under System Settings → Privacy & Security → Microphone and run again.

Before you measure

Run calibration once per interface + sample rate + knob setting. It takes the pedal out of the loop (straight cable out → in), measures the interface’s own latency, tone, and distortion floor, and subtracts them from everything you measure afterwards.