Calibration

Every audio interface adds its own fingerprint to a measurement: a delay, a slight tone shape, a little distortion of its own. Calibration measures all of that once — with a plain cable in place of the pedal — so PedalScope can subtract the interface from every pedal measurement that follows. It’s how the app can honestly claim a chart shows the pedal, not the pedal plus your converter.

Gain staging, in order

Calibration measures the loop including your knob positions, so the knobs get set first and the calibration comes second. Four steps, in this order:

TS cables everywhere. Use the same plain TS (“mono” guitar) cables for the loopback that your pedal measurements use — calibration must measure the same electrical path the pedal will drive. A TRS cable in the loopback is a different path: into a balanced input it typically arrives about 6 dB hotter than the TS path, and the loop response — including phase — it calibrates isn’t the one your measurements go through, silently corrupting the compensation baked into everything that follows.
Set the input gain for the pedal, not the loop. 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 — loud enough to use the converter’s resolution, with headroom to spare.
Calibrate at that same gain. Take the pedal out, patch the TS cable straight from output to input, and run Calibrate. If the direct loop clips or runs hot, lower the calibration sweep level (Settings → Calibration) — not the input gain you just staged for the pedal.
Then leave the knobs alone. The calibration is only a picture of the loop as it was; any knob change between calibration and measurement silently invalidates it. If a knob moves, recalibrate — it’s two minutes.

The sweep level

The calibration sweep’s digital level is a preference (Settings → Calibration; default −26 dBFS — the standard measurement drive level; range −40…−6 dBFS). The loopback chain is linear, so calibration is equally valid anywhere in that range: the level changes the loop’s signal-to-noise ratio, not its measured response. Trim it so the direct loop peaks in the −12…−6 dBFS window at the input gain you staged for the pedal — the app’s earlier fixed −12 dBFS sweep clipped direct loops at input gains that are perfectly healthy for pedal measurements, which is exactly why the level is now yours to set. Every calibration records the level it ran at and the loop gain it measured. Very low levels trade away signal-to-noise; runs under 20 dB SNR are rejected outright.

Running it

Patch a short TS cable straight from the interface output you’ll use to the input you’ll use. No pedal in the loop.
In the measure flow, pick your device, input, and sample rate on the Setup step, then run Calibrate. It plays a sweep through the cable and takes about ten seconds. The sweep level in use is shown next to the button.
Re-patch the pedal into the loop and carry on measuring. The calibration is saved and reused until you change something.

Recalibrate when anything in the chain changes: interface knobs (input gain, output/monitor level), the instrument/line switch, sample rate, which jacks you use, cables, or the interface itself. The app stores calibrations per device and sample rate and warns on a mismatch, but it can’t see your knobs — keeping them where they were calibrated is on you.

What the numbers mean

A calibration reports a few figures of merit; here’s how to read them:

Round-trip latency — how long a signal takes to leave the app and come back, in samples and milliseconds. The value itself doesn’t matter (a few ms is normal); its stability does. The same rig should report the same latency within a sample on every run — if it jumps around, something (usually the interface’s buffer settings) is unstable.
Loop response — how flat the cable loop measures after compensation. Expect essentially flat (within about ±0.1 dB) across the audio band. A big tilt or ripple means a bad cable or an accidental processing stage (direct-monitor mix, loopback DSP) in the path.
Loop gain — the measured digital-out → digital-in gain of the loop, stamped alongside the sweep level it was measured at. Your interface’s knob positions are invisible to software, but the gain they produce is measured — this pair is the calibration’s settings fingerprint. If a fresh calibration reports a different loop gain than the last one on the same rig, a knob moved.
Interface distortion floor — the harmonics the interface itself adds, typically around 0.01 % THD on a decent interface. This is the measurement’s honesty line: PedalScope will not claim a pedal is cleaner than the interface measuring it. If a pedal reading approaches the floor, the chart is showing you the interface, not the pedal.
Noise / SNR — hiss picked up from the loop, measured from the silent lead-in. Drives the level recommendations in the measure flow.

For the curious

Latency comes from cross-correlating the captured sweep against the emitted one — polarity-agnostic, sub-sample interpolated. The loop’s frequency response is measured from the same sweep and divided out of later measurements (its inverse is regularized outside the sweep band so noise doesn’t blow up). The distortion floor uses the same synchronized-sweep harmonic separation as pedal measurements, so “interface THD” and “pedal THD” are directly comparable numbers. Level-invariance isn’t taken on faith either: the test suite calibrates a simulated loop at two very different sweep levels and requires the results to agree.

Level habits that keep calibrations honest

Calibrate at the same input gain you measure at — that’s the whole point of the staging order above. The sweep level is free to differ from your measurement drive levels; the knobs are not.
If the calibration reports clipping, lower the sweep level in Settings and run it again — leave the input gain where you staged it for the pedal. A clipped calibration is invalid, and the app will say so rather than store it.

The low-level warning

Clipping isn’t the only way to waste a converter. If a capture comes in far below the target window, PedalScope warns that you are spending converter resolution on silence — under-driven runs make the static transfer curve jittery near the zero crossing and put a worse noise floor under every derived number. The check projects each capture’s peak to the loudest planned drive using the level that run was actually played at, so deliberately quiet runs — a gain map’s soft steps — never false-alarm.

The warning is advice, not a gate: the run completes either way. It appears when a calibration comes in low, and again at measurement start — there it’s computed from the selected calibration’s measured loop gain, before any audio plays. The remedy is the staging order above: raise the interface input gain for the pedal case, then recalibrate at that gain. (A clipping loopback has the opposite remedy — lower the sweep level in Settings, and leave the input gain alone.)

Once calibrated, you’re two clicks from a real measurement — see your first measurement.