Results on hardware
Every number here was measured on the pedal: the conversions were uploaded, a signal was played into the pedal’s input, and its USB audio output was recorded. That recording was compared with the original model or IR run in software. Lower is better everywhere.
NAM: sweep measurements
The Ampeg SVT “Gain 10 Ultra Lo and Hi MD 421” model, converted three ways and re-amped in one session: sweeps at four input levels and bass tones. Each upload is compared with the model run in software on the pedal’s own dry take of the same signal. Scores are rms dB, with levels matched:
| Upload | Frequency response | Low end (40–160 Hz) | Note dynamics | Harmonics |
|---|---|---|---|---|
| Official app | 4.75 dB | 4.95 dB | 9.34 dB | 12.16 dB |
| open-stomp 0.4 | 3.05 dB | 5.35 dB | 5.84 dB | 10.85 dB |
| open-stomp 0.5 | 2.14 dB | 3.51 dB | 2.88 dB | 8.60 dB |
Note dynamics is how the level of each note changes with how hard it’s played (fundamental levels of bass tones at three levels, after removing one overall offset). Harmonics compares the 3rd and 5th harmonics relative to each note’s fundamental.
The graphs compare tone, so each upload is level-matched to the model over 100 Hz–8 kHz. The official conversion is off by up to 10–13 dB in the 400 Hz–1 kHz mids at low input levels, 5–8 dB in the upper mids and up to 20 dB in the top octave. Both open-stomp versions stay within 4 dB from 100 Hz to 10 kHz. Below that, this model has a +20 dB resonance at 63–80 Hz that 512 taps can’t fully follow: 0.5 gets within 4–8 dB of it, 0.4 within 7.5–10.
NAM: real playing
A 3-minute bass DI re-amped through the pedal with the Ampeg SVT-VR Classic (Normal) model and a cab IR, against the model and IR in software on exactly what the pedal heard. Fixing the low end has the details, and the interactive comparison has every graph.
| Official app | open-stomp 0.4 | open-stomp 0.5 | |
|---|---|---|---|
| Overall tone (40 Hz–10 kHz) | 9.4 dB | 3.5 dB | 2.1 dB |
| Low end (40–160 Hz) | 3.0 dB | 5.2 dB | 3.3 dB |
| Moment to moment (median) | 6.2 dB | 4.3 dB | 3.0 dB |
| Closest to the original in | 8% | 2% | 87% of 100 ms windows |
IR: cab measurements
The CAB block on its own, re-amped in the same session and measured by sweep, against the full IR file in software (31.5 Hz–16 kHz, levels matched):
| IR | Official app | open-stomp 0.4 | open-stomp 0.5 |
|---|---|---|---|
| Ampeg 8x10 57 AH | 5.28 dB rms | 1.36 dB rms | 2.11 dB rms |
| Ampeg SVT Neumann | 16.13 dB rms, and 45 dB quieter | 5.13 dB rms | 5.23 dB rms |
The SVT Neumann file starts with 21 ms of silence, so the official upload plays almost none of the cab.
In 0.5 the IR fit covers 30 Hz–16 kHz instead of 25 Hz–20 kHz. In software, on fifteen Ampeg cab IRs, that lowered the average error of what the pedal plays from 2.42 to 2.18 dB rms. On the pedal the picture is mixed: on the Neumann it is up to 1.8 dB closer at 50–100 Hz, but both IRs lose 1–4 dB in the 32–40 Hz bands, below a bass’s low E, and that’s where these two scores get worse. In the real-playing test above, the 0.5 IR conversion of the Beta52 cab improved the overall tone from 2.8 to 2.1 dB.
Reproducing these
The scripts are in tools/pedalsim:
readme_plots.py <session> measures a re-amp session (the reamp_run.py test signal, one block at a time) and draws
the graphs above; readme_plots.py --cached redraws them from tools/pedalsim/results. The real-playing comparison is in
tools/pedalsim/reamp_compare.