BellyDSP
A guitar amp simulator and multi-effects processor in C++ on JUCE, with neural amp captures (NAM) running on NeuralAmpModelerCore, the official NAM engine. Built as a personal tool for practicing and writing, and free software: AGPL-3.0-or-later (see License).
Current target: a standalone macOS app that plays live through a Focusrite Scarlett Solo 4th Gen. VST3/AU plugin export comes later from the same JUCE target.
Features
Everything in the plan is built (see docs/PROGRESS.md for the measurements behind each item, and docs/ASSUMPTIONS.md for the choices waiting on Sean's review):
- Amps. Eight built-in amps (Glass, Ember, Monolith, Lantern, Basalt, Comet, Forge, Quartz), each with its own head and knobs, plus your own
.nam capture as a ninth, picked from the stage's shelf of minis or a MIDI footswitch (program change 1 to 9). Only the selected amp runs: a switch warms the incoming amp up unheard, then crossfades, so it never clicks and lands about 0.1 s later. Each amp's Gain blends five captures across the knob. The built-in captures are trained from the project's own amp models, not captures of real amps.
- Cab. Two close mics and a stereo room mic, each an IR, with level, pan, polarity, delay, and mute; auto phase alignment; low and high cuts. A close mic loaded with a cab pack moves on a position pad, morphing between captures.
- Gates. Gate A before the amp and Gate B after it, linked by default (one decision applied at both points), detecting from the clean DI, with adaptive release, hysteresis, hold, and Learn.
- Drive. A boost (Clean, Tight, Screamer) and an overdrive (Mid Drive, Distortion, Transparent, Fuzz), modelled from the schematics and validated against circuit simulations, oversampled 4x or 8x with no added latency.
- Pre FX (reorderable): Gate A, auto-wah (an envelope filter driven by the clean DI, Up or Down), compressor (Studio and Pedal), boost, overdrive, EQ (accurate 9-band graphic or 5-band parametric, plus cuts).
- Post FX (reorderable): EQ, compressor, a key-aware 4-voice harmonizer, an 8-voice multivoicer (Poly or Mono), Bloom (bitcrush, phaser with Classic, Modern, and Vibe, flanger with through-zero), chorus (Classic, Dimension, Tri), delay (digital, analog, tape; stereo, ping-pong, dual; ducking), and reverb (Room, Hall, Plate; freeze; shimmer). Delay and reverb tails spill over when bypassed.
- Tuner. An overlay built to be read from a step back: the note at 200 px, a 41-segment scale over +-50 cents, in tune within 3 cents, the six strings of four tunings as chips, A4 from 430 to 450 Hz, and an output mute. The footer shows the note and a small cents meter while it's open.
- Tempo. A global tempo, tapped from the panel or a footswitch; the delay, chorus, phaser, flanger, and reverb pre-delay can sync to it.
- Presets and scenes. JSON presets with library-relative files found again by content hash when moved, eight scenes per preset switched from a footswitch, undo/redo, A/B, and five factory style presets (Modern Prog, Math Rock, Tech Death, Metal, Midwest Emo) on the built-in captures and cabs.
- MIDI. Right-click any control to MIDI-learn a footswitch (toggle or momentary) or an expression pedal; mappings are saved with presets.
- Real time. Zero added latency (only the opt-in through-zero flanger reports 5 ms), no allocation, freeing, or locking on the audio thread (tested on every feature), and the whole rig with every block on at about 18% of a 128-sample buffer's time.
The GUI (docs/UI_DESIGN.md, DESIGN.md) is one main screen on a fixed 1280 x 760 canvas that scales with the window, every pixel drawn in code:
- Top bar: the brand's menu, A/B, undo and redo, the preset and its scene between the previous and next arrows, Save, the CPU, the In and Out meters, and Tuner, Match tone, and Settings.
- Stage: the amp on a dark stage lit from below, with the shelf of eight mini heads to pick it; or the cab (the library, the speaker with its mics, the room mic); or the EQ's graph over its spectrum.
- Board: the pedals as rendered gear under the stage, in three tabs: Pre FX, Amp & Cab (the amp, Gate B, and the cab), and Post FX. Each tab shows an LED per block, so a closed tab still shows what's on. Click a footswitch to switch a block, click a pedal to select it, drag a pedal along its row to reorder it, and switch a whole section off with its switch.
[ and ] step the tabs.
- Inspector: a 300 px column at the right with the selected block's every control.
- Scenes: the eight scenes along the bottom, with Store and the tuner's reading.
- Overlays: the tuner, Match tone (six numbered steps), and Settings (Input, Output and MIDI, Audio device, About).
Keys: Left and Right step presets, 1 to 8 recall scenes, T opens the tuner, Escape closes an overlay, Cmd-Z and Shift-Cmd-Z undo and redo.
Building
You need Apple's command line tools (xcode-select --install) and CMake (installed in ~/.local/bin). Full setup, including the Scarlett Solo gotchas, is in docs/SETUP.md.
git submodule update --init --recursive # JUCE and NeuralAmpModelerCore
cmake -B build -DCMAKE_BUILD_TYPE=Release # configure
cmake --build build -j # compile
ctest --test-dir build --output-on-failure # run the tests
open build/BellyDSP_artefacts/Release/Standalone/BellyDSP.app
Releasing
Releases are published on the public repo, https://github.com/SeanSnaider/BellyDSP/releases, and the app updates itself (Sparkle on the Mac, WinSparkle on Windows). Every release is built from a commit that's on the public repo's main and tagged there, so the exact source of every download is public. One command checks that, builds, tests, signs, packages, tags, pushes the tag, and publishes: tools/release/release.sh <version>. docs/RELEASING.md has the one-time setup and the details; docs/INSTALL.md is the install guide.
Repo layout
| Path | What it is |
|---|
src/dsp/ | The DSP: the Block interface, the chain, every amp, cab, filter, and effect |
src/PluginProcessor.* | Parameters, state, file loading, MIDI, and the audio callback, which just drives the chain |
src/BlockParameters.*, src/Presets.* | Parameter glue for each effect block, and the preset format |
src/PluginEditor.* | The editor: lays out the GUI and runs presets, scenes, MIDI learn menus, and undo |
src/platform/ | What the shipped app knows about itself: its version, its bundled files, and the auto-updater |
src/ui/ | The GUI: tokens and fonts, LookAndFeel, controls, meters, the top bar, the stage, the board and its pedal art, the amp heads, the cab, every block's inspector, the scenes, and the overlays (tuner, Match tone, Settings) |
presets/factory/ | The factory presets, embedded in the app |
docs/ui/amp-ui-handoff/ | The original GUI handoff, retired as a source of truth by the redesign (docs/UI_DESIGN.md) |
resources/fonts/ | Geist and Fraunces (SIL OFL, licences alongside), embedded in the app |
tools/render/ | ampsim_render: runs a WAV through the chain offline, with CPU timing |
tools/device_probe/ | ampsim_device_probe: opens the interface and reports timing and input levels |
tools/capture/, tools/train_capture.sh, tools/fetch_nam_input.sh | Capturing your own gear for NAM: ampsim_capture plays NAM's input file through the gear and records it, and the trainer wrapper runs NAM's official trainer on it (docs/CAPTURING.md) |
tools/content/ | The scripts that imported the bundled cab IRs and ranked them |
tools/release/, tools/fetch_deps.sh, tools/notices/ | The release pipeline: signing, packaging, appcasts, publishing, the update test, the licence notices |
installer/ | The DMG's read-me and the Windows installer script (Inno Setup) |
Amp models
No .nam models are bundled. Download amp-only captures (no cab baked in) from TONE3000, or train your own with NAM's trainer, and keep them in models/, which is gitignored. NAM core's example models in third_party/NeuralAmpModelerCore/example_models/ work for testing.
Cab IRs
21 public-domain (CC0) IRs from Jester Dyne Productions are bundled (content/, listed under "Built in" in the cab's library on the stage). Load your own .wav files too (free and paid packs are widely available); keep them in irs/, which is gitignored.
License
BellyDSP Copyright (C) 2026 Sean Snaider
This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version (SPDX: AGPL-3.0-or-later). This program comes with ABSOLUTELY NO WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See LICENSE for the full text.
Source: https://github.com/SeanSnaider/BellyDSP. The app's brand menu links to the exact source of each version.
- Every source file carries an SPDX header (
tools/spdx_headers.py).
- JUCE is used under its AGPLv3 option, which is why the whole program is AGPL.
- Sean's own NAM captures (in
content/, as they're added) are licensed under CC BY 4.0, attribution "Sean Snaider". The bundled cab IRs are CC0 1.0.
- Third-party components keep their own licences, listed with their full texts in
THIRD_PARTY_NOTICES.txt (generated by tools/notices/make_notices.cmake, shipped in the app and the downloads).
