SquelchBox

A FOSS TB-303-style acid bassline synthesizer plugin. VST3 + CLAP + standalone.

Built with Rust, nih-plug, and egui.

Listen

Hear it in your browser — 320 kbps demo.

Features

• Oscillator -- bandlimited saw + square (BLIT/polyBLEP), drift LFO for analog warmth
• 3-pole diode ladder filter -- self-oscillating, with 2x oversampling via half-band polyphase
• Envelopes -- exponential amp, one-shot filter (attack-decay), dedicated accent envelope
• 16-step sequencer -- per-step pitch/accent/slide/rest, pattern length 1-16, swing, 4-bank pattern memory
• Sync modes -- Internal (free-run), Host (DAW transport slave), MIDI (keyboard only)
• FX chain -- Diode distortion, tempo-synced delay (analog/clean), Schroeder reverb, brickwall limiter
• Slide/glide -- portamento between legato steps, authentic 303 slide behavior
• Computer keyboard -- chromatic note input, step editing, tap tempo, pattern randomizer
• MIDI export -- dump patterns as .mid files

Install (prebuilt binaries)

Download the latest release for your platform from the Releases page.

Each archive contains VST3, CLAP, and standalone binaries plus an install script.

Linux

tar xzf squelchbox-linux-x86_64.tar.gz
./install.sh
# Rescan plugins in your DAW

Installs to ~/.vst3/, ~/.clap/, and ~/.local/bin/.

macOS

tar xzf squelchbox-macos-arm64.tar.gz   # or macos-x86_64
./install.sh
# Rescan plugins in your DAW

Installs to ~/Library/Audio/Plug-Ins/VST3/, ~/Library/Audio/Plug-Ins/CLAP/, and ~/.local/bin/.

Binaries are unsigned. On first launch: right-click > Open, or run:

xattr -dr com.apple.quarantine squelchbox-standalone

Standalone note: Use the included squelchbox-macos.sh launcher instead of running squelchbox-standalone directly. CoreAudio delivers variable-sized buffers that can exceed the configured size, causing a panic in nih-plug's CPAL backend. The launcher passes --period-size 4096 to accommodate this. See nih-plug#266.

Windows

Extract the .zip and run install.bat (may need Administrator for the VST3/CLAP paths).

Installs VST3 to %CommonProgramFiles%\VST3\, CLAP to %CommonProgramFiles%\CLAP\, and standalone to %LocalAppData%\SquelchBox\.

Standalone note: Use SquelchBox.bat (not squelchbox-standalone.exe directly). WASAPI in shared mode delivers buffers in the device's native period (often 1056-1266 samples on Windows 11), exceeding nih-plug's default 512-sample buffer. The launcher passes --period-size 2048 to avoid this.

Arch Linux (AUR)

paru -S squelchbox     # or yay, makepkg, etc.

Installs VST3 and CLAP system-wide to /usr/lib/vst3/ and /usr/lib/clap/, standalone to /usr/bin/squelchbox.

Verify checksums

Each release includes a SHA256SUMS.txt. After downloading:

sha256sum -c SHA256SUMS.txt

Build from source

Requires Rust stable 1.75+ (or nightly).

# Check / test
cargo check
cargo test

# Bundle VST3 + CLAP
cargo xtask bundle squelchbox --release

# Install (Linux)
rm -rf ~/.vst3/squelchbox.vst3
cp -r target/bundled/squelchbox.vst3 ~/.vst3/
cp -f target/bundled/squelchbox.clap ~/.clap/

# Standalone (Linux — match your PipeWire/JACK sample rate)
cargo run --release -- --sample-rate 44100 --period-size 512

Project structure

src/
  lib.rs              -- crate root
  plugin.rs           -- nih-plug Plugin impl, process loop
  params.rs           -- parameter definitions (knobs, toggles, FX)
  kbd.rs              -- keyboard/MIDI event queue (GUI <-> audio)
  main.rs             -- standalone entry point

  dsp/
    oscillator.rs     -- bandlimited saw + square
    envelope.rs       -- amp, filter, accent envelopes
    filter_diode.rs   -- 3-pole diode ladder (bilinear transform)
    oversampler.rs    -- 2x half-band polyphase up/downsample
    voice.rs          -- monophonic voice: osc + filter + envelopes
    fx/               -- distortion, delay, reverb, limiter, FxChain

  sequencer/
    clock.rs          -- tempo clock with swing
    pattern.rs        -- 16-step pattern + 4-slot bank
    runtime.rs        -- sequencer state machine

  ui/
    mod.rs            -- editor entry point (create())
    ids.rs            -- centralized egui ID registry
    palette.rs        -- colors + layout constants
    widgets.rs        -- param knob, button painters
    keyboard.rs       -- keyboard input, pattern persistence
    panels/           -- faceplate, band1, band2, fx_dist, fx_time, toast
      lower/          -- left strip, pitch row, step grid, transpose, right strip

  util/
    paths.rs          -- XDG data/config/preset directories
    midi_export.rs    -- pattern-to-MIDI file export

Dependencies

Crate	Purpose
nih-plug	Plugin framework (VST3/CLAP/standalone)
nih_plug_egui	egui integration for plugin GUIs
parking_lot	Fast mutexes for audio/GUI sync
rtrb	Lock-free SPSC ring buffer
serde + serde_json	Pattern bank persistence
tracing	Structured logging

License

GPL-3.0-or-later -- required by nih-plug's license.