A modular synthesizer written in C, available as a standalone app (SDL2 for graphics and audio, MIDI input via libremidi) and as a VST3 instrument plug-in (Linux, Windows and macOS).

Live demo at: https://koppi.github.io/sdl2-synth

Features

• 4-oscillator synth: Each with independent waveform, pitch, detune, gain, phase, pulse width, and unison controls
• Mixer: Control the mix and master volume of each oscillator with bus compression
• Effects: Flanger, delay, reverb, and analog filter with real-time controls
• Arpeggiator: Multiple modes, adjustable tempo, octave control, and multi-octave chord arpeggiation
• Oscilloscope & Spectrum: Visualize output waveform and frequency spectrum
• MIDI input: Map MIDI CC to synth parameters for external control
• Interactive Keyboard: On-screen piano keyboard with visual feedback
• VST3 plug-in: The same engine and user interface inside your DAW, with host automation, state saving, MIDI input and tempo sync (see below)

Build Dependencies

CMake downloads and builds these automatically:

• SDL2 (graphics, audio, events) - standalone app
• Dear ImGui (GUI library)
• libremidi (MIDI input) - standalone app
• Steinberg VST 3 SDK (MIT licensed) - VST3 plug-in only

You need CMake 3.15 or newer (3.25 for the plug-in), a C and C++ compiler, and on Linux the usual development packages for X11, OpenGL and ALSA.

Build Instructions

Linux

sudo apt-get install git cmake build-essential libx11-dev libxext-dev libxcursor-dev libxi-dev \
    libxrandr-dev libxss-dev libxfixes-dev libxkbcommon-dev libgl1-mesa-dev libasound2-dev \
    libpulse-dev libudev-dev
git clone https://github.com/koppi/sdl2-synth && cd sdl2-synth
cmake --preset release
cmake --build --preset release && build/release/synth

macOS

brew install git cmake
# Then build as above

Windows

Install CMake and Visual Studio 2022 (C++ workload), then from a developer prompt:

cmake --preset release-windows
cmake --build --preset release-windows

VST3 plug-in

cmake --preset vst3                    # Windows: vst3-windows, macOS: vst3-macos-universal
cmake --build --preset vst3
ctest --test-dir build/vst3 -C Release # smoke test: audio, automation, state

The plug-in bundle is written to build/vst3/VST3/Release/sdl2-synth.vst3; copy it to your VST3 folder (~/.vst3, ~/Library/Audio/Plug-Ins/VST3 or C:\Program Files\Common Files\VST3). The build also runs Steinberg's VST3 validator on the plug-in. Pass -DSYNTH_BUILD_STANDALONE=OFF to build only the plug-in.

Running

build/release/synth

MIDI Control

MIDI input is mapped to numerous parameters (see src/midi.c for mapping). All GUI controls respond to mouse drag and mouse wheel. The oscilloscope is fed directly from the audio callback thread for real-time visualization.

VST3 plug-in

Download a ready-made plug-in for your platform from the releases page and follow the INSTALL.txt inside. It shows up as an instrument called SDL2 Synth.

• Parameters: every knob in the editor (about 115) is an automatable host parameter, grouped by section.
• MIDI: notes (sample accurate), pitch bend (+/-2 semitones) and these controllers, which the host maps to parameters: CC1 LFO 1 depth, CC7 master volume, CC10 master pan, CC71 filter resonance, CC72/73/75 envelope release/attack/decay, CC74 filter cutoff, CC91 reverb mix, CC93 flanger depth, CC94 delay mix.
• Tempo: the arpeggiator and the multi-tap delay follow the host tempo while Host Sync is on.
• State: saved with the project; presets of the host work as usual.
• Editor: the same Dear ImGui interface as the standalone app, resizable. It is drawn by a small software renderer, so it needs neither OpenGL nor extra libraries and behaves the same on every OS. On Linux the host has to provide the VST3 IRunLoop, as all Linux hosts do.
• Not in the plug-in: the standalone app's computer-keyboard input, MIDI Learn (use your host's mapping instead) and the chord progression generator.

Releases

Pushing a version tag builds the plug-in for all platforms, validates it and publishes a GitHub release:

git tag v1.2.3 && git push origin v1.2.3

macOS packages are ad-hoc signed. To ship them signed and notarized with an Apple Developer ID, add the repository secrets APPLE_CERTIFICATE_P12 (base64), APPLE_CERTIFICATE_PASSWORD, APPLE_SIGNING_IDENTITY, APPLE_ID, APPLE_TEAM_ID and APPLE_APP_PASSWORD; the workflow then signs and notarizes automatically.

VST is a registered trademark of Steinberg Media Technologies GmbH. See THIRD-PARTY-NOTICES.md for the licenses of the bundled components.