Comprehensive DSP graph and synthesis library for developing a modular synthesizer in Rust, such as HexoSynth.
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HexoDSP - Comprehensive DSP graph and synthesis library for developing a modular synthesizer in Rust, such as HexoSynth.

This project contains the complete DSP backend of the modular synthesizer HexoSynth.

It's aimed to provide a toolkit for everyone who wants to develop a synthesizer in Rust. You can use it to quickly define a DSP graph that you can change at runtime. It comes with a (growing) collection of already developed DSP modules/nodes, such as oscillators, filters, amplifiers, envelopes and sequencers.

The DSP graph API also provides multiple kinds of feedback to track what the signals in the DSP threads look like. From monitoring the inputs and outputs of single nodes to get the current output value of all nodes.

There is also an (optional) JIT compiler for defining custom pieces of DSP code that runs at native speed in a DSP graph module/node.

Here a short list of features:

  • Runtime changeable DSP graph.
  • Serialization and loading of the DSP graph and the parameters.
  • Full monitoring and feedback introspection into the running DSP graph.
  • Provides a wide variety of modules.
  • (Optional) JIT (Just In Time) compiled custom DSP code for integrating your own DSP algorithms at runtime. One possible frontend language is the visual "BlockCode" programming language in HexoSynth.
  • Extensible framework for quickly adding new nodes to HexoDSP.
  • A comprehensive automated test suite covering all modules in HexoDSP.

And following DSP nodes:

For a comprehensive list checkout the HexoDSP DSP Node Reference.

Category Name Function
IO Util Out Audio output (to DAW or Jack)
Osc Sampl Sample player
Osc Sin Sine oscillator
Osc BOsc Basic bandlimited waveform oscillator (waveforms: Sin, Tri, Saw, Pulse/Square)
Osc VOsc Vector phase shaping oscillator
Osc Noise Noise oscillator
Osc FormFM Formant oscillator based on FM synthesis
Signal Amp Amplifier/Attenuator
Signal SFilter Simple collection of filters, useable for synthesis
Signal FVaFilt Collection of virtual analog filters (Moog, EDP Wasp, Korg MS20)
Signal Delay Single tap signal delay
Signal PVerb Reverb node, based on Dattorros plate reverb algorithm
Signal AllP All-Pass filter based on internal delay line feedback
Signal Comb Comb filter
Signal Code JIT (Just In Time) compiled piece of custom DSP code.
N->M Mix3 3 channel mixer
N->M Mux9 9 channel to 1 output multiplexer/switch
Ctrl SMap Simple control signal mapper
Ctrl Map Control signal mapper
Ctrl CQnt Control signal pitch quantizer
Ctrl Quant Pitch signal quantizer
Mod TSeq Tracker/pattern sequencer
Mod Ad Attack-Decay (AD) envelope
Mod Adsr Attack-Decay-Sustain-Release (ADSR) envelope
Mod TsLFO Tri/Saw waveform low frequency oscillator (LFO)
Mod RndWk Random walker, a Sample & Hold noise generator
IO Util FbWr / FbRd Utility modules for feedback in patches
IO Util Scope Oscilloscope for up to 3 channels
IO Util MidiP MIDI Pitch/Note input from plugin host, DAW or hardware
IO Util MidiCC MIDI CC input from plugin host, DAW or hardware
IO Util ExtA - ExtF Access to plugin parameter sets A to F

API Examples


The development documentation with all private fields and functions can be found separately hosted: HexoDSP API Developer Documentation.

High Level SynthConstructor API

HexoDSP offers a high level API for constructing DSP graphs, which is known as the [crate::SynthConstructor]. In combination with the [crate::build] module it allows you to define DSP graphs that connect DSP nodes in Rust.

HexoDSP offers you a set of readily available DSP nodes. You can check out the reference documentation of the nodes here: HexoDSP DSP Node Reference.

Additionally you can check out the DynamicNode1x1 DSP node, that allows you define your own DSP nodes to plug into a HexoDSP graph.

Here is a short example that shows how this [crate::SynthConstructor] API works:

use hexodsp::*;
use hexodsp::build::*;

let mut sc = SynthConstructor::new();


// Define a sine oscillator at 110Hz. The `0` is used to identify
// the oscillator instance. `sin(1)` would be a second and independent sine oscillator.
// `sin(2)` a third sine oscillator and so on... You may use up to 256 sine oscillators
// if your CPU is fast enough.
let sin = sin(0).set().freq(110.0);

// (Bandlimited) Sawtooth oscillator at 220Hz
let saw = bosc(0).set().wtype(2).set().freq(220.0);

// Setup a mixer node to sum the two oscillators:
let mix = mix3(0).input().ch1(&sin.output().sig());
let mix = mix3(0).input().ch2(&saw.output().sig());

// Turn down the initial output volume of the mixer a bit:
let mix = mix3(0).set().ovol(0.7);

// Plug the mixer into the audio output node:
let out = out(0).input().ch1(&mix.output().sig());

// Upload the DSP graph:

// start some frontend loop here, or some GUI or whatever you like....

// Later at runtime you might want to change the oscillator
// frequency from the frontend:

fn spawn_audio_thread(exec: NodeExecutor) {
    // Some loop here that interfaces with [NodeExecutor::process] and regularily
    // calls [NodeExecutor::process_graph_updates].
    // Please refer to the examples that come with HexoDSP!

Hexagonal Matrix API

This is a short overview of the API provided by the hexagonal Matrix API, which is the primary API used inside HexoSynth.

This only showcases the direct generation of audio samples, without any audio device playing it. For a real time application of this library please refer to the examples that come with this library.

use hexodsp::*;

let (node_conf, mut node_exec) = new_node_engine();
let mut matrix = Matrix::new(node_conf, 3, 3);

let sin = NodeId::Sin(0);
let amp = NodeId::Amp(0);
let out = NodeId::Out(0);, 0, Cell::empty(sin)
                   .out(None, None, sin.out("sig")));, 1, Cell::empty(amp)
                   .input(amp.inp("inp"), None, None)
                   .out(None, None, amp.out("sig")));, 2, Cell::empty(out)
                   .input(out.inp("inp"), None, None));

let gain_p = amp.inp_param("gain").unwrap();
matrix.set_param(gain_p, SAtom::param(0.25));

let (out_l, out_r) = node_exec.test_run(0.11, true, &[]);
// out_l and out_r contain two channels of audio
// samples now.

Simplified Hexagonal Matrix API

There is also a simplified version for easier setup of DSP chains on the hexagonal grid, using the [crate::MatrixCellChain] abstraction:

use hexodsp::*;

let (node_conf, mut node_exec) = new_node_engine();
let mut matrix = Matrix::new(node_conf, 3, 3);
let mut chain = MatrixCellChain::new(CellDir::B);

chain.node_out("sin", "sig")
    .node_io("amp", "inp", "sig")
    .set_atom("gain", SAtom::param(0.25))
    .node_inp("out", "ch1")
    .place(&mut matrix, 0, 0);

let (out_l, out_r) = node_exec.test_run(0.11, true, &[]);
// out_l and out_r contain two channels of audio
// samples now.

State of Development

As of 2022-07-30: The architecture and it's functionality have been mostly feature complete by now. The only part that is still lacking is the collection of modules/nodes, this is the area of current development. Adding lots of nodes.

Make sure to follow Weird Constructors Mastodon account or the releases of this project to be notified of updates.

Running the Jack Example:

To run the example:

    cargo run --release --example jack_demo_node_api

You might need following dependencies (Ubuntu Linux):

    sudo apt install libjack0 libjack-jackd2-dev qjackctl

These might work on Debian too:

    sudo apt install libjack0 libjack-dev

Running the Automated Testsuite:

There exists an automate test suite for the DSP and backend code:

    cargo test

Known Bugs

  • The ones you encounter and create as issues on GitHub.


  • Dimas Leenman (aka Skythedragon) contributed the FormFM node.
  • Frederik Halkjær (aka Fredemus, aka RocketPhysician) contributed the DSP algorithms for the FVaFilt virtual analog filter node.


I currently have a quite precise vision of what I want to achieve and my goal is to make music with this project eventually.

The projects is still young, and I currently don't have that much time to devote for project coordination. So please don't be offended if your issue rots in the GitHub issue tracker, or your pull requests is left dangling around for ages.

If you want to contribute new DSP nodes/modules to HexoDSP/HexoSynth, please look into the guide at the start of the [crate::dsp] module.

I might merge pull requests if I find the time and think that the contributions are in line with my vision.

Please bear in mind, that I can only accept contributions under the License of this project (GPLv3 or later).

Contact the Author

You can reach me via Discord or Mastodon. I'm joined most public Rust Discord servers, especially the "Rust Audio" Discord server. I am also sometimes on, for instance in the #lad channel (nick weirdctr).


This project is licensed under the GNU General Public License Version 3 or later.

Why GPL?

The obivious reason is that this project copied and translated code from many other free software / open source synthesis projects. The sources will show the origin and license of the individual parts.

My Reasons

Picking a license for my code bothered me for a long time. I read many discussions about this topic. Read the license explanations. And discussed this matter with other developers.

First about why I write code for free at all, the reasons are:

  • It's my passion to write computer programs. In my free time I can write the code I want, when I want and the way I want. I can freely allocate my time and freely choose the projects I want to work on.
  • To help a friend or member of my family.
  • To solve a problem I have.
  • To learn something new.

Those are the reasons why I write code for free. Now the reasons why I publish the code, when I could as well keep it to myself:

  • So that it may bring value to users and the free software community.
  • Show my work as an artist.
  • To get into contact with other developers.
  • To exchange knowledge and help other developers.
  • And it's a nice change to put some more polish on my private projects.

Most of those reasons don't yet justify GPL. The main point of the GPL, as far as I understand: The GPL makes sure the software stays free software until eternity. That the end user of the software always stays in control. That the users have the means to adapt the software to new platforms or use cases. Even if the original authors don't maintain the software anymore. It ultimately prevents "vendor lock in". I really dislike vendor lock in, especially as developer. Especially as developer I want and need to stay in control of the computers and software I use.

Another point is, that my work (and the work of any other developer) has a value. If I give away my work without any strings attached, I effectively work for free. This compromises the price I (and potentially other developers) can demand for the skill, workforce and time.

This makes two reasons for me to choose the GPL:

  1. I do not want to support vendor lock in scenarios for free. I want to prevent those when I have a choice, when I invest my private time to bring value to the end users.
  2. I don't want to low ball my own (and other developer's) wage and prices by giving away the work I spent my scarce private time on with no strings attached. I do not want companies to be able to use it in closed source projects to drive a vendor lock in scenario.

We can discuss relicensing of my code or project if you are interested in using it in a closed source project. Bear in mind, that I can only relicense the parts of the project I wrote. If the project contains GPL code from other projects and authors, I can't relicense it.

License: GPL-3.0-or-later