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VCV Prototype

Scripting language host for VCV Rack containing:

  • 6 inputs
  • 6 outputs
  • 6 knobs
  • 6 lights (RGB LEDs)
  • 6 switches with RGB LEDs

Supported scripting languages:

Discussion thread

Scripting API

This is the reference API for the JavaScript script engine, along with default property values. Other script engines may vary in their syntax (e.g. block.inputs[i][j] vs block.getInput(i, j) vs input(i, j)), but the functionality should be similar.

/** Display message on LED display.

/** Skip this many sample frames before running process().
For CV generators and processors, 256 is reasonable.
For sequencers, 32 is reasonable since process() will be called every 0.7ms with
a 44100kHz sample rate, which will capture 1ms-long triggers.
For audio generators and processors, 1 is recommended, but use `bufferSize` below.
If this is too slow for your purposes, consider writing a C++ plugin, since
native VCV Rack plugins have 10-100x better performance.
config.frameDivider // 32

/** Instead of calling process() every sample frame, hold this many input/output
voltages in a buffer and call process() when it is full.
This decreases CPU usage, since processing buffers is faster than processing one
frame at a time.
The total latency of your script in seconds is
`config.frameDivider * config.bufferSize * block.sampleTime`.
config.bufferSize // 1

/** Called when the next block is ready to be processed.
function process(block) {
	/** Engine sample rate in Hz. Read-only.

	/** Engine sample timestep in seconds. Equal to `1 / sampleRate`. Read-only.
	Note that the actual time between process() calls is
	`block.sampleTime * config.frameDivider`.

	/** The actual size of the input/output buffers.

	/** Voltage of the input port of column `i`. Read-only.
	block.inputs[i][bufferIndex] // 0.0

	/** Voltage of the output port of column `i`. Read/write.
	block.outputs[i][bufferIndex] // 0.0

	/** Value of the knob of column `i`. Between 0 and 1. Read/write.
	block.knobs[i] // 0.0

	/** Pressed state of the switch of column `i`. Read-only.
	block.switches[i] // false

	/** Brightness of the RGB LED of column `i`, between 0 and 1. Read/write.
	block.lights[i][0] // 0.0 (red)
	block.lights[i][1] // 0.0 (green)
	block.lights[i][2] // 0.0 (blue)

	/** Brightness of the switch RGB LED of column `i`. Read/write.
	block.switchLights[i][0] // 0.0 (red)
	block.switchLights[i][1] // 0.0 (green)
	block.switchLights[i][2] // 0.0 (blue)

The Vult API is slightly different than Prototype's scripting API. See examples/template.vult for a reference of the Vult API.

Build dependencies

Set up your build environment like described here, including the dependencies:



pacman -S mingw-w64-x86_64-premake


brew install premake

Ubuntu 16.04+

sudo apt install premake4

Arch Linux

sudo pacman -S premake


Add path to Rack-SDK

export RACK_DIR=/set/path/to/Rack-SDK/

load submodules

git submodule update --init --recursive


make dep

Adding a script engine

  • Add your scripting language library to the build system so it builds with make dep, following the Duktape example in Makefile.
  • Create a MyEngine.cpp file (for example) in src/ with a ScriptEngine subclass defining the virtual methods, using src/DuktapeEngine.cpp as an example.
  • Build and test the plugin.
  • Add a few example scripts and tests to examples/. These will be included in the plugin package for the user.
  • Add your name to the Contributors list below.
  • Send a pull request. Once merged, you will be added as a repo maintainer. Be sure to “watch” this repo to be notified of bug reports and feature requests for your engine.