Michał Kalbarczyk

Real-Time Audio Processing with Ruby

19 December 2016 - Reading time: 10 minutes

Real-Time Audio Processing with Ruby

When I first read about MRuby, I just want to play with it. The first thing that came to my mind was a VST plugin. I could build a little wrapper, and pass all function to ruby interpreter. I’ll describe how I’ve done this.


First of all, we need to setup few things. Let’s start with MRuby.


We need to clone MRuby repo:

$ git clone https://github.com/mruby/mruby
$ cd mruby

After that, we need to configure our MRuby build. We need to modify build_config.rb. At the end of the file, let’s add our build specification.

MRuby::Build.new('mrubyvst') do |conf|
  toolchain :gcc
  conf.gembox 'default'
  conf.gem :core => 'mruby-eval'
  conf.gem :github => 'iij/mruby-dir'
  conf.gem :github => 'iij/mruby-io'

In this case, we specify that we will use a gcc toolchain. Also, we will include all default gems and few other. In MRuby gems are compiled in. So if you want to change a gem set, you have to recompile mruby build. We will need mruby-eval to allow ruby script load other ruby scripts, mruby-dir to list directory content and mruby-io to access to files etc.

We’re done, we should compile it.

$ rake

At the end of build process we, will see

      Config Name: mrubyvst
 Output Directory: build/mrubyvst
    Included Gems:
             mruby-sprintf - standard Kernel#sprintf method
             mruby-print - standard print/puts/p
             mruby-math - standard Math module
             mruby-time - standard Time class
             mruby-struct - standard Struct class
             mruby-enum-ext - Enumerable module extension
             mruby-string-ext - String class extension
             mruby-numeric-ext - Numeric class extension
             mruby-array-ext - Array class extension
             mruby-hash-ext - Hash class extension
             mruby-range-ext - Range class extension
             mruby-proc-ext - Proc class extension
             mruby-symbol-ext - Symbol class extension
             mruby-random - Random class
             mruby-object-ext - Object class extension
             mruby-objectspace - ObjectSpace class
             mruby-fiber - Fiber class
             mruby-enumerator - Enumerator class
             mruby-enum-lazy - Enumerator::Lazy class
             mruby-toplevel-ext - toplevel object (main) ...
             mruby-compiler - mruby compiler library
             mruby-bin-mirb - mirb command
               - Binaries: mirb
             mruby-bin-mruby - mruby command
               - Binaries: mruby
             mruby-bin-strip - irep dump debug section ...
               - Binaries: mruby-strip
             mruby-kernel-ext - Kernel module extension
             mruby-class-ext - class/module extension
             mruby-eval - standard Kernel#eval method

Great we just built MRuby. Let’s move on.


We want to build the VST plugin. We have to download SDK from Steinberg Website and unpack it somewhere on your computer.


We will also need a DAW that will load our plugin. I’ve tested it with Ableton Live 9. There is a trial version, but also a demo. In demo mode, you can’t save you project, but we don’t need this feature. There are 32-bit and 64-bit builds, we will build 64-bit version, so we will need 64-bit daw.

Building VST

Next thing is a VST plugin itself. We will clone the repository, set it up and launch it within the DAW.

$ git clone https://github.com/fazibear/mrubyvst
$ cd mrubyvst
$ rake init

Take a look at Rakefile, on the top there are constants.

MRUBY_DIR = File.expand_path('../mruby')
VST_SDK_DIR = File.expand_path('../vst-sdk')
SCRIPT_PATH = File.expand_path('./mrubyvst.rb')

We have to change MRUBY_DIR and VST_SDK_DIR to correct place of these libraries on your computer. SCRIPT_PATH points to a file that will load into Ruby VM on initialising, and VST_CLASS is the class name that MRuby will instantiate. Now we can build it!

$ rake

We’ve just build it. Does it work? No. We need to copy or link it to a special directory, so DAW can find it. On Mac VST plugins are in ~/Library/Audio/Plug-Ins/VST/. But wait. There is a script that will do it for you.

$ rake link

Will link your VST build to that directory. You can also unlink it with.

$ rake unlink

Starting a DAW

We are ready to launch DAW. If you don’t know how to use VST plugin here is a little tutorial. Good luck.

This is how VST look like in Ableton Live (in other DAW will look different)


Great! The plugin works, now we will look at implementation details.

Implementation Take a look at mrubyvst.h file.

#include "public.sdk/source/vst2.x/audioeffectx.h"

#include <mutex>

#include "mruby.h"
#include "mruby/compile.h"
#include "mruby/string.h"
#include "mruby/array.h"
#include "mruby/class.h"
#include "mruby/variable.h"

class MRubyVst : public AudioEffectX {

  MRubyVst(audioMasterCallback audioMaster);

  // Processing
  virtual void processReplacing(float** inputs, float** outputs, VstInt32 sampleFrames);

  // Program
  virtual void setProgram(VstInt32 index);
  virtual bool getProgramNameIndexed(VstInt32 category, VstInt32 index, char* text);

  // Parameters
  virtual void setParameter(VstInt32 index, float value);
  virtual float getParameter(VstInt32 index);
  virtual void getParameterLabel(VstInt32 index, char* label);
  virtual void getParameterDisplay(VstInt32 index, char* text);
  virtual void getParameterName(VstInt32 index, char* text);

  // Info
  virtual bool getEffectName(char* name);
  virtual bool getVendorString(char* text);
  virtual bool getProductString(char* text);
  virtual VstInt32 getVendorVersion();

  // Helpers
  void log(const char* txt);

  std::mutex m;
  mrb_state *mrb;
  mrb_value vst_instance;

In VST there are 4 groups of things that we need to implement.

Most important is processing. In this method, we will change an input audio data. But we will cover this method later. The second group is programs. The program is a state of all parameters. And parameters are single variables that sound processing depends on. The last one is info. VST have to return some pieces of information to identify yourself.

Let’s visualize them. On top we see mrubyvst, it’s a name of our plugin. Dropdown with empty.rb is a programs field. There is a list of programs. And on the right, there are 4 parameters with names and values. We need to decide how many programs and parameters we will need before compilation. In our example, there are 10 programs and 4 parameters.

But wait. What about Ruby ? This is C++ header file! That’s correct, we need to create a little wrapper that passes information to and from Ruby VM. Stay calm.


The constructor is a great place to initialise our Ruby land. After that, we need to load Ruby script and instantiate our MRubyVST class. Also, we need to set few constants in that class.

MRubyVst::MRubyVst(audioMasterCallback audioMaster): AudioEffectX(audioMaster, PROGRAMS_COUNT, PARAMETERS_COUNT) {


  mrb = mrb_open();

  FILE *file = fopen(SCRIPT_PATH, "r");
  if (file != NULL) {
    mrb_load_file(mrb, file);

    mrb_value vst_class = mrb_vm_const_get(mrb, mrb_intern_lit(mrb, VST_CLASS));
    mrb_const_set(mrb, vst_class, mrb_intern_lit(mrb, "PROGRAMS_COUNT"), mrb_fixnum_value(PROGRAMS_COUNT));
    mrb_const_set(mrb, vst_class, mrb_intern_lit(mrb, "PARAMETERS_COUNT"), mrb_fixnum_value(PARAMETERS_COUNT));
    mrb_const_set(mrb, vst_class, mrb_intern_lit(mrb, "SAMPLE_RATE"), mrb_float_value(mrb, getSampleRate()));
    mrb_const_set(mrb, vst_class, mrb_intern_lit(mrb, "SCRIPT_PATH"), mrb_str_new_cstr(mrb, (SCRIPT_PATH)));

    vst_instance = mrb_instance_new(mrb, vst_class);


The most important part is line 8. This is where we start our embedded Ruby VM. Next, we are opening a file and if it exists loads into our VM. Now our Ruby script is loaded. Line 14 will get our Ruby class, and allow us to set few constants. After that, we can instantiate it, and close file.

The Ruby Script

class MRubyVST
  attr_reader :vendor, :product, :effect_name, :version

  def initialize
    @vendor = 'Mruby'
    @product = 'MrubyVST'
    @effect_name = 'MRubyEffects'
    @version = 0
    @programs_dir = "#{File.dirname(SCRIPT_PATH)}/programs"

  def programs
    Dir.entries(@programs_dir) - ['.', '..']

  def load_program(path)

  def change_program(index)
    @program = load_program("#{@programs_dir}/#{programs[index]}") if programs[index]

  def program_name(index)
    programs[index] || "-empty-"

  def set_parameter(index, value)
    @program.set_parameter(index, value) if @program

  def parameter_name(index)
    @program.parameter_name(index) if @program

  def parameter_value(index)
    @program.parameter_value(index) if @program

  def parameter_display_value(index)
    @program.parameter_display_value(index) if @program

  def parameter_label(index)
    @program.parameter_label(index) if @program

  def process(data)
    @program.process(data) if @program

  def log(str)
    io = File.open('/tmp/mrubyvst.log', 'a')
    io.write(str + "\n")

This is the Ruby script. All information are here. Vendor, product, version etc. Our plugin will use programs to load various Ruby modules. When user will change a program, we will instantiate the new class and pass most methods to this module. Programs list is just a files from programs directory. Simple submodule looks like this:

class GainVST
  def initialize
    @gain = 1.0

  def set_parameter(index, value)
    @gain = value if index = 0

  def parameter_name(index)
    index == 0 ? 'Gain' : 'empty'

  def parameter_value(index)
    @gain if index == 0

  def parameter_display_value(index)
    @gain.to_s if index == 0

  def parameter_label(index)
    'dB' if index == 0

  def process(data)
    data[0].map!{ |left | left * @gain}
    data[1].map!{ |right| right * @gain }


It’s a simple gain plugin. That will change the volume of input audio, depending on how gain is set. The default is 1. What are these methods do ? We will use only one parameter with index 0 (zero)

  • parameter_name: return a name of parameter with given index
  • parameter_value: return a value of parameter with given index
  • parameter_display_value: return a value of parameter with given index as a string, you can format it as you like
  • parameter_label: return label of our parameter
  • set_parameter: set a value of parameter with given index
  • process: process input data and return output. In this case, will multiply all input values with a gain value.

MRuby Wrapper

Now we need to move back to our wrapper. And implement methods that will pass data to ruby and back. Most methods look very similar, look at one here:

bool MRubyVst::getProgramNameIndexed(VstInt32 category, VstInt32 index, char* text) {
  if(!mrb_nil_p(vst_instance) && mrb_respond_to(mrb, vst_instance, mrb_intern_lit(mrb, "program_name"))){
    mrb_value mrb_name = mrb_funcall(mrb, vst_instance, "program_name", 1, mrb_fixnum_value(index));
    if (!mrb_nil_p(mrb_name)) {
      vst_strncpy(text, RSTRING_PTR(mrb_name), kVstMaxProgNameLen);
      return true;

  return false;

Because MRuby is not thread-safe we need to use Mutex. Lock it when we start using MRuby and unlock at the end. Check if our VST instance is not nil, and if it responds to program_name method. Now we can invoke it if result is not nil we can decode it and copy it so VST can read it. Returning true means that we copied something. It is not very complicated to figure out when and how data are converted. Now take a look at the most important thing. Audio processing.

void MRubyVst::processReplacing(float** inputs, float** outputs, VstInt32 sampleFrames) {
  if(!mrb_nil_p(vst_instance) && mrb_respond_to(mrb, vst_instance, mrb_intern_lit(mrb, "process"))){
    int ai = mrb_gc_arena_save(mrb);

    float* in1  =  inputs[0];
    float* in2  =  inputs[1];
    float* out1 = outputs[0];
    float* out2 = outputs[1];

    mrb_value mrb_inputs = mrb_ary_new(mrb);
    mrb_value mrb_input_1 = mrb_ary_new(mrb);
    mrb_value mrb_input_2 = mrb_ary_new(mrb);

    for (int i=0;i<sampleFrames;i++) {
      mrb_ary_push(mrb, mrb_input_1, mrb_float_value(mrb, (*in1++)));
      mrb_ary_push(mrb, mrb_input_2, mrb_float_value(mrb, (*in2++)));

    mrb_ary_push(mrb, mrb_inputs, mrb_input_1);
    mrb_ary_push(mrb, mrb_inputs, mrb_input_2);

    mrb_value mrb_outputs = mrb_funcall(mrb, vst_instance, "process", 1, mrb_inputs);

    //mrb_value mrb_outputs = mrb_inputs;

    if (!mrb_nil_p(mrb_outputs)) {
      mrb_value mrb_output_1 = mrb_ary_shift(mrb, mrb_outputs);
      mrb_value mrb_output_2 = mrb_ary_shift(mrb, mrb_outputs);

      for (int i=0;i<sampleFrames;i++) {
        (*out1++) = mrb_float(mrb_ary_shift(mrb, mrb_output_1));
        (*out2++) = mrb_float(mrb_ary_shift(mrb, mrb_output_2));
    mrb_gc_arena_restore(mrb, ai);

Few things happening here. We need to instruct Ruby Garbage Collector to remove unused data. This is what line 4 and 36 do. All objects created between these lines can be garbage collected, so we will not have any memory leaks. Next, we need to convert an array of floats into Ruby array of floats. Creating a Ruby arrays for each input and add a converted float to it. Now we can invoke a process method with parameters. The result of that method is converted back to C array of floats and then passed to DAW.

Final Words

Yes, it works. Of course, the simple gain plugin takes a lot more CPU than written in C. But as you know, we need to pack it into Ruby object and extract at the end. Now you can do whatever you want with the audio stream in Ruby. There are few example plugins like pan and stereo enhancer. But also you can create your own. Try to run web server within. Anything. In Ruby, it’s simpler and more fun.


It was hard to debug MRuby behavior in VST. So I’ve made a little script to test it out. If you want to play with MRuby you can check out this repo, and look at this documentation. Maybe you’ll find something interesting there.