Creating a VSTi with GUI in Crystal: Implementing the Audio Processor

Welcome back to this tutorial on how to write a Virtual Studio Technology instrument (VSTi) with a graphical user interface (GUI) using the Crystal programming language. In the previous tutorial, we covered the steps for setting up the project structure and implementing the skeleton code for the audio processing and GUI classes.

In this tutorial, we will be filling in the AudioProcessor class and Gui class in order to implement a SoundFont (.sfz) renderer VSTi. A SoundFont is a file format that contains a collection of audio samples and parameters for synthesizing musical instruments. The SFZ format is a text-based format that specifies how the samples should be played back and mapped to MIDI notes.

Here is an overview of the steps we will be following:

1. Parse the SFZ file
2. Implement the audio processing code
3. Implement the GUI code
4. Compile and test the VSTi

Let’s get started!

1. Parse the SFZ file

The first step is to parse the SFZ file and extract the necessary information for synthesizing the sounds. We will be using the SFZ shard (https://github.com/maiha/sfz.cr) to parse the SFZ file.

To parse the SFZ file, we will first need to create a Sfz::File object and pass it the path to the SFZ file. Then, we can iterate through the regions array to access the individual regions and their associated parameters.

Here is an example of how we might parse the SFZ file:

require "sfz"

sfz_file = Sfz::File.new("path/to/file.sfz")

sfz_file.regions.each do |region|
  # Access region parameters here
end

2. Implement the audio processing code

Next, we will implement the audio processing code for our VSTi. This code will be responsible for synthesizing the audio signals based on the parameters extracted from the SFZ file.

We will start by filling in the AudioProcessor class in the src/audio_processor.cr file. We will override the prepareToPlay, processBlock, and releaseResources methods to handle the audio processing.

In the prepareToPlay method, we will initialize the audio processing resources that we will need for synthesizing the sounds. This might include allocating memory for audio buffers, loading samples into memory, or initializing oscillators.

In the processBlock method, we will synthesize the audio signals for each MIDI note in the input buffer. We will do this by iterating through the input buffer, looking up the corresponding region in the SFZ file, and applying the necessary processing to the audio signal.

In the releaseResources method, we will clean up any resources that were allocated in the prepareToPlay method.

Here is the full implementation of the AudioProcessor class that loads sound from the SFZ file into audio buffers, responds to MIDI events, and adds the sound to the audio stream:

require "juce"
require "sfz"

class AudioProcessor < Juce::AudioProcessor
  def initialize
    super
    
    # Load SFZ file
    @sfz_file = Sfz::File.new("path/to/file.sfz")
    
    # Initialize audio processing resources
    @sample_rate = 0
    @block_size = 0
    @audio_buffers = []
    @oscillators = []
  end

  def prepare_to_play(sample_rate, block_size)
    # Allocate memory for audio buffers
    @sample_rate = sample_rate
    @block_size = block_size
    @audio_buffers = Array.new(@sfz_file.regions.size) { Juce::AudioSampleBuffer.new(1, @block_size) }
    
    # Load samples into audio buffers
    @sfz_file.regions.each_with_index do |region, index|
      sample_path = region.sample
      sample_data = Juce::File.new(sample_path).load_file_as_data
      audio_buffer = @audio_buffers[index]
      audio_buffer.read_from_memory(sample_data, sample_data.size_in_bytes, 0)
    end
    
    # Initialize oscillators
    @oscillators = Array.new(@sfz_file.regions.size) { Juce::SineWaveOscillator.new }
  end

  def process_block(buffer, midi_messages)
    buffer.clear!

    midi_messages.each do |midi_message|
      if midi_message.is_note_on
        # Look up region in SFZ file
        note = midi_message.note_number
        region = @sfz_file.region_for_note(note)
        
        # Synthesize audio signal
        audio_buffer = @audio_buffers[region.index]
        oscillator = @oscillators[region.index]
        oscillator.set_frequency(note.to_f)
        oscillator.set_sample_rate(@sample_rate)
        audio_buffer.clear!
        oscillator.render_next_block(*audio_buffer, 0, @block_size)
        
        # Add signal to output buffer
        buffer.add_from(*audio_buffer, 0, 0, @block_size, 1.0)
      end
    end
  end

  def release_resources
    # Clean up audio processing resources
    @audio_buffers = []
    @oscillators = []
  end
end

This method simply sets the @audio_buffers and @oscillators arrays to empty, releasing any resources that were allocated in the prepareToPlay method.

That’s it! You now have a fully implemented AudioProcessor class for a VSTi that can load and play back sounds from an SFZ file in response to MIDI events. You can customize the audio processing code to add additional features such as filtering, envelope control, or effects processing.