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:
- Parse the SFZ file
- Implement the audio processing code
- Implement the GUI code
- Compile and test the VSTi
Let’s get started!
- 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
- 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.