mb-sound-jackffi

An UNSTABLE (as in it occasionally crashes, hangs, or drops audio) Ruby FFI interface for the JACK Audio Connection Kit. I've only tested this on Linux.

This FFI interface can be used by my mb-sound gem, a companion library to an educational video series I'm making about sound.

The mb-sound gem uses standalone command line tools for playing and recording audio, via popen. This works well enough, but has high and unpredictable latency. Some of the things I want to do with audio and video will require tighter control over latency and synchronization between audio and video.

There was another FFI interface for JACK, but the homepage no longer exists and it hasn't been updated in ages.

There are some base packages you'll probably want first, though they might not be required for your system:

# Debian-/Ubuntu-based Linux (macOS/Arch/CentOS will differ)
sudo apt-get install libffi-dev libjack-jackd2-dev

Then you'll want to install Ruby 2.7.2 or newer (I recommend RVM).

Finally, you can add the Gem (via Git) to your Gemfile:

# your-project/Gemfile
gem 'mb-sound-jackffi', git: 'git@github.com:mike-bourgeous/mb-sound-jackffi.git'

The MB::Sound::JackFFI class represents a connection to a JACK server with a specific client name. Its input and output instance methods will create input or output ports on the JACK client.

The JACKFFI_INPUT_CONNECT and JACKFFI_OUTPUT_CONNECT environment variables will override :physical or nil connection parameters when creating input and output objects. If you are creating multiple inputs and outputs in your code (or both audio and MIDI), pass an Array or a connection string to whichever I/O you don't want to use environment variables.

For more complex connections, you can separate named ports for a single port with commas, and connections for multiple ports with semicolons.

# Connect to the first ports on a named client
JACKFFI_INPUT_CONNECT='zynaddsubfx'
JACKFFI_OUTPUT_CONNECT='system'
bin/passthrough.rb

# Connect one to one (semicolon separates connections for multiple ports)
export JACKFFI_INPUT_CONNECT='zynaddsubfx:out_1;zynaddsubfx:out_2'
export JACKFFI_OUTPUT_CONNECT='system:playback_1;system:playback_2'
bin/passthrough.rb

# Connect many to many (comma separates multiple connections to one port)
export JACKFFI_INPUT_CONNECT='zynaddsubfx:out_1,zynaddsubfx:out_2;zynaddsubfx:out_2'
export JACKFFI_OUTPUT_CONNECT='system:playback_1,system:playback_2;system:playback_5'
bin/passthrough.rb

require 'mb-sound-jackffi' # Or 'mb/sound/jack_ffi'

# Enjoy silence
out = MB::Sound::JackFFI['my app'].output(port_names: ['left', 'right'], connect: :physical)
loop do
  out.write([Numo::SFloat.zeros(out.buffer_size)] * out.channels)
end

Also check out bin/passthrough.rb and bin/invert.rb.

Check out bin/midi_thru.rb and bin/midi_invert.rb.

See bin/meter.rb and bin/passthrough.rb for examples of retrieving and displaying the names of ports connected to JackFFI.

Testing is mostly manual. I might eventually figure out a way to install jackd in a CI environment and run loopback tests. If that happens, you will be able to run the integrated test suite with rspec, but not yet.

Pull requests are welcome.

This project is released under a 2-clause BSD license. See the LICENSE file.