Tag Archives: gsoc08

1, 2, 3, 4, A2DP Stream

After a few people asking me how to use the A2DP Sink with BlueZ, I’ve decided to write this mini-tutorial with a step-by-step on how to establish a A2DP stream from any device to a BlueZ-enabled host.

Before we start the hands-on, let’s see a little bit of nomenclature. In our use-case we have an stream being transmitted between two devices through a pipeline. Our pipeline is constructed of the remote host, BlueZ and PulseAudio. Each element of this pipeline has a Source (SRC) and a Sink (SNK) interface. The stream is handled between different elements by being sent from one element’s source to another element’s sink. So, the big picture of our pipeline is something like this (with the stream being represented by an arrow):

Remote-SRC ----> SNK-BlueZ-SRC ----> SNK-PulseAudioBT-SRC

With the PulseAudio bluetooth source being available for applications to connect to it and serve themselves with fresh, just-decoded PCM frames of pure stereo audio. In our example, we gonna add another element to the pipeline tho. In order to able to hear the audio we’re receiving, we gonna connect the PulseAudio bluetooth source to a PulseAudio ALSA sink which pushes the audio out through our soundcard speakers.

PulseAudioBT-SRC ----> SNK-PulseAudioALSA ----> SoundCard

So after these lines on theory, let’s get out hands dirty. First of all, you need a recent version of BlueZ and PulseAudio. BlueZ 4.46 and PulseAudio 0.9.16 should be enough, but I strongly recommend getting the newest releases available by the time you read this. Right now the most recent releases are BlueZ 4.58 and PulseAudio 0.9.21.

  1. PulseAudio must be running with module-bluetooth-discover loaded;
    • This module is automatically loaded on startup with the default configuration;
    • If it’s not loaded you can use pactl to load it manually or configure PulseAudio to load it on startup in /etc/pulse/default.pa;
  2. Enable the Source interface on BlueZ:
    • edit /etc/bluetooth/audio.conf and add the following line: Enable=Source ;
    • restart bluetoothd;
  3. Pair both devices;
  4. On the object path representing the remote host, on org.bluez D-Bus service, call the org.bluez.AudioSource.Connect() method;
    • For the connection to succeed you’ll probably have to grant access on the remote host;
    • When the connection is established, a new card should show up on PulseAudio with one source available;
  5. Connect PulseAudio bluetooth source to PulseAudio ALSA sink;
    • load-module module-loopback source=<name> sink=<name>
    • You can use the line above to load the module-loopback, using pactl (on the command line) or inside pacmd shell;
    • To find out the name of the source and the sink, you can use the commands list-sources and list-sinks inside pacmd shell;
    • This step is only needed if you want to send the audio coming through A2DP to your speakers.

It’s important to remember that the current implementation is working but could see some improvements, so expect to hear some glitches if you go through signal variation. Happy bluetooth audio streaming!

Bluez-PulseAudio is now mainstream

With the PulseAudio 0.9.13 being release on Oct 10th, my bluetooth module for PulseAudio has gone upstream! Check out the relese notes.

Com o lançamento do PulseAudio 0.9.13 em 10/Oct, meu módulo bluetooth entrou na linha de desenvolvimento principal! Veja as notas de lançamento.

Con el lanzamiento del PulseAudio 0.9.13 en 10/Oct, mi modulo bluetooth ha entrado en la linea de desarrollo principal! Vea las notas de lanzamiento.

GSoC 2008

This year I’ve participated in the Google Summer of Code program. Despite the fact that wasn’t summer in the southern hemisphere, everything happened as expected. I’ve worked with BlueZ as my mentoring organization, and my project was to add bluetooth support to the PulseAudio sound server. The abstract of my application can be found here.

The implementation was made through 2 modules: module-bluetooth-device and module-bluetooth-discover. The latter connects to BlueZ through D-Bus to find out what devices have already been paired with each adapter present on the system, and the loads one instance of the module-bluetooth-device for each device found (in contrast to linux kernel modules, PulseAudio modules can be loaded more than once at the same time). It also keeps watching for new adapters and devices, so it can load a module to take care of each new device that shows up.

The former, module-bluetooth-device, is the one who actually does the job of creating the bluetooth audio channel in PulseAudio. First, it connects to the BlueZ audio service through one unix socket to obtain the device capabilities. Then it configures the device according to it’s capabilities, setup the SBC encoder (if applicable) and obtain a file descriptor to write audio data to the device. This fd is passed to a PulseAudio I/O thread (which runs with real time priorities if the user has real time privileges properly set), which gets the audio data coming to this sink (sink is the name of an output channel in PulseAudio), encode it (if applicable) and write it on the device fd. Also, the I/O thread has to take care of the clock synchronization between PulseAudio an the device.

I really enjoyed doing this job. Working with an open source community is outstanding! I’ve learned a lot during the program, made good contacts, and the most important: had a lot of fun! I would like to thank a lot my mentor, Luiz Augusto von Dentz, and the PulseAudio maintainer, Lennart Poettering. Without the help of these two guys I wouldn’t have been able to finish this project. And of course I have to thank Google for helping FLOSS develpment throug this program and LH for making the program happen and for being so kind and patient with all the students. LH, you rock!

I’ll continue working on this project, since there still a lot to be done. Time synchronization is not the best and the usability is far from ideal. I have a git repository for this on gitorious, on an branch called bt. Feel free to point bugs and make sugestions. Also, if you really enjoyed the project and want to help more, I accept donations of A2DP, HSP, or HFP bluetooth audio devices, for test and development. All of this was made so far with a borrowed device from my good friend João Eduardo Ferreira Bertacchi (thanks JE!). The oficial release note on the BlueZ website about this project can be found here.