Bluetooth Audio ALSA Backend

Important

This repository contains experimental code testing ideas for enhancements to BlueALSA. The code is not fully tested and not yet ready for submitting upstream. None of the branches are stable, and may be force-pushed at any time.

At present, the proposed enhancements collected together in the main branch are:

1. Support for multiple programs to connect to the same BlueALSA PCM simultaneously, with mixing of the streams for playback and duplication of the streams for capture. Must be enabled at run time with bluealsad --multi-client.

2. Support for additional ALSA PCM software parameters:

   • period_event
   • stop threshold
   • silence_size / silence_threshold

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Warning

The latest source uses new names for some core components:

• The bluealsa daemon is now called bluealsad
• The bluealsa-cli utility is now called bluealsactl

See the wiki guide Migrating from release 4.3.1 or earlier for more information.

About BlueALSA

This project is a rebirth of a direct integration between BlueZ and ALSA. Since BlueZ >= 5, the built-in integration has been removed in favor of 3rd party audio applications. From now on, BlueZ acts as a middleware between an audio application, which implements Bluetooth audio profile, and a Bluetooth audio device.

The current status quo is, that in order to stream audio from/to a Bluetooth device, one has to install a general-purpose audio server such as PipeWire or PulseAudio, or use BlueZ version 4 which is deprecated and unmaintained.

This project created and maintains a product called BlueALSA, with which one can achieve the same Bluetooth audio profile support as with PulseAudio, but with fewer dependencies and at a lower level in the software stack. BlueALSA registers all known Bluetooth audio profiles in BlueZ, so in theory every Bluetooth device (with audio capabilities) can be connected.

BlueALSA is designed specifically for use on small, low-powered, dedicated audio or audio/visual systems where the high-level audio management features of PulseAudio or PipeWire are not required. The target system must be able to function correctly with all its audio applications interfacing directly with ALSA, with only one application at a time using each Bluetooth audio stream. In such systems BlueALSA adds Bluetooth audio support to the existing ALSA sound card support. Note this means that the applications are constrained by the capabilities of the ALSA API, and the higher-level audio processing features of audio servers such as PulseAudio and PipeWire are not available.

BlueALSA consists of the daemon bluealsad, ALSA plug-ins, and a number of utilities. The basic context is shown in this diagram:

flowchart TD
classDef external fill:#eee,stroke:#333,stroke-width:4px,color:black;
classDef bluealsa fill:#bbf,stroke:#333,stroke-width:4px,color:black;

A[Bluetooth Adapter] <--> B((bluetoothd<br/>daemon))
A <--> C((bluealsad daemon))
B <-- dbus --> C
C <-- dbus --> G((bluealsactl))
C <-- dbus --> D((bluealsa-aplay))
D --> E([ALSA libasound])
E --> K[Speakers]
C <-- dbus --> F((bluealsa<br/>ALSA plug-ins))
F <--> H([ALSA libasound])
H <--> I((ALSA<br/>applications))
C <-- dbus --> J(("other D-Bus<br/>clients"))
C <--> L((ALSA MIDI<br/>sequencer))
L <--> M([ALSA libasound])
M <--> N((ALSA MIDI<br/>application))

class A,B,E,H,I,J,K,L,M,N external;
class C,D,F,G bluealsa;

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The heart of BlueALSA is the daemon bluealsad which interfaces with the BlueZ Bluetooth daemon bluetoothd and the local Bluetooth adapter. It handles the profile connection and configuration logic for A2DP, HFP and HSP and presents the resulting audio streams to applications via D-Bus.

BlueALSA includes ALSA plug-ins which hide all the D-Bus specifics and permit applications to use the ALSA PCM and mixer interfaces, so that existing ALSA applications can access Bluetooth audio devices in the same way as they use sound card PCMs and mixers.

In case of BLE MIDI, the daemon creates a simple MIDI port directly in ALSA MIDI sequencer, so that ALSA MIDI application can connect to the remote BLE MIDI device in the same way as it would connect to a local MIDI device.

BlueALSA also includes a number of utility applications. Of particular note are:

• bluealsactl
  an application to allow command-line management of the BlueALSA system.

• bluealsa-aplay
  an application to simplify the task of building a Bluetooth speaker using BlueALSA.

• bluealsa-rfcomm
  a command-line application which provides access to the RFCOMM terminal for HFP/HSP devices.

Installation

Build and install instructions are included in the file INSTALL.md and more detailed guidance is available in the wiki.

Usage

bluealsad daemon

The main component of BlueALSA is a program called bluealsad. By default, this program shall be run as a root during system startup. It will register org.bluealsa service in the D-Bus system bus, which can be used for accessing configured audio devices. In general, BlueALSA acts as a proxy between BlueZ and ALSA.

The bluealsad daemon must be running in order to pair, connect, and use remote Bluetooth audio devices. In order to stream audio to e.g. a Bluetooth headset, firstly one has to connect the device. If you are not familiar with the Bluetooth pairing and connecting procedures on Linux, there is a basic guide in the wiki: Bluetooth pairing and connecting.

For details of command-line options to bluealsad, consult the bluealsad manual page.

ALSA plug-ins

When a Bluetooth audio device is connected one can use the bluealsa virtual PCM device with ALSA applications just like any other PCM device:

aplay -D bluealsa Bourree_in_E_minor.wav

If there is more than one Bluetooth device connected, the target one can be specified as a parameter to the PCM:

aplay -D bluealsa:XX:XX:XX:XX:XX:XX, Bourree_in_E_minor.wav

Please note that this PCM device is based on the ALSA software PCM I/O plug-in - it has no associated sound card, and it will not be available in the ALSA Kernel proc interface.

Setup parameters of the bluealsa PCM device can be set in the local .asoundrc configuration file like this:

cat ~/.asoundrc
defaults.bluealsa.service "org.bluealsa"
defaults.bluealsa.device "XX:XX:XX:XX:XX:XX"
defaults.bluealsa.profile "a2dp"
defaults.bluealsa.delay 10000

BlueALSA also allows to capture audio from the connected Bluetooth device. To do so, one has to use the capture PCM device, e.g.:

arecord -D bluealsa -f s16_le -c 2 -r 48000 capture.wav

In addition to A2DP profile, used for high quality audio, BlueALSA also allows to use phone audio connection via SCO link. One can use either built-in HSP/HFP support, which implements only audio related part of the specification, or use oFono service as a back-end. In order to open SCO audio connection one shall switch to sco profile like follows:

aplay -D bluealsa:DEV=XX:XX:XX:XX:XX:XX,PROFILE=sco Bourree_in_E_minor.wav

In order to control input or output audio level, one can use provided bluealsa control plug-in. This plug-in allows adjusting the volume of the audio stream or simply mute/unmute it, e.g.:

amixer -D bluealsa sset '<control name>' 70%

where the control name is the name of a connected Bluetooth device with a control element suffix, e.g.:

amixer -D bluealsa sset 'Jabra MOVE v2.3.0 A2DP' 50%

For full details of the BlueALSA ALSA PCM device and mixer device consult the BlueALSA plug-ins manual page.

There are also a number of articles on the bluez-alsa project wiki giving more examples of using these plug-ins.

For more advanced ALSA configuration, consult the asoundrc on-line documentation provided by the AlsaProject wiki page.

bluealsa-aplay

It is possible to create Bluetooth-powered speaker using BlueALSA. For this it is required to forward the audio signal from the BlueALSA capture PCM to some other playback PCM (e.g. built-in audio card). In order to simplify this task, BlueALSA includes a program called bluealsa-aplay, which acts as a simple BlueALSA player. Connect your Bluetooth device (e.g. smartphone) and do as follows:

bluealsa-aplay XX:XX:XX:XX:XX:XX

For details of command-line options to bluealsa-aplay, consult the bluealsa-aplay manual page. There are also some articles on the bluez-alsa project wiki giving examples of its use.

The list of available BlueALSA PCMs (provided by connected Bluetooth devices with audio capabilities) can be obtained directly from BlueALSA D-Bus API or using bluealsa-aplay as a convenient wrapper as follows:

bluealsa-aplay -L

Contributing

This project welcomes contributions of code, documentation and testing.

Please see the CONTRIBUTING guide for details.

Bug reports, feature requests, and requests for help

The most commonly encountered errors are discussed in the TROUBLESHOOTING guide. Please check that file to see if there is already a solution for your issue.

If you are unable to find a solution in that document or by reading the manual pages, then please search previous issues (both open and closed), and consult the wiki before raising a new issue. Unfortunately the wiki is not indexed by web search engines, so searching on-line for your issue will not discover the information in there.

If reporting a problem as a new issue, please use the appropriate bluez-alsa GitHub issue reporting template and complete each section of the template as fully as possible.

License

BlueALSA is licensed under the terms of the MIT license. See the LICENSE file for details.

Resources

1. Bluetooth Adopted Specifications
2. Bluetooth LE MIDI Specification
3. Bluetooth Design Guidelines
4. RTP Payload Format for MPEG-4
5. Coding of MPEG-4 Audio
6. ALSA project library reference