Bluetooth-alsa

Bluetooth audio for Linux

The Linux drivers for audio have gone through some experimental stages. It's worth explaining where each of these experiments fits into the big picture and what we learned.

Early work

Development started with the btsco daemon and the snd-bt-sco kernel module. The daemon opens a data connection to a HSP headset and works in conjunction with the kernel audio driver snd-bt-sco to provide an alsa device to send and receive headset audio. btsco2 takes a slightly different approach and runs in the place of the original btsco daemon.

Work with A2DP started with a basic SBC encoder that used floating-point math to compress the audio stream. Negotiating the connection to the headset and transmitting the stream was tested and realized using a standalone application, a2play. This application does not work well with other apps; audio is typically piped in using standard input rather than working as a normal audio device. There is another application, a2recv, that listens for incoming streaming audio and plays the stream on the computer's built-in speakers.

Some rudimentary experimenting with AVRCP resulted in two different test applications: avrecv listens for an AVRCP connection from a headset and displays received messages on the console. avsnd makes the outbound connection to the specified headset and similarly listens for messages.

Limitations

It became apparent that the audio driver inside the kernel was not the best solution to the problem of making a driver. Audio architectures like the userspace library side of alsa provide a way for audio drivers like bluetooth to be implemented entirely in userspace which is proving to be a better fit.

A2DP similarly needed a userspace alsa plugin in order to make it work with mp3 players and movie players.

An additional complication is the fact that the alsa interface is too simplistic to cover the advanced signaling that bluetooth provides, like ringing a headset and waiting for an answer or responding to a track advance event from an A2DP/AVRCP set.

Alsa plugin progress

A first pass at the A2DP plugin resulted in a plugin that communicates directly with the headset without any intermediate application. This proved to be too simplistic since only one application could use the set. Some audio applications would lock up the headphones by repeatedly opening and closing the audio device because these operations were translated into opening and closing the bluetooth audio link.

The first attempt at SCO alsa plugin fortunately involved an intermediate daemon to mitigate some of these issues.

As a second iteration on A2DP alsa plugins, a daemon ("a2dpd") was designed to control connections to the headphones and to mix audio from multiple applications together.

SBC encoding

Adaptations in the SBC encoder allowed it to run on small computers without a floating point coprocessor. For the first time, our A2DP transmission was scaled down enough to operate on cellphone-class devices. Unfortunately, some quality problems were introduced, such as low volume and occasional overflows when the input volume is high (manifest as a popping sound).

The fixed-point-only SBC encoder was moved into its own project on sourceforge, and unfortunately the continued presence of the (messier) combined fixed-point/floating-point code in the btsco project has been a source for some confusion.