OLS: Audio Streaming over Bluetooth

On July 23 Marcel Holtmann delivered a presentation on the state of Audio Streaming over Bluetooth at the 2008 Linux Symposium in Ottawa. Holtmann's background involves working on improving Linux Bluetooth audio support for laptops and embedded systems such as cell phones.

Marcel expressed frustration with the complexity of the Bluetooth specifications which include approximately 20 protocols and 40 profiles. Profiles include things like mono headsets, in-car usage and high quality stereo headphones. There are protocols for serial device emulation, phone book access, caller ID information, text messaging and multiple options for audio and video.

Bluetooth defines separate protocols for streaming and control, such as skipping tracks, seeking within tracks, and displaying ID3 information. Having these aspects split into different protocols was called "messy" because they are always used together.

Mono headsets are supported by the Synchronous Connection Oriented link (SCO), while the Advanced Audio Distribution Profile (A2DP) is designed for high quality stereo audio. For audio compression Bluetooth defines a royalty-free SubBand-Codec (SBC) to avoid fees for use of common codecs like MP3 and AAC. All A2DP devices must support SBC, but many also support decoding MP3 and AAC as well. Linux's SBC support was initially very poor, but some developers from the Instituto Nokia de Tecnologia in Brazil stepped up to improve encoding and now the the LGPL SBC implementation rivals some of the best commercial implementations.

Early Bluetooth headset support in Linux involved copying all the audio data over sockets from the application to the Bluetooth daemon. The daemon would then copy the data again to the device, causing unnecessary CPU usage and increasing latency. The current design works by setting up channels and connecting external applications directly to the device sockets. Marcel also mentioned investigating a shared memory approach for better performance at the cost of some extra complexity.

Adding support for a Bluetooth audio device is quite different than for standard audio hardware — compressed data must be sent directly to the devices, possibly with ID3 and other information. If the audio being played is in a format that a device does not support it must be decoded and re-encoded first. Bluetooth devices will also appear and disappear while audio is being played.

Marcel on ALSA: "I won't touch it anymore." ALSA's primary failing is that it wasn't designed to support virtual devices. He is also not convinced that the current direction of PulseAudio is suitable for Bluetooth audio, in particular there is no support for changing codecs while audio is being sent to a device. GStreamer, however can support the concept of virtual devices, sending out encoded data and sending ID3 information when required. If a file format is supported by a Bluetooth device, GStreamer can easily be told to send it as-is without re-encoding it. It can also handle the passing off of the encoding and decoding tasks to special hardware, which is commonly required for embedded systems.

Future work includes adding more intelligence to the handling of control signals. When the user presses Pause and there are multiple devices and streams active, which stream should be affected? The current implementation applies the action to all streams, but it may be better to be able to tell which control device is associated with which stream.

There is also ongoing work to support new hardware. Marcel has had some issues with headsets that are very sensitive to timing, but don't provide enough timing information to reliably fix. There have also been some problems supporting "Enhanced" Synchronous Connection-oriented (eSCO) Links due to vendors that are unwilling to cooperate with the developers.

For more information on Bluetooth development see Marcel's OLS Paper [pdf] and BlueZ.org, the site for the official Linux Bluetooth protocol stack.