Android Bluetooth Stack: Bluedroid（五岁以下儿童）：The analysis of A2DP Source

1. A2DP Introduction

The Advanced Audio Distribution Profile (A2DP) defines the protocols and procedures that realize distribution of audio content of high-quality in mono or stereo on ACL channels. As indicated in the diagram of 'Protocol Model', A2DP depends on AVDTP and SDP. 

Protocol Model

The AVDTP define procedures required to set up an audio streaming. The A2DP defines parameters and procedures that are specific foraudio streaming, including audio codec, SDP parameters. It's the pre-condition of analyzing A2DP Source implementation to familiar with the two specs: AVDTP and A2DP. The block diagrams of sending/receiving audio streaming and created packet format are shown in the diagram below. In the analysis, I only focus on MP and Media PL. AVDTP spec describes MP, while A2DP spec describes Media PL. This profile support a mandatory codec: SBC, 3 optional codecs: MPEG-1,2 Audio, MPEG-2,4 AAC and ATRAC family, and the vendor specific A2DP codecs, eg. aptX. I will only talk about the mandatory codec: SBC in the article.

Block diagram of Audio Streaming Procedures and the Packet Format

The following roles are defined for devices that implement A2DP:

Source(SRC) – A device is the SRC when it acts as a source of a digital audio stream
that is delivered to the SNK of the piconet, eg. a smartphone which sends audio stream.
Sink (SNK) – A device is the SNK when it acts as a sink of a digital audio stream
delivered from the SRC on the same piconet, eg. a Bluetooth headset which receives audio stream.

In Bluedroid, only the source role is implemented, so I analysis the implementation of A2DP source in Bluedroid.(But both of source and sink are implemented in Bluez)

2. Code Analysis

I will not list very detailed function callings(The best way to understand the code is reading the code by yourself), while just describe the process. So you will not read a lot of the boring source code.

2.1 Files Structure

I list the important files for A2DP from the upper layer to lower layger.

• btif/src/btif_av.c                     Bluedroid AV HAL implementation which implements the interface defined in AOSP/hardware/bt_av.h.
• btif/src/btif_media_task.c    This is the multimedia module for the BTIF system. It contains task implementations AV, HS and HF profiles' audio&video processing.
• btif/co/bta_av_co.c               This is the advanced audio/video call-out function implementation for BTIF.
• bta/av/bta_av_ci.c                This is the implementation for advanced audio/video call-in functions which are called from BTIF.
• bta/av/bta_av_api.c             This is the implementation of the API for the advanced audio/video(AV) subsystem of BTA. This interface is called from btif_av.c.
• bta/av/bta_av_mian.c          This is the main implementation file for BTA advanced audio/video.
• bta/av/bta_av_ssm.c            This is the stream state machine for the BTA advanced audio/video.
• bta/av/bta_av_aact.c            This file contains action functions for advanced audio/video stream.
• bta/av/bta_av_sbc.c             This module contains utility functions for dealing with SBC data frames and codec capabilities.
• stack/a2dp/a2d_api.c           This is the implementation of the API for the Advanced Audio Distribution Profile(A2DP)
• stack/a2dp/a2d_sbc.c          This file contains utility functions to help build and parse SBC codec information element and media payload.
• embdrv/sbc/sbc/encoder     This folder contains the files which implement SBC decoder.

2.2 Transfer Audio data to AVDTP via A2DP

The transferring process is handle with 'btif_media_task' which is A2DP media task for SBC encoder. The task is created when Bluetooth is enabled(see btif_enable_bluetooth_evt() in btif/src/btif_core.c). A timer drives the task to encoder and transfer audio stream. The timer is started in btif_media_task_aa_start_tx() when the 'audio_stream_out'  interface has a PCM data buffer ready to write(see hardware/libhardware/include/hardware/audio.h).  On receiving the timer event, the task handles all ready PCM data buffers. If stream is started, run the SBC encoder on each chunk of PCM samples and build an output packet consisting of one or more encoded SBC frames. 

The diagram is the flowchart of transferring audio data to AVDTP. The blue blocks are in 'btif', the red blocks are in 'bta/av', and the yellow blocks are in 'stack/avdt'.

1. BTIF reads PCM data from audio flinger via Audio HAL.(Step 6)
2. BTIF calls SBC encoder to encode PCM data to SBC frames which are put in a queue.(Step 7 and 8)
3. BTIF notifies BTA that the source data is ready in the queue.(Step 9~13)
4. BTA gets the SBC frames from the queue, then adds SBC Header. Media PL is constructed now.(Step 15~17)
5. BTA writes Media PL to AVDTP.(Step 18)
6. AVDTP adds Media Packet Header.(Step 19)

2.3 Implement Audio HAL for A2DP audio device

audio_stream_out  in Audio HAL defines the abstract interface for the audio output hardware. It provides information about various properties of the audio output hardware driver. We must implement audio_stream_out for A2DP audio device to output the audio from Audio Flinger to Bluetooth stack. The interface is defined in AOSP/hardware/libhardware/audio.h. Bluedroid implements the interface in AOSP/external/bluetooth/bluedroid/audio_a2dp_hw.

socket is used for IPC between Audio HAL and BTIF.  Two sockets are defined in audio_a2dp_hw.h:

#define A2DP_CTRL_PATH "/data/misc/bluedroid/.a2dp_ctrl"
#define A2DP_DATA_PATH "/data/misc/bluedroid/.a2dp_data"

A2DP_CTRL_PATH is a command socket, while A2DP_DATA_PATH is a data socket. BTIF defines two channels which are mapped to the two sockets in UIPC_Open() in udrv/ulinux/uipc.c. The channel IDs are UIPC_CH_ID_AV_CTRL and UIPC_CH_ID_AV_AUDIO. Setp 6 in the flowchart reads the PCM data from Audio HAL to BTIF with the channel ID: UIPC_CH_ID_AV_AUDIO.

3 Summary

Bluedroid in AOSP does not support all the features of A2DP. Source role is implemented, but Sink role is not implemented(Bluez implements both of them). SBC is supported in Bluedroid, while other codecs are not supported. The vendor can not add a new codec plugin easily, we have to modify the code in Bluedroid. And there are many small bugs in Bluedroid. So the improvements are needed for Bluedroid.

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