蓝牙A2DP规格详解：高级音频分布协议的关键要素

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"A2DP Profile 规格"是蓝牙技术的核心组成部分，专门针对高质量音频分布设计，旨在确保蓝牙设备间在音频分发使用场景中的互操作性。这个规范于2015年7月14日通过，版本为1.3.1，由AudioVideoWG小组负责准备，其电子邮件地址为avv-main@bluetooth.org。蓝牙设备采用A2DP Profile时，必须满足高级音频分发所需的特定要求。这些要求是以最终用户服务的形式表达的，目标是定义设备之间交互的共同特性和流程，确保在音频播放、同步和其他音频分发功能方面的一致体验。A2DP主要应用于蓝牙基础速率/增强数据速率 (BR/EDR) 和低功耗 (LE) 两种类型的部署中。对于BR/EDR设备，A2DP提供短距离、持续的无线连接，适合于如音频流媒体这样的应用，其特点是传输速度快，但可能对电池消耗较大。相比之下，LE蓝牙（如蓝牙4.0/4.1/4.2）适合长时间、远距离的连接，适用于物联网(IoT)设备，如智能手表或智能家居设备，这些设备通常需要较低的功耗。双模蓝牙芯片可以同时支持BR/EDR和LE设备，使智能手机和平板电脑等设备能够无缝连接音频耳机和传感器等不同类型的设备。A2DP Profile的实现涉及多个层次，包括物理(PHY)层、链路层、逻辑链路控制和适配协议(L2CAP)、属性协议(ATT)以及安全管理器，它们协同工作以提供安全、高效的音频传输。物理层管理射频通信，BR/EDR和LE分别使用不同的信道配置。链路层则负责数据包的结构、连接管理和数据传输。直接测试模式允许测试者直接控制射频层的数据包传输。主机控制器接口(HCI)作为蓝牙控制器和主机之间的桥梁，提供了标准的通信途径。逻辑链路控制和适配协议(L2CAP)提供协议多路复用等功能，用于数据包的传输和重组。属性协议(ATT)主要用于LE部署，定义设备间的连接后数据交换的客户端/服务器模式，并通过通用属性配置文件(GATT)组织设备性能和服务。GATT将服务组织成有意义的服务类别，支持服务发现、读取、写入和通知等功能，主要用于LE蓝牙设备。安全管理器确保设备间的配对、身份验证和加密，为各种安全等级的应用提供工具。通用访问配置文件(GAP)则与GATT配合，定义设备发现和连接过程中的规则，进一步增强了蓝牙设备的音频分发能力。 A2DP Profile是蓝牙音频分发的核心技术规范，它定义了蓝牙设备在音频传输中的行为和交互标准，确保了不同品牌和类型的蓝牙设备在音质、连接稳定性和功耗等方面的兼容性，推动了蓝牙音频设备市场的繁荣。

BLUETOOTH SPECIFICATION Page 13 of 75

Advanced Audio Distribution Profile (A2DP)

Bluetooth SIG Proprietary

Portable Player (SRC)

Headphones (SNK)

Audio Stream

Portable Recorder (SNK)

Microphone (SRC)

Audio Stream

Figure 2.2: Examples of Configuration

2.3 User Requirements and Scenarios

The following scenario is covered by this profile:

 Setup/control/manipulate a streaming of audio data from the SRC to the SNK(s).

The following restrictions are applied to this profile:

1. The profile does not support a synchronized point-to-multipoint distribution.

2. There exists certain delay between the SRC and the SNK due to radio signal

processing, data buffering, and encode/decode of the stream data. Countering the

effects of such delays depends on implementation.

The following requirements are set in this profile:

1. The audio data rate should be sufficiently smaller than usable bit rate on the

Bluetooth link. This is to allow retransmission schemes to reduce the effects of

packet loss.

2. The profile does not exclude any content protection method.

2.4 Profile Fundamentals

The profile fundamentals are same as defined in the GAVDP in addition to the following

requirement.

BLUETOOTH SPECIFICATION Page 15 of 75

Advanced Audio Distribution Profile (A2DP)

Bluetooth SIG Proprietary

3 Application Layer

This section describes the feature requirements on units complying with the A2DP.

Table 3.1 shows the feature requirements for this profile.

Item

No.

Feature

Support in

SRC

Support in

SNK

Audio Streaming

Table 3.1: Application Layer Features

Table 3.2 maps each feature to the procedures used for that feature, and shows

whether the procedure is optional, mandatory, or conditional. The procedures are

described in the reference section.

Item

No.

Feature

Procedure

Ref.

Support

in SRC

Support

in SNK

Audio Streaming

Send Audio Stream

3.2.1

N/A

Receive Audio Stream

3.2.2

N/A

Table 3.2: Application Layer Feature to Procedure Mapping

3.1 Audio Streaming Set Up

If the AVDTP version of the remote device is unknown, the device shall perform a SDP

query to get the AVDTP version on the remote device. This shall be performed before

the GAVDP_ConnectionEstablishment procedure is performed. This is required since

some commands in the audio streaming setup procedure depend on the AVDTP

version.

When a device wishes to start streaming of audio content, the device firstly needs to set

up a streaming connection. Signaling procedures and typical signaling flows are

illustrated in Section 4.1 and Appendix A of GAVDP [3], respectively. During such set up

procedure, the devices select the most suitable audio streaming parameters. There are

two kinds of services configured; one is an application service capability, and the other

is a transport service capability. (For details, see section 4.4 in AVDTP [4].) This profile

specifies audio-specific parameters necessary for these signaling procedures. An

example of how the session signaling is performed is described in Chapter 14 of

GAVDP [3] and in Chapter 13 of this specification.

The application service capability for A2DP consists of audio codec capability and

content protection capability. Requirements for audio codec interoperability and details

of codec parameters such as mode, sampling frequency, and bit rate are described in

Chapter 4. The content protection capability is described in Appendix A as informative.

The transport service capability is provided by AVDTP in order to manipulate the

streaming packets more intelligently. Appropriate configuration of these services

increases channel throughput. Available services are listed in Section 5.1.2.

3.2 Audio Streaming

Once streaming connection is established and Start Streaming procedure in GAVDP is

executed, both SRC and SNK are in the STREAMING state, in which the SRC (SNK) is

ready to send (receive) audio stream. (See section 4.1 in GAVDP [3].) The SRC uses

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