蓝牙A2DP协议规范v1.3.1解读

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"Bluetooth A2DP_v1.3.1 是一份关于蓝牙高级音频分布配置文件（Advanced Audio Distribution Profile）的规格文档，旨在定义蓝牙设备支持高质量音频分发所需的要求。该文档由蓝牙技术联盟（Bluetooth SIG）在2015年7月14日采纳，修订版本为v1.3.1。它详细阐述了用户服务的需求，并规定了音频分布使用模型中蓝牙设备间互操作所需的特性与流程。文档贡献者包括来自不同公司的专业人士，如Broadcom、CSR、Denso、Ericsson和Marvell等公司的工程师。" Bluetooth A2DP 是蓝牙技术的一个关键部分，主要用于无线传输高质量的音频流。以下是该协议的一些核心知识点： 1. **音频质量**：A2DP的目标是提供无损或接近无损的音频体验，这使得它成为音乐爱好者和专业音频应用的理想选择。相比早期的蓝牙音频标准，如Basic Audio Profile (BAP)，A2DP 提供了显著提升的音质。 2. **双向通信**：A2DP 支持点对点的通信模式，允许音频源设备（如手机或电脑）向音频 sink 设备（如耳机或扬声器）发送音频流。 3. **编解码器支持**：A2DP 支持多种编解码器，如SBC（Subband Coding）作为强制性编码器，以及AAC、aptX 和 LDAC 等高级编码器，这些编码器能提供更好的音质和更低的延迟。 4. **流媒体控制**：A2DP 规范定义了如何控制音频流，如播放、暂停、快进、后退和音量控制等。 5. **兼容性与互操作性**：A2DP 旨在确保不同制造商的蓝牙设备能够无缝协作，确保音频传输的稳定性和一致性。 6. **服务质量（QoS）**：为了保证音频流的流畅，A2DP 规范中包含了服务质量机制，以确保数据包传输的及时性和完整性。 7. **电源管理**：考虑到蓝牙设备的电池寿命，A2DP 还考虑了电源效率，允许设备在保持良好音质的同时，有效地管理能耗。 8. **安全性**：虽然A2DP主要关注音频质量，但作为蓝牙的一部分，它也遵循蓝牙的安全协议，例如加密和身份验证，以保护用户的隐私和数据安全。 9. **版本更新**：从v1.3.1的修订可以看出，A2DP 经历了多次改进和优化，以适应不断变化的技术需求和市场环境。蓝牙A2DP v1.3.1 的发布，意味着蓝牙设备间的音频传输得到了进一步的标准化和优化，提升了用户体验。这份文档对于蓝牙设备开发者、音频工程师和相关行业的专业人士来说，是一份重要的参考资料。

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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