Overview

Audio audio subsystem is one of the important subsystems of the Android Framework and one of the most complex subsystems. It is responsible for the audio data streaming and control functions in Android, as well as the management of audio equipment. A complete Android audio system architecture is shown below:

audio frame layering

application layer

Application layer, call the app or settings of audio service, such as: audio and video player, making phone calls, recording/video recording, volume setting, etc.

Framework layer

The Framework layer includes java and c++, and calls each other through the jni mechanism. It is mainly divided into the following three layers:

Framework java

Framework java layer provides java interface called by APP. The code location of this layer is **[frameworks/base/media/java/android/media]**, which mainly includes:

• MediaPlayer.java: Provide an interface called by the playing APP, supporting audio files in various formats;
• MediaRecorder.java: Provide an interface for calling audio recording APP;
• AudioTrack.java: Provide an interface called by the audio playback APP, this can only be used to play audio files in wav format;
• AudioRecorder.java: Responsible for the collection of recording data, belonging to the Android application framework API class;
• AudioManager.java: provides sound control;
• AudioService.java: channel selection;
• AudioSystem: sound effect settings;
  Among them, MediaPlayer and MediaRecorder have similar functions compared to AudioTrack and AudioRecorder. They are powerful and easy to use, but they are highly packaged and not suitable for in-depth development.

jni

The Audio JNI layer is a bridge between the audio application framework and the local framework. The C++ layer code can be called through the JNI layer. It is equivalent to a bridge that makes the java function interface and the c++ function interface one-to-one correspondence. The corresponding directory is **[framewoks/ base/core/jni and frameworks/base/core/jni】**

• android_media_MediaPlayer.cpp
• android_media_AudioRecord.cpp
• android_media_AudioTrack.cpp
• android_media_AudioSystem.cpp

Native Framework

Native Framework is written in C++ language, and is called by the Framework java layer through jni conversion. It is divided into client and server parts, which correspond to the playback, recording interface, sound control, and settings in the Framework java layer.
Client：【frameworks/av/media/libaudioclient】

• AudioRecord.cpp: Responsible for the collection of recording data, belonging to the Android native framework API class
• AudioTrack.cpp: Responsible for the output of playback data, belonging to the Android native framework API class;
• AudioSystem.cpp: Responsible for the comprehensive management of audio affairs, belonging to the Android native framework API class

Server: [Frameworks/av/services/audioflinger] compiled into libaudioflinger library, running in AudioServer system process

• AudioFlinger.cpp
• AudioHwDevice.cpp
• AudioStreamOut.cpp

HAL layer

The HAL layer is the object that AudioFlinger accesses downwards. The manufacturer will implement its own interface layer at this layer, bridge the hardware driver and the upper framework, and the formed file will be compiled into audio.primary., audio.a2dp. and other so library files for audio Module loading.

• HAL interface layer: [hardware/interfaces/audio]
• HAL implementation layer: [hardware/libhardware/modules/audio]

Specific call process