https://source.android.com/devices/audio/index.html
Audio
Android's audio Hardware Abstraction Layer (HAL) connects the higher-level, audio-specific framework APIs in android.media to the underlying audio driver and hardware. This section includes implementation instructions and tips for improving performance.
Audio Architecture
Android audio architecture defines how audio functionality is implemented and points to the relevant source code involved in the implementation.
Figure 1. Android audio architecture
Application framework
The application framework includes the app code, which uses the
android.media APIs to interact with audio hardware. Internally, this code calls corresponding JNI glue classes to access the native code that interacts with audio hardware.
JNI
The JNI code associated with
android.media calls lower level native code to access audio hardware. JNI is located in
frameworks/base/core/jni/ and
frameworks/base/media/jni.
Native framework
The native framework provides a native equivalent to the
android.media package, calling Binder IPC proxies to access the audio-specific services of the media server. Native framework code is located in
frameworks/av/media/libmedia.
Binder IPC
Binder IPC proxies facilitate communication over process boundaries. Proxies are located in
frameworks/av/media/libmedia and begin with the letter "I".
Media server
The media server contains audio services, which are the actual code that interacts with your HAL implementations. The media server is located in
frameworks/av/services/audioflinger.
HAL
The HAL defines the standard interface that audio services call into and that you must implement for your audio hardware to function correctly. The audio HAL interfaces are located in
hardware/libhardware/include/hardware. For details, see
hardware/audio.h.
Kernel driver
The audio driver interacts with your hardware and HAL implementation. You can use Advanced Linux Sound Architecture (ALSA), Open Sound System (OSS), or a custom driver (HAL is driver-agnostic).
Note: If you use ALSA, we recommend external/tinyalsa for the user portion of the driver because of its compatible licensing (the standard user-mode library is GPL-licensed).
Android native audio based on Open SL ES
(not shown)
This API is exposed as part of
Android NDK and is at the same architecture level as
android.media.
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