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.
The following figure and list describe how audio functionality is implemented and the relevant source code that is involved in the implementation:
- Application framework
- At the application framework level is the app code, which utilizes the android.media APIs to interact with the audio hardware. Internally, this code calls corresponding JNI glue classes to access the native code that interacts with the audio hardware.
The JNI code associated with android.media is located in the
frameworks/base/media/jnidirectories. This code calls the lower level native code to obtain access to the audio hardware.
- Native framework
The native framework is defined in
frameworks/av/media/libmediaand provides a native equivalent to the android.media package. The native framework calls the Binder IPC proxies to obtain access to audio-specific services of the media server.
- Binder IPC
The Binder IPC proxies facilitate communication over process boundaries. They are located in
frameworks/av/media/libmediadirectory and begin with the letter "I".
- Media Server
The audio services in the media server, located in
frameworks/av/services/audioflinger, is the actual code that interacts with your HAL implementations.
The HAL defines the standard interface that audio services call into
and that you must implement to have your audio hardware function correctly. The audio HAL
interfaces are located in
hardware/libhardware/include/hardware. See <hardware/audio.h> for additional details.
- Kernel Driver
The audio driver interacts with the hardware and your implementation of the HAL. You can choose
to use ALSA, OSS, or a custom driver of your own at this level. The HAL is driver-agnostic.
Note: If you do choose ALSA, we recommend using
external/tinyalsafor the user portion of the driver because of its compatible licensing (The standard user-mode library is GPL licensed).
See the rest of the pages within the Audio section for implementation instructions and ways to improve performance.