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.

The following figure and list describe how audio functionality is implemented and the relevant source code that is involved in the implementation:

Audio architecture

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.
JNI
The JNI code associated with android.media is located in the frameworks/base/core/jni/ and frameworks/base/media/jni directories. 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/libmedia and 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 the frameworks/av/media/libmedia directory 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.
HAL
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 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/tinyalsa for 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.