Audio Implementation

This section explains how to implement the audio Hardware Abstraction Layer (HAL), provides details about configuring an audio policy (file formats, code organization, pre-processing effects), and describes how to configure the shared library (creating the Android.mk file).

Audio HAL capabilities

The Android 10 release includes the following new capabilities for audio HAL:

  • AudioSource
    • VOICE_PERFORMANCE
    • ECHO_REFERENCE
  • AudioFormat
    • AAC_LATM (LC, HE V1, and V2)
    • CELT
    • APTX_ADAPTIVE
    • LHDC
    • LHDC_LL
  • AudioChannelMask
    • OUT_HAPTIC_A
    • OUT_HAPTIC_B

Those HAL capabilities should be declared in the device audio_policy_configuration.xml for the framework to use them.

Audio HAL and subsystem requirements

The Android 10 release includes the following requirements for audio HAL and subsystem implementation:

  • If separate input stream profiles are declared in audio_policy_configuration.xml , it must be possible to activate them all simultaneously for any device selection.
  • If capture for sound trigger (capture from hotword DSP buffer) is supported by one input profile, the implementation must support a number of active streams on this profile corresponding to the number of concurrent sessions supported by sound trigger HAL.
  • Concurrency of DSP hotword detection and capture from the AP.
  • Concurrency of voice call TX and capture from the AP.

Implementing audio HAL

The audio HAL is composed of the following interfaces:

  • hardware/libhardware/include/hardware/audio.h. Represents the main functions of an audio device.
  • hardware/libhardware/include/hardware/audio_effect.h. Represents effects that can be applied to audio such as downmixing, echo, or noise suppression.

You must implement all interfaces.

Audio header files

For a reference of the properties you can define, refer to the audio header files:

  • In Android 6.0 and higher, see system/media/audio/include/system/audio.h.
  • In Android 5.1 and lower, see system/core/include/system/audio.h.

For an example, refer to the implementation for the Galaxy Nexus at device/samsung/tuna/audio.

Next steps

In addition to implementing the audio HAL, you must also create an audio policy configuration file that describes your audio topology and package the HAL implementation into a shared library. You can also configure pre-processing effects such as automatic gain control and noise suppression.