What Is the Audio Architecture in Android?
The Android audio architecture has evolved significantly since the first version of Android was released in 2008. In the early days of Android, basic audio support was provided through the OpenCORE media framework carried over from Linux. This provided basic audio playback and recording capabilities, but lacked higher-level features like audio routing and audio effects (Wikipedia, 2022).
Over time, Google developed its own custom audio architecture for Android, building on top of the low-level Linux audio components. This included the introduction of the Android Audio HAL in Android 2.3 Gingerbread, the Audio Policy Manager in Android 4.1 Jelly Bean, and other components like the native AudioFlinger mixing layer (Android Source, 2022).
With each new version of Android, the audio architecture has expanded with new features like multi-channel audio, USB audio support, audio chaining, and enhancements to audio latency. The goal has been to provide a robust, full-featured audio system capable of meeting the needs of modern mobile applications and use cases.
Audio HAL
The Audio HAL (Hardware Abstraction Layer) is the layer that connects the higher-level, audio-specific framework APIs in android.media to the underlying audio hardware drivers. It allows these higher-level APIs to interact with the audio hardware while abstracting away the lower-level implementation details of the drivers.
The role of the Audio HAL is to provide interfaces that can be called by the higher framework layers to access various audio hardware capabilities. This includes functions like opening and closing audio devices, reading and writing audio data to/from devices, adjusting audio routing between devices, controlling audio volume and mute, querying audio hardware capabilities, etc.
By abstracting this functionality in the HAL, Android is able to provide a consistent API surface in the higher layers like android.media across devices with different underlying audio hardware. The Audio HAL handles interfacing the hardware-specific drivers to those common higher level APIs.
Audio Policy Manager
The Audio Policy Manager is a service that mediates all audio requests in Android [1]. It works with the AudioFlinger to route audio streams to appropriate outputs based on use case priorities and audio capability of each audio hardware module present on the device [2].
Some key responsibilities of the Audio Policy Manager include:
- Receiving audio routing requests from client apps and services
- Resolving any conflicts between multiple simultaneous audio requests
- Determining the most appropriate audio output path based on rules and priorities
- Communicating routing decisions back to the AudioFlinger to connect audio streams to outputs
- Exposing audio configuration options to the system and providing interfaces for changing audio settings
The Audio Policy Manager allows dynamic audio policy changes even while media playback is ongoing. It is a key component that enables advanced audio routing capabilities in Android.
Audio Flinger
AudioFlinger is the heart of the Android audio management system. It is the audio mixing engine in the Android audio architecture and resides in Android System Server, between the Audio Policy Manager and audio drivers.
According to the Android developer blog, AudioFlinger is responsible for several key audio mixing tasks:
- Mixing multiple audio streams from applications into a final mix that is sent to audio hardware
- Applying audio effects like equalization and bass boost
- Converting audio formats
- Resampling audio streams
AudioFlinger contains audio “tracks” that correspond to audio streams from applications. It mixes and applies effects to these tracks before passing the final audio mix to the hardware drivers. This audio mixing stage is a core part of Android’s audio architecture.
Native Audio APIs
Android provides native audio APIs to build high-performance audio applications. The key native audio APIs are:
OpenSL ES
OpenSL ES (Open Sound Library for Embedded Systems) is a lightweight C-based audio API developed primarily for games. It provides features for audio playback, recording, effects like 3D spatialization, and MIDI playback. OpenSL ES avoids audio latency issues by bypassing the Android audio framework and interacting directly with the audio drivers. It is useful for games and applications requiring low latency audio (https://developer.android.com/ndk/guides/audio/opensl).
OpenMAX AL
OpenMAX AL (Open Media Acceleration Library) is another lightweight C-based API that provides features for media playback and capture. It includes audio effects like 3D spatialization, pre-processing features like acoustic echo cancellation, and the ability to access underlying audio drivers. OpenMAX AL is useful for applications that need to apply audio effects or pre-processing (https://developer.android.com/ndk/guides/audio/opensl).
Together, OpenSL ES and OpenMAX AL provide low-level access to audio functionality for high-performance applications. They allow bypassing the Android audio framework to reduce latency and offer advanced audio capabilities.
Java Audio APIs
Android provides several Java APIs for handling audio playback and management in applications (Audio and video overview). Some of the key APIs include:
AudioManager – This class provides access to volume and ringer mode control. It allows adjusting the volume for music, alarms, notifications and system sounds (AudioManager reference). Key methods include setStreamVolume() for setting volume levels and setRingerMode() for changing between silent, vibrate and normal modes.
MediaPlayer – For music and audio playback, the MediaPlayer class provides the ability to control playback of audio/video files and streams (MediaPlayer guide). It supports features like pausing, seeking, setting the volume, querying duration and more. MediaPlayer allows applications to play audio resources from raw files, URIs and other sources.
Together, AudioManager and MediaPlayer provide the fundamental building blocks for audio handling in Android apps written in Java. These APIs allow both audio configuration/management as well as playback capabilities for common audio use cases.
Audio Drivers
The audio drivers are software components that communicate with the audio hardware. They act as intermediaries between the Android audio framework and the actual speakers, microphones, and other hardware components on a device.
The drivers handle low-level audio functionality like:
- Converting audio data between digital and analog formats
- Controlling audio volume and mute settings
- Managing audio routes between different components
- Applying audio effects like equalization and noise cancellation
The drivers expose controls and capabilities of the audio hardware to the higher-level Android audio framework while abstracting away hardware differences between devices. This allows Android to have a consistent audio architecture across devices with different audio hardware components.
Common audio drivers used on Android devices include ALSA (Advanced Linux Sound Architecture), HAL (Hardware Abstraction Layer) audio drivers, and vendor-specific proprietary drivers. The drivers are packaged into firmware images that get loaded onto devices during manufacturing or system updates.
Audio Hardware
Android devices typically contain a range of audio hardware to handle input and output of audio signals. Key components include the microphone, speaker, and headphone jack.
The microphone is used to record audio input and convert sound waves into an electrical signal. Android supports built-in microphones as well as wired headset microphones. The audio signal from the microphone passes through an analog-to-digital converter and into the audio driver for processing.
Speakers convert electrical audio signals back into sound waves so the audio can be heard. Most Android devices have built-in speakers, but wired and wireless headsets can also be used for audio output. The speaker audio signal comes from the audio driver and passes through a digital-to-analog converter on the way to the speaker.
Many Android devices also have a standard 3.5mm headphone jack for connecting wired headphones or headsets. This allows private audio listening and microphone input via a wired connection. The headphone jack connects to the audio driver hardware directly.
By providing this combination of microphone, speaker, and headphone jack, the audio hardware in Android devices is designed to handle both audio input and output signals efficiently in a mobile form factor.
Sources:
https://source.android.com/docs/core/audio
https://source.android.com/docs/core/audio/implement
Audio Enhancements
Android provides various audio enhancements to improve the listening experience, such as noise suppression and equalization. Noise suppression helps remove background noise during calls and recordings. The Equalizer FX app allows adjusting bass, 3D surround, reverb and other effects. There are many equalizer apps like Equalizer, Music Volume EQ, and Bass Booster that let you customize the sound profile.
Adaptive Sound is a feature in Pixel phones that automatically adjusts the equalizer based on the content playing. Users can also manually tweak the sound settings like enabling Dolby Atmos for more spatial audio. Audio modification apps like DFX and FxSound can enhance the clarity, bass, and volume. Overall, Android offers various ways to suppress noise and adjust the frequency levels to create the perfect listening experience.
Conclusion
Android provides a rich set of audio APIs and architecture to enable high quality audio experiences on mobile devices. The key components covered in this article include the Audio HAL, Audio Policy Manager, Audio Flinger, native audio APIs, Java audio APIs, audio drivers, and audio hardware.
Looking ahead, Android audio capabilities will continue to advance with support for lossless and spatial audio, as evidenced by announcements for the Pixel 8 series and Qualcomm’s Snapdragon Sound platform. As mobile processors and audio codecs improve, Android will be able to deliver studio-quality audio, multi-channel surround sound, and more immersive listening experiences.
The audio architecture in Android strikes a balance between performant native code and accessible Java APIs to give developers a flexible audio framework. As audio technology evolves, Android is well-positioned to take advantage of new opportunities while maintaining backwards compatibility. The future is bright for mobile audio as Android pushes the boundaries of what’s possible on a smartphone or tablet.
