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- Audio devices visible to Android for both internal and external streams are defined in audio_policy_configuration.xml ([1] [2])
- configuration is provided for each audio hardware module/driver (represented by Audio HAL module),
- there should be at least Primary Audio HAL that will provide primary output from Android to the vehicle via AUDIO_DEVICE_OUT_BUS and also that will expose primary input devices like FM TUNER or AUX IN,
- if there is other audio output hardware, it is usually represented by other Audio HAL module (e.g. A2DP Audio HAL, USB Audio HAL, EAVB Audio HAL) - it also comes with corresponding input device, if available (e.g. A2DP Out, A2DP In).
- Raw streams:
- Streams have properties. It seems any of these encodings might be used in the HAL interface: https://source.android.com/devices/audio/data_formats
- Not all compressions might be supported in a particular platform, it should be expressed in the audio policy configuration which devices support direct output of offloading of compressed data to HW. PCM is considered as default, especially in Android Automotive.
- By default all Android internal streams are going through Framework Mixer. This can be omitted by:
- specific audio policy configuration where some devices are marked to handle direct output - then audio streams might be routed to that device (either by AudioPolicyManager decision or application preference) - ref. example for HDMI multi-channel
- the app when using AAudio API in EXCLUSIVE mode based on MMAP buffers - Audio HAL implementation must support this.
- Android Automotive is expected to be configured in fixed-volume, i.e. all internal streams have fixed volume. Volume control requests are sent directly to Audio HAL by separate API (setAudioPortConfig) - gain controllers are also defined in the audio policy configuration. Ref: Configure volume . NOTE: there's also dedicated API for controlling fader/balance (AudioControl HAL - extension to Audio HAL) but implementation is hardware specific and it's to the vendor how to utilize this.
- What are referred to as metadata match the Android term "attributes", the two main attributes are :
- Usage : (Communication, Alarm, Notification,...) the list is relatively long
- Content type : (Unknown, Movie, Music, Sonification, Speech) and this one is important for post processing configuration
- Streams have properties. It seems any of these encodings might be used in the HAL interface: https://source.android.com/devices/audio/data_formats
- Input streams to Android
- Audio HAL exposes ALSA input device via abstracted audio device with particular type (e.g. AUDIO_DEVICE_IN_FM_TUNER) and properties defined in the audio policy configuration.
- Can be handled via:
- createAudioPatch/releaseAudioPatch API (no longer recommended since Android10)
- HwAudioSource Player (aimed to replace above-mentioned API)
- AudioRecord/AudioTrack API - kind of loopback, representing external stream via internal stream
- No high-level limitation on the number of streams, limitation might come from particular kernel audio driver implementation or hardware.
- A small presentation about it can be found uploaded here
Analysis of both approaches
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