A user application requests to open an audio role, which is bound to a stream, which is bound to a zone. This allows the application to only care about the audio role.
For example, a navigation application can request the navigation role, which is bound to the navigation stream defined by the HAL. This stream is bound to the driver zone, which is the closest speaker to the driver.
The high level API allows applications to open roles such as emergency, navigation or multimedia. A role is bound to a stream, which is basically a device URI. When a role is opened, then the policy engine is notified and executes an interrupt on every other opened role with a lower priority. An interrupt is a policy engine function that can change the volume, mute or unmute, change the stream’s state.
This behaviour allows the policy engine to take actions like lowering the radio volume when an application wants to play something on the emergency role.
A stream is basically a device URI that you can open to write audio data. For example, it can be “hw:2,0,1”, which means that you have to use this as an alsa device URI.
Multiple speakers are spread around inside a vehicule, they are named depending on their position, like front-center, front-left, front-right, rear-left, rear-right, etc…
Zones are an abstraction of positional audio. A zone is made of one or more speakers and describes logical audio areas like driver, front, rear, etc.