Car audio architecture

Phone audio has one output and a volume key. Car audio has a dozen speakers, an external amplifier, chimes that must never be ducked, a rear-seat zone with its own volume, and a navigation prompt that has to be audible over music without stopping it.

The model that handles this is zones, contexts and buses.

The three concepts#

Audio context — what kind of sound is this?#

A semantic category derived from the stream's AudioAttributes: MUSIC, NAVIGATION, VOICE_COMMAND, CALL, ALARM, NOTIFICATION, SYSTEM_SOUND, EMERGENCY, SAFETY, VEHICLE_STATUS.

Context is not a channel. It is a statement of intent, and it is what the routing and ducking rules act on.

Output device (bus) — where does it physically go?#

A bus is an output the audio HAL exposes — bus0_media_out, bus1_nav_out and so on. Behind each is a path through the amplifier to real speakers.

Multiple buses matter because an external amplifier can then mix them independently: navigation can be attenuated relative to music by the amp, in hardware, with no software ducking at all.

Zone — where in the vehicle?#

A set of buses plus its own volume groups and its own focus state. Zone 0 is the main cabin. A rear-seat entertainment system is another zone, where a passenger can play something at their own volume while the driver hears navigation.

The configuration file decides everything#

car_audio_configuration.xml is where the whole topology is declared. Get it wrong and audio either does not play or plays out of the wrong speakers.

<carAudioConfiguration version="3">
  <zones>
    <zone name="primary" isPrimary="true" occupantZoneId="0">
      <volumeGroups>
 
        <group>
          <device address="bus0_media_out">
            <context context="music"/>
            <context context="announcement"/>
          </device>
        </group>
 
        <group>
          <device address="bus1_navigation_out">
            <context context="navigation"/>
          </device>
          <device address="bus2_voice_command_out">
            <context context="voice_command"/>
          </device>
        </group>
 
        <group>
          <device address="bus3_call_out">
            <context context="call"/>
            <context context="call_ring"/>
          </device>
        </group>
 
        <!-- Safety chimes get their own group so they can never be ducked
             along with media. -->
        <group>
          <device address="bus4_alarm_out">
            <context context="alarm"/>
            <context context="emergency"/>
            <context context="safety"/>
          </device>
        </group>
 
      </volumeGroups>
    </zone>
 
    <zone name="rear_seat" occupantZoneId="1">
      <volumeGroups>
        <group>
          <device address="bus100_rear_media_out">
            <context context="music"/>
            <context context="navigation"/>
            <context context="voice_command"/>
            <context context="call"/>
            <context context="alarm"/>
            <context context="notification"/>
            <context context="system_sound"/>
            <context context="emergency"/>
            <context context="safety"/>
            <context context="vehicle_status"/>
            <context context="announcement"/>
          </device>
        </group>
      </volumeGroups>
    </zone>
  </zones>
</carAudioConfiguration>

Volume groups#

A volume group is a set of devices that move together under one control. The driver's volume knob adjusts the group for whatever is currently dominant, not one global level — which is why turning the knob during a navigation prompt changes navigation volume, not music.

val audio = car.getCarManager(Car.AUDIO_SERVICE) as CarAudioManager
 
val zoneId = CarAudioManager.PRIMARY_AUDIO_ZONE
val groupCount = audio.getVolumeGroupCount(zoneId)
 
val max = audio.getGroupMaxVolume(zoneId, groupId)
val current = audio.getGroupVolume(zoneId, groupId)
 
audio.setGroupVolume(zoneId, groupId, current + 1, /* flags = */ 0)

setGroupVolume needs android.car.permission.CAR_CONTROL_AUDIO_VOLUME, which is signature-level. Ordinary apps change their own stream volume; only system apps drive the vehicle's volume model.

Getting your audio into the right context#

Your app controls its context through AudioAttributes. This is the single most important audio decision an automotive app makes.

// Music player
val musicAttrs = AudioAttributes.Builder()
    .setUsage(AudioAttributes.USAGE_MEDIA)
    .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
    .build()
 
// Navigation prompt — routed to the nav bus, ducks media rather than pausing it
val navAttrs = AudioAttributes.Builder()
    .setUsage(AudioAttributes.USAGE_ASSISTANCE_NAVIGATION_GUIDANCE)
    .setContentType(AudioAttributes.CONTENT_TYPE_SPEECH)
    .build()

Declare what the sound is. An app that marks navigation prompts as USAGE_MEDIA will have them routed to the media bus, ducked like music, and possibly silenced by a rear passenger's volume control.

Debugging#

adb shell dumpsys car_service --services CarAudioService
adb shell dumpsys audio_flinger
adb shell dumpsys audio_policy
 
# What devices does the HAL actually expose?
adb shell lshal | grep -i audio

The CarAudioService dump shows zones, volume groups, device addresses and the current focus holders. When audio comes out of the wrong speaker, compare that dump against your configuration XML — the mismatch is almost always there.

Common failures#

That last one is the classic bring-up bug: the configuration references bus0_media_out and the audio HAL calls it bus0_output. Nothing errors; the routing simply never matches.

Next#

Focus and ducking — the policy layer that decides who is audible when.