Bluetooth in Android 4.2 and 4.3 (2): avrcp Evolution

Avrcp (audio/video remote control profile) provides the following functions:

• Use Bluetooth headsets (such as Sony wm600 and sbh50) or the on-board console to control music playing on your mobile phone
• Display the music playing status, song name, artist and other information on the mobile phone on the Bluetooth headset or the car-mounted console
• View music files on your mobile phone on the Bluetooth headset or on-board console to display the playlist

There are several versions of avrcp. With the evolution of the version, features are constantly increased. The following are specific features of each version:

• 1.0-Basic remote control commands (play/pause/stop, etc .)
• 1.3-all of 1.0 plus metadata and media-player state support
  • The status of the music source (playing, stopped, etc .)
  • Metadata Information on the track itself (artist, track name, etc .).
• 1.4-all of 1.0 and 1.3 plus media browsing capabilities for multiple media players
  • Browsing and manipulation of multiple players
  • Browsing of media metadata per Media Player, including a "Now playing" list
  • Basic search capabilities
• 1.5-all of 1.0, 1.3 and 1.4 plus specification corrections and clarifications to absolute volume control, browsing and other features

Currently, mainstream devices support avrcp 1.3, and a small number of devices support avrcp 1.4. However, avrcp 1.4 has many problems and the standards are not clear enough. Therefore, avrcp 1.5 is introduced to fix avrcp 1.4. Avrcp 1.4 should eventually be replaced by avrcp 1.5. Avrcp 1.5 is not added in terms of functionality compared with avrcp 1.4. At present, avrcp is still evolving, such as displaying the album art. For more details, please register and download th.org.

Avrcp defines two types of Role: controller and target. The Bluetooth headset or on-board console is controller, while the mobile phone is target. Before Android 4.3, aosp (Android open source project) only supported avrcp 1.0. Some vendors, such as Sony and third-party Rom, such as cyanogenmod, in Android, support for avrcp 1.3/1.4 is implemented based on bluez. In Android 4.2, after replacing Bluetooth stack with bluedroid, only avrcp is supported.
1.0, causing dissatisfaction among some mobile phone users who upgraded to Android 4.2.

In the latest Android 4.3, bluedroid and Media Player finally provide support for avrcp 1.3. However, it is not clear when avrcp 1.4/1.5 can be supported. To support avrcp 1.4/1.5, you must make improvements to bluedroid and music player. The development difficulty is higher than avrcp 1.3. Especially for browsing, a lot of work is on the music player side. In addition, bluez has supported avrcp 1.4/1.5, but it is still being improved.

New content added to avrcp 1.4 spec is as follows:

• New scenarios
  • Remote control from car audio system

The Controller (CT) is the car audio system and the mobile phone is the target (TG ). the user browsers the available media on the mobile phone via the car interface. the user may then perform actions triggering retrieval of media metadata from the phone, and perform other control operations.

• New feature support
  • Connection establishment for browsing
  • Release connection for browsing
  • Absolute Volume
  • Media Player Selection
    • Supports multiple players
  • Browsing
    • Database aware players
  • Search
  • Now playing
    • Playable folders

• Sections related new features
  • 4.1.6 procedure of avrcp browsing commands
  • 4.4.1 pass through command
  • 6.3.2 avrcp specific browsing commands
  • 6.7.2 registernofitcation
    • Volume changed 6.13.3
  • 6.9 media player selection
    • Setadressedplayer
    • Addressed player changed notification
    • Setbrowsedplayer
    • Available players changed notification
    • Y now playing content changed
  • 6.10 media content navigation
    • Scope: Media Player list, Media Player virtual filesystem, search results, now playing
    • Browsable items
    • UIDs
      • Database unaware/aware players (without/with UID change detection)
      • UIDs changed notification
    • Browsing commands
  • 6.11 search
  • 6.12 item operations
    • Play item by uid
    • Add-to now playing
  • 6.13 volume handling
    • Setabsolutevomume
    • Notify volume change
  • 6.15.2 error handling for breowsing commands
  • 8 service discovery interoperability requirements
  • 9 L2CAP interoperability requirements
    • Retransmission and Flow Control
    • Configuration of the browsing Channel
  • 25 list of defined notification events
  • 28 uid Scheme

Note: The Bluetooth headsets mentioned above refer to Bluetooth headsets with LCD display and remote control.