Building Audio Accessories

In this document

In implementing an audio accessory, such as a headset, headphone amplifier, microphone, DAC/ADC, or dock, you should consider how your accessory will connect with Android devices. In particular, you should decide if your accessory will use wired a 3.5 mm headset connector, Universal Serial Bus (USB), or a Bluetooth connection to stream music or other audio content.

Audio over 3.5 mm headset connector

Many Android-based devices include a 3.5 mm (“mini”) headset connector. In addition to the traditional stereo output and mono input features, the Wired audio headset specification defines standard impedances and functions so a range of Android devices and headsets can inter-operate.

Audio over USB

Android can use USB in several modes:

  • development
  • accessory
  • host

In the development mode, there is no audio capability.

Accessory mode is provided by the Open Accessory (AOA) protocol version 2.0. There is limited audio capability in accessory mode, as described in Connecting custom audio over USB.

Host mode enables the Android device to drive the USB bus and operate with a wide range of USB-based peripherals, including audio interfaces. Host mode audio is described in USB Digital Audio.

Audio over Bluetooth

An accessory that connects with Android over Bluetooth can use an Advanced Audio Distribution Profile (A2DP) connection stream music for playback. Playing audio over a Bluetooth with A2DP is supported on Android 1.5 (API Level 3) and higher. An Android user can connect to an accessory that supports this profile using the system Settings > Bluetooth and play music directly to the accessory without the need for a secondary application.

Note: If you want to provide a custom application for output to your audio accessory, note that the Android 3.0 (API Level 11) allows applications to operate an A2DP connection using the BluetoothA2dp class.

Next steps

To get started on building an audio accessory that uses a Bluetooth connection:

  • Select a hardware platform or build an hardware device that can support Bluetooth communications and the A2DP connection profile.
  • Review the ADK 2012 firmware source code (<adk-src>/adk2012/board/library/ADK2/), which includes an example implementation of an audio playback accessory using a Bluetooth connection.

Note: The ADK 2012 source code includes an open source Bluetooth stack that is built for the Texas Instruments CC2564 chip, but can work with any Bluetooth chip that implements a standard Host/Controller Interface (HCI).

MIDI over USB and Bluetooth LE

Both USB and Bluetooth Low Energy can be used as transports for the MIDI protocol. See MIDI for more information.