page.title=Building Audio Accessories
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    <h2>In this document</h2>
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<p>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.</p>
<h2 id="audio-over-35mm">Audio over 3.5 mm headset connector</h2>
<p>Many Android-based devices include a 3.5 mm (“mini”) headset connector. In
addition to the traditional stereo output and mono input features, the <a
href="headset-spec.html">Wired audio headset specification</a> defines standard
impedances and functions so a range of Android devices and headsets can inter-operate.</p>

<h2 id="audio-over-usb">Audio over USB</h2>
<p>Android can use USB in several modes:</p>
  <ul>
    <li>development
    <li>accessory
    <li>host
  </ul>
<p>In the development mode, there is no audio capability.</p>
<p>Accessory mode is provided by the Open Accessory (AOA) protocol version 2.0.
There is limited audio capability in accessory mode, as described in <a
href="custom.html#audio-over-usb">Connecting custom audio over USB</a>.</p>
<p>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 <a href="{@docRoot}devices/audio/usb.html">USB Digital Audio</a>

<h2 id="audio-over-bluetooth">Audio over Bluetooth</h2>
<p>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 &gt; Bluetooth and play music directly to the
accessory without the need for a secondary application.</p>
<p class="note"><strong>Note:</strong> 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
<a href="http://developer.android.com/reference/android/bluetooth/BluetoothA2dp.html"><code>BluetoothA2dp</code></a>
class.</p>
<h3 id="next-steps_1">Next steps</h3>
<p>To get started on building an audio accessory that uses a Bluetooth connection:</p>
<ul>
<li>Select a hardware platform or build an hardware device that can support Bluetooth
  communications and the A2DP connection profile.</li>
<li>Review the ADK 2012
  <a href="http://developer.android.com/tools/adk/adk2.html#src-download">firmware source code</a>
  (<code>&lt;adk-src&gt;/adk2012/board/library/ADK2/</code>), which includes an example implementation
  of an audio playback accessory using a Bluetooth connection.</li>
</ul>
<p class="note"><strong>Note:</strong> 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).</p>

<h2 id="midi-over-usb">MIDI over USB and Bluetooth LE</h2>
<p>Both USB and Bluetooth Low Energy can be used as transports for the
<a href="http://en.wikipedia.org/wiki/MIDI">MIDI</a> protocol.
See <a href="{@docRoot}devices/audio/midi.html">MIDI</a> for more information.
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