src/devices/audio.jd

page.title=Audio
@jd:body

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    <h2>In this document</h2>
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<p>
  Android's audio Hardware Abstraction Layer (HAL) connects the higher level, audio-specific
  framework APIs in <a href="http://developer.android.com/reference/android/media/package-summary.html">android.media</a>
  to the underlying audio driver and hardware. 
</p>

<p>
  The following figure and list describe how audio functionality is implemented and the relevant
  source code that is involved in the implementation:
</p>
<p>
  <img src="images/audio_hal.png">
</p>
<dl>
  <dt>
    Application framework
  </dt>
  <dd>
    At the application framework level is the app code, which utilizes the
    <a href="http://developer.android.com/reference/android/media/package-summary.html">android.media</a>
    APIs to interact with the audio hardware. Internally, this code calls corresponding JNI glue
    classes to access the native code that interacts with the audio hardware.
  </dd>
  <dt>
    JNI
  </dt>
  <dd>
    The JNI code associated with <a href="http://developer.android.com/reference/android/media/package-summary.html">android.media</a> is located in the
    <code>frameworks/base/core/jni/</code> and <code>frameworks/base/media/jni</code> directories.
    This code calls the lower level native code to obtain access to the audio hardware.
  </dd>
  <dt>
    Native framework
  </dt>
  <dd>
    The native framework is defined in <code>frameworks/av/media/libmedia</code> and provides a
    native equivalent to the <a href="http://developer.android.com/reference/android/media/package-summary.html">android.media</a> package. The native framework calls the Binder
    IPC proxies to obtain access to audio-specific services of the media server.
  </dd>
  <dt>
    Binder IPC
  </dt>
  <dd>
    The Binder IPC proxies facilitate communication over process boundaries. They are located in
    the <code>frameworks/av/media/libmedia</code> directory and begin with the letter "I".
  </dd>
  <dt>
    Media Server
  </dt>
  <dd>
    The audio services in the media server, located in
    <code>frameworks/av/services/audioflinger</code>, is the actual code that interacts with your
    HAL implementations.
  </dd>
  <dt>
    HAL
  </dt>
  <dd>
    The HAL defines the standard interface that audio services call into
    and that you must implement to have your audio hardware function correctly. The audio HAL
    interfaces are located in
<code>hardware/libhardware/include/hardware</code>. See <a
href="http://source.android.com/devices/reference/audio_8h_source.html">audio.h</a> for additional details.
  </dd>
  <dt>
    Kernel Driver
  </dt>
  <dd>
    The audio driver interacts with the hardware and your implementation of the HAL. You can choose
    to use ALSA, OSS, or a custom driver of your own at this level. The HAL is driver-agnostic.
    <p>
  <strong>Note:</strong> If you do choose ALSA, we recommend using <code>external/tinyalsa</code>
  for the user portion of the driver because of its compatible licensing (The standard user-mode
  library is GPL licensed).
</p>
  </dd>
</dl>

<p>
   See the rest of the pages within the Audio section for implementation
   instructions and ways to improve performance.
</p>