src/devices/audio/implement.jd

page.title=Audio Implementation
@jd:body

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  <div id="qv">
    <h2>In this document</h2>
    <ol id="auto-toc">
    </ol>
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<p>This page explains how to implement the audio Hardware Abstraction Layer (HAL) and configure the
shared library.</p>

<h2 id="implementing">Implementing the HAL</h2>

<p>The audio HAL is composed of three different interfaces that you must implement:</p>

<ul>
<li><code>hardware/libhardware/include/hardware/audio.h</code> - represents the main functions
of an audio device.</li>
<li><code>hardware/libhardware/include/hardware/audio_policy.h</code> - represents the audio policy
manager, which handles things like audio routing and volume control policies.</li>
<li><code>hardware/libhardware/include/hardware/audio_effect.h</code> - represents effects that can
be applied to audio such as downmixing, echo, or noise suppression.</li>
</ul>

<p>For an example, refer to the implementation for the Galaxy Nexus at
<code>device/samsung/tuna/audio</code>.</p>

<p>In addition to implementing the HAL, you need to create a
<code>device/&lt;company_name&gt;/&lt;device_name&gt;/audio/audio_policy.conf</code> file that
declares the audio devices present on your product. For an example, see the file for the Galaxy
Nexus audio hardware in <code>device/samsung/tuna/audio/audio_policy.conf</code>. Also, see the
audio header files for a reference of the properties that you can define.</p>

<p>In the Android M release and later, the paths are:<br />
<code>system/media/audio/include/system/audio.h</code><br />
<code>system/media/audio/include/system/audio_policy.h</code></p>

<p>In Android 5.1 and earlier, the paths are:<br />
<code>system/core/include/system/audio.h</code><br />
<code>system/core/include/system/audio_policy.h</code></p>

<h3 id="multichannel">Multi-channel support</h3>

<p>If your hardware and driver supports multichannel audio via HDMI, you can output the audio
stream  directly to the audio hardware. This bypasses the AudioFlinger mixer so it doesn't get
downmixed to two channels.</p>

<p>The audio HAL must expose whether an output stream profile supports multichannel audio
capabilities. If the HAL exposes its capabilities, the default policy manager allows multichannel
playback over HDMI.</p>

<p>For more implementation details, see the <code>device/samsung/tuna/audio/audio_hw.c</code> in
the Android 4.1 release.</p>

<p>To specify that your product contains a multichannel audio output, edit the
<code>audio_policy.conf</code> file to describe the multichannel output for your product. The
following is an example from the Galaxy Nexus that shows a "dynamic" channel mask, which means the
audio policy manager queries the actual channel masks supported by the HDMI sink after connection.
You can also specify a static channel mask like <code>AUDIO_CHANNEL_OUT_5POINT1</code>.</p>

<pre>
audio_hw_modules {
  primary {
    outputs {
        ...
        hdmi {
          sampling_rates 44100|48000
          channel_masks dynamic
          formats AUDIO_FORMAT_PCM_16_BIT
          devices AUDIO_DEVICE_OUT_AUX_DIGITAL
          flags AUDIO_OUTPUT_FLAG_DIRECT
        }
        ...
    }
    ...
  }
  ...
}
</pre>

<p>AudioFlinger's mixer downmixes the content to stereo automatically when sent to an audio device
that does not support multichannel audio.</p>

<h3 id="codecs">Media codecs</h3>

<p>Ensure the audio codecs your hardware and drivers support are properly declared for your
product. For details on declaring supported codecs, see <a href="{@docRoot}devices/media.html#expose">Exposing Codecs
to the Framework</a>.</p>

<h2 id="configuring">Configuring the shared library</h2>

<p>You need to package the HAL implementation into a shared library and copy it to the appropriate
location by creating an <code>Android.mk</code> file:</p>

<ol>
<li>Create a <code>device/&lt;company_name&gt;/&lt;device_name&gt;/audio</code> directory to
contain your library's source files.</li>
<li>Create an <code>Android.mk</code> file to build the shared library. Ensure that the Makefile
contains the following line:
<pre>
LOCAL_MODULE := audio.primary.&lt;device_name&gt;
</pre>

<p>Notice your library must be named <code>audio.primary.&lt;device_name&gt;.so</code> so
that Android can correctly load the library. The "<code>primary</code>" portion of this filename
indicates that this shared library is for the primary audio hardware located on the device. The
module names <code>audio.a2dp.&lt;device_name&gt;</code> and
<code>audio.usb.&lt;device_name&gt;</code> are also available for bluetooth and USB audio
interfaces. Here is an example of an <code>Android.mk</code> from the Galaxy Nexus audio hardware:
</p>

<pre>
LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := audio.primary.tuna
LOCAL_MODULE_RELATIVE_PATH := hw
LOCAL_SRC_FILES := audio_hw.c ril_interface.c
LOCAL_C_INCLUDES += \
        external/tinyalsa/include \
        $(call include-path-for, audio-utils) \
        $(call include-path-for, audio-effects)
LOCAL_SHARED_LIBRARIES := liblog libcutils libtinyalsa libaudioutils libdl
LOCAL_MODULE_TAGS := optional

include $(BUILD_SHARED_LIBRARY)
</pre>

</li>

<li>If your product supports low latency audio as specified by the Android CDD, copy the
corresponding XML feature file into your product. For example, in your product's
<code>device/&lt;company_name&gt;/&lt;device_name&gt;/device.mk</code> Makefile:

<pre>
PRODUCT_COPY_FILES := ...

PRODUCT_COPY_FILES += \
frameworks/native/data/etc/android.hardware.audio.low_latency.xml:system/etc/permissions/android.hardware.audio.low_latency.xml \
</pre>

</li>

<li>Copy the <code>audio_policy.conf</code> file that you created earlier to the
<code>system/etc/</code> directory in your product's
<code>device/&lt;company_name&gt;/&lt;device_name&gt;/device.mk</code> Makefile. For example:

<pre>
PRODUCT_COPY_FILES += \
        device/samsung/tuna/audio/audio_policy.conf:system/etc/audio_policy.conf
</pre>

</li>

<li>Declare the shared modules of your audio HAL that are required by your product in the
product's <code>device/&lt;company_name&gt;/&lt;device_name&gt;/device.mk</code> Makefile. For
example, the Galaxy Nexus requires the primary and bluetooth audio HAL modules:

<pre>
PRODUCT_PACKAGES += \
        audio.primary.tuna \
        audio.a2dp.default
</pre>

</li>
</ol>

<h2 id="preprocessing">Audio pre-processing effects</h2>

<p>The Android platform provides audio effects on supported devices in the
<a href="http://developer.android.com/reference/android/media/audiofx/package-summary.html">audiofx
</a> package, which is available for developers to access. For example, on the Nexus 10, the
following pre-processing effects are supported:</p>

<ul>
<li>
<a href="http://developer.android.com/reference/android/media/audiofx/AcousticEchoCanceler.html">
Acoustic Echo Cancellation</a></li>
<li>
<a href="http://developer.android.com/reference/android/media/audiofx/AutomaticGainControl.html">
Automatic Gain Control</a></li>
<li>
<a href="http://developer.android.com/reference/android/media/audiofx/NoiseSuppressor.html">
Noise Suppression</a></li>
</ul>


<p>Pre-processing effects are paired with the use case mode in which the pre-processing is requested
. In Android app development, a use case is referred to as an <code>AudioSource</code>; and app
developers request to use the <code>AudioSource</code> abstraction instead of the actual audio
hardware device. The Android Audio Policy Manager maps an <code>AudioSource</code> to the actual
hardware with <code>AudioPolicyManagerBase::getDeviceForInputSource(int inputSource)</code>. The
following sources are exposed to developers:</p>

<ul>
<li><code>android.media.MediaRecorder.AudioSource.CAMCORDER</code></li>
<li><code>android.media.MediaRecorder.AudioSource.VOICE_COMMUNICATION</code></li>
<li><code>android.media.MediaRecorder.AudioSource.VOICE_CALL</code></li>
<li><code>android.media.MediaRecorder.AudioSource.VOICE_DOWNLINK</code></li>
<li><code>android.media.MediaRecorder.AudioSource.VOICE_UPLINK</code></li>
<li><code>android.media.MediaRecorder.AudioSource.VOICE_RECOGNITION</code></li>
<li><code>android.media.MediaRecorder.AudioSource.MIC</code></li>
<li><code>android.media.MediaRecorder.AudioSource.DEFAULT</code></li> </ul>

<p>The default pre-processing effects applied for each <code>AudioSource</code> are specified in
the <code>/system/etc/audio_effects.conf</code> file. To specify your own default effects for every
<code>AudioSource</code>, create a <code>/system/vendor/etc/audio_effects.conf</code> file and
specify the pre-processing effects to turn on. For an example, see the implementation for the Nexus
10 in <code>device/samsung/manta/audio_effects.conf</code>. AudioEffect instances acquire and
release a session when created and destroyed, enabling the effects (such as the Loudness Enhancer)
to persist throughout the duration of the session. </p>

<p class="warning"><strong>Warning:</strong> For the <code>VOICE_RECOGNITION</code> use case, do
not enable the noise suppression pre-processing effect. It should not be turned on by default when
recording from this audio source, and you should not enable it in your own audio_effects.conf file.
Turning on the effect by default will cause the device to fail the <a
href="{@docRoot}compatibility/index.html"> compatibility requirement</a> regardless of whether this was on by
default due to configuration file , or the audio HAL implementation's default behavior.</p>

<p>The following example enables pre-processing for the VoIP <code>AudioSource</code> and Camcorder
<code>AudioSource</code>. By declaring the <code>AudioSource</code> configuration in this manner,
the framework will automatically request from the audio HAL the use of those effects.</p>

<pre>
pre_processing {
   voice_communication {
       aec {}
       ns {}
   }
   camcorder {
       agc {}
   }
}
</pre>

<h3 id="tuning">Source tuning</h3>

<p>For <code>AudioSource</code> tuning, there are no explicit requirements on audio gain or audio
processing with the exception of voice recognition (<code>VOICE_RECOGNITION</code>).</p>

<p>The requirements for voice recognition are:</p>

<ul>
<li>"flat" frequency response (+/- 3dB) from 100Hz to 4kHz</li>
<li>close-talk config: 90dB SPL reads RMS of 2500 (16bit samples)</li>
<li>level tracks linearly from -18dB to +12dB relative to 90dB SPL</li>
<li>THD &lt; 1% (90dB SPL in 100 to 4000Hz range)</li>
<li>8kHz sampling rate (anti-aliasing)</li>
<li>Effects/pre-processing must be disabled by default</li>
</ul>

<p>Examples of tuning different effects for different sources are:</p>

<ul>
<li>Noise Suppressor
<ul>
<li>Tuned for wind noise suppressor for <code>CAMCORDER</code></li>
<li>Tuned for stationary noise suppressor for <code>VOICE_COMMUNICATION</code></li>
</ul>
</li>
<li>Automatic Gain Control
<ul>
<li>Tuned for close-talk for <code>VOICE_COMMUNICATION</code> and main phone mic</li>
<li>Tuned for far-talk for <code>CAMCORDER</code></li>
</ul>
</li>
</ul>

<h3 id="more">More information</h3>

<p>For more information, see:</p>

<ul>
<li>Android documentation for
<a href="http://developer.android.com/reference/android/media/audiofx/package-summary.html">
audiofx package</a></li>

<li>Android documentation for
<a href="http://developer.android.com/reference/android/media/audiofx/NoiseSuppressor.html">
Noise Suppression audio effect</a></li>

<li><code>device/samsung/manta/audio_effects.conf</code> file for the Nexus 10</li>
</ul>