src/devices/audio/implement-pre-processing.jd

page.title=Configuring Pre-Processing Effects
@jd:body

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    <h2>In this document</h2>
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<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, the Nexus 10
supports the following pre-processing effects:</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>

<h2 id=audiosources>Pairing with AudioSources</h2>
<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 a
given capture path configuration (device, gain, pre processing, etc.) according
to product-specific rules. 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>

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

<h2 id=tuning>Source tuning</h2>

<p><code>AudioSource</code> tuning does not have explicit requirements on audio
gain or audio processing with the exception of voice recognition
(<code>VOICE_RECOGNITION</code>). Requirements for voice recognition include:</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>Near-ultrasound requirements (for testing, see
<a href="{@docRoot}compatibility/cts/near-ultrasound.html">Near Ultrasound
Tests</a>):
<ul>
<li>Support for SUPPORT_PROPERTY_MIC_NEAR_ULTRASOUND as defined in section 7.8.3
of the CDD.</li>
<li>Support one or both of 44100 or 48000 sampling rates with no band-pass or
anti-aliasing filters.</li>
</ul></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>

<h2 id="resources">Resources</h2>

<p>For more information, refer to the following resources:</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>