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 <a name="androidAudioSubSystemTitle"></a><h1>Audio Subsystem</h1>


 <a name="toc"/>
 <div style="padding:10px">
 <a href="#androidAudioSubSystemIntroduction">Introduction</a><br/>
 <a href="#androidAudioSubSystemPorting">Porting Android to other Audio Stacks</a><br/><div style="padding-left:40px">

 <a href="#androidAudioSubSystemAbstract">AudioHardwareInterface Abstract Class</a><br/>
 <a href="#androidAudioSubSystemAudioHardwareInterfaceModification">Modifying AudioHardwareInterface</a><br/></div></div></font></div>

 <a name="androidAudioSubSystemIntroduction"></a><h2>Introduction</h2>

 <p>Android provides two native layers that handle audio software:</p>
 <p><ul><li><b>Audio Flinger</b>: the audio software implementation that provides the minimum required audio functions (as illustrated in the diagram below).</li>
 <li><b>AudioHardwareInterface</b>: the hardware abstraction layer that hides driver-specific audio implementations from the Android platform.</li>
 </ul>
 </p>
 <p><img src="android_audio_architecture.gif"></p>

 Solid elements represent Android blocks and dashed elements represent partner-specific proprietary blocks.


 <a name="androidAudioSubSystemPorting"></a><h2>Porting Android to other Audio Stacks</h2>

 <p>Porting Android to other audio stacks (OSS, ALSA, proprietary user-space audio libraries, etc.) requires inheriting from and modifying <code>AudioHardwareInterface</code> to support the driver-specific implementation.</p>


 <a name="androidAudioSubSystemAbstract"></a><h3>AudioHardwareInterface Abstract Class</h3>

 <p><code>AudioHardwareInterface</code> (<code>//device/servers/audio/flinger</code>) contains several pure virtual functions that the audio driver class being ported needs to implement.</p>


 <a name="androidAudioSubSystemAudioHardwareInterfaceModification"></a><h3>Modifying AudioHardwareInterface</h3>

 <p>Once the audio driver that inherits <code>AudioHardwareInterface</code> is ready, modify the static function <code>AudioHardwareInterface::create()</code> in order to link/load the driver in Android.</p>


 <p>Assume the manufacturer audio driver inherits from <code>AudioHarddwareInterface</code> and that it is compiled into a native shared library (<code>libaudio.so</code>). In this case, use <code>dlopen</code> to load the library.</p>
 <p>You can find an example of a similar implementation in <code>//device/libs/media/mediaplayer.cpp</code>. (Note that the example below uses <code>libpv.so</code> because this is a real code snippet from <code>mediaplayer.cpp</code>. If your native shared library is called <code>libaudio.so</code>, replace <code>libpv.so</code> with your <code>libaudio.so</code>.)
 <pre box="prettify">
 // load PV library and create PV player
 mLibHandle = dlopen("libpv.so", RTLD_NOW);
 if (!mLibHandle) {
    LOGE("dlopen failed on libpv.so\n");
    return UNKNOWN_ERROR;
 }
 createPlayer_f createPlayer = reinterpret_cast<createPlayer_f>(dlsym(mLibHandle, "createPlayer"));
 if (!createPlayer) {
    LOGE("dlsym failed on createPlayer in libpv.so\n");
    return UNKNOWN_ERROR;
 }
 </pre>
 <p>Load the libraries with a call from <code>AudioHardwareInterface::create()</code>, as illustrated below (full code found in <code>//device/servers/audioflinger</code>).</p>
 <pre class="prettify">
 if (property_get("ro.kernel.qemu", value, 0)) {
    LOGD("Running in emulation - using generic audio driver");
    hw = new AudioHardwareGeneric();
 }
 else {
    // Insert calling of dynamic loading of driver here...
 }
 if (hw->initCheck() != NO_ERROR) {
    LOGW("Using stubbed audio hardware. No sound will be produced.");
    delete hw;
    hw = new AudioHardwareStub();
 }
 </pre>


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	<a name="androidAudioSubSystemTitle"></a><h1>Audio Subsystem</h1>



	<a name="toc"/>
	<div style="padding:10px">
	<a href="#androidAudioSubSystemIntroduction">Introduction</a><br/>
	<a href="#androidAudioSubSystemPorting">Porting Android to other Audio Stacks</a><br/><div style="padding-left:40px">

	<a href="#androidAudioSubSystemAbstract">AudioHardwareInterface Abstract Class</a><br/>
	<a href="#androidAudioSubSystemAudioHardwareInterfaceModification">Modifying AudioHardwareInterface</a><br/></div></div></font></div>

	<a name="androidAudioSubSystemIntroduction"></a><h2>Introduction</h2>

	<p>Android provides two native layers that handle audio software:</p>
	<p><ul><li><b>Audio Flinger</b>: the audio software implementation that provides the minimum required audio functions (as illustrated in the diagram below).</li>
	<li><b>AudioHardwareInterface</b>: the hardware abstraction layer that hides driver-specific audio implementations from the Android platform.</li>
	</ul>
	</p>
	<p><img src="android_audio_architecture.gif"></p>

	Solid elements represent Android blocks and dashed elements represent partner-specific proprietary blocks.



	<a name="androidAudioSubSystemPorting"></a><h2>Porting Android to other Audio Stacks</h2>

	<p>Porting Android to other audio stacks (OSS, ALSA, proprietary user-space audio libraries, etc.) requires inheriting from and modifying <code>AudioHardwareInterface</code> to support the driver-specific implementation.</p>


	<a name="androidAudioSubSystemAbstract"></a><h3>AudioHardwareInterface Abstract Class</h3>

	<p><code>AudioHardwareInterface</code> (<code>//device/servers/audio/flinger</code>) contains several pure virtual functions that the audio driver class being ported needs to implement.</p>


	<a name="androidAudioSubSystemAudioHardwareInterfaceModification"></a><h3>Modifying AudioHardwareInterface</h3>

	<p>Once the audio driver that inherits <code>AudioHardwareInterface</code> is ready, modify the static function <code>AudioHardwareInterface::create()</code> in order to link/load the driver in Android.</p>


	<p>Assume the manufacturer audio driver inherits from <code>AudioHarddwareInterface</code> and that it is compiled into a native shared library (<code>libaudio.so</code>). In this case, use <code>dlopen</code> to load the library.</p>
	<p>You can find an example of a similar implementation in <code>//device/libs/media/mediaplayer.cpp</code>. (Note that the example below uses <code>libpv.so</code> because this is a real code snippet from <code>mediaplayer.cpp</code>. If your native shared library is called <code>libaudio.so</code>, replace <code>libpv.so</code> with your <code>libaudio.so</code>.)
	<pre box="prettify">
	// load PV library and create PV player
	mLibHandle = dlopen("libpv.so", RTLD_NOW);
	if (!mLibHandle) {
	LOGE("dlopen failed on libpv.so\n");
	return UNKNOWN_ERROR;
	}
	createPlayer_f createPlayer = reinterpret_cast<createPlayer_f>(dlsym(mLibHandle, "createPlayer"));
	if (!createPlayer) {
	LOGE("dlsym failed on createPlayer in libpv.so\n");
	return UNKNOWN_ERROR;
	}
	</pre>
	<p>Load the libraries with a call from <code>AudioHardwareInterface::create()</code>, as illustrated below (full code found in <code>//device/servers/audioflinger</code>).</p>
	<pre class="prettify">
	if (property_get("ro.kernel.qemu", value, 0)) {
	LOGD("Running in emulation - using generic audio driver");
	hw = new AudioHardwareGeneric();
	}
	else {
	// Insert calling of dynamic loading of driver here...
	}
	if (hw->initCheck() != NO_ERROR) {
	LOGW("Using stubbed audio hardware. No sound will be produced.");
	delete hw;
	hw = new AudioHardwareStub();
	}
	</pre>


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