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144
145<a name="androidAudioSubSystemTitle"></a><h1>Audio Subsystem</h1>
146
147
148
149<a name="toc"/>
150<div style="padding:10px">
151<a href="#androidAudioSubSystemIntroduction">Introduction</a><br/>
152<a href="#androidAudioSubSystemPorting">Porting Android to other Audio Stacks</a><br/><div style="padding-left:40px">
153
154<a href="#androidAudioSubSystemAbstract">AudioHardwareInterface Abstract Class</a><br/>
155<a href="#androidAudioSubSystemAudioHardwareInterfaceModification">Modifying AudioHardwareInterface</a><br/></div></div></font></div>
156
157<a name="androidAudioSubSystemIntroduction"></a><h2>Introduction</h2>
158
159<p>Android provides two native layers that handle audio software:</p>
160<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>
161<li><b>AudioHardwareInterface</b>: the hardware abstraction layer that hides driver-specific audio implementations from the Android platform.</li>
162</ul>
163</p>
164<p><img src="android_audio_architecture.gif"></p>
165
166Solid elements represent Android blocks and dashed elements represent partner-specific proprietary blocks.
167
168
169
170<a name="androidAudioSubSystemPorting"></a><h2>Porting Android to other Audio Stacks</h2>
171
172<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>
173
174
175<a name="androidAudioSubSystemAbstract"></a><h3>AudioHardwareInterface Abstract Class</h3>
176
177<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>
178
179
180<a name="androidAudioSubSystemAudioHardwareInterfaceModification"></a><h3>Modifying AudioHardwareInterface</h3>
181
182<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>
183
184
185<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>
186<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>.)
187<pre box="prettify">
188// load PV library and create PV player
189mLibHandle = dlopen("libpv.so", RTLD_NOW);
190if (!mLibHandle) {
191 LOGE("dlopen failed on libpv.so\n");
192 return UNKNOWN_ERROR;
193}
194createPlayer_f createPlayer = reinterpret_cast<createPlayer_f>(dlsym(mLibHandle, "createPlayer"));
195if (!createPlayer) {
196 LOGE("dlsym failed on createPlayer in libpv.so\n");
197 return UNKNOWN_ERROR;
198}
199</pre>
200<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>
201<pre class="prettify">
202if (property_get("ro.kernel.qemu", value, 0)) {
203 LOGD("Running in emulation - using generic audio driver");
204 hw = new AudioHardwareGeneric();
205}
206else {
207 // Insert calling of dynamic loading of driver here...
208}
209if (hw->initCheck() != NO_ERROR) {
210 LOGW("Using stubbed audio hardware. No sound will be produced.");
211 delete hw;
212 hw = new AudioHardwareStub();
213}
214</pre>
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