| page.title=Audio |
| @jd:body |
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| <div id="qv-wrapper"> |
| <div id="qv"> |
| <h2>In this document</h2> |
| <ol id="auto-toc"> |
| </ol> |
| </div> |
| </div> |
| |
| <p> |
| This page exlains 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>See the implementation for the Galaxy Nexus at <code>device/samsung/tuna/audio</code> for an example.</p> |
| |
| <p>In addition to implementing the HAL, you need to create a |
| <code>device/<company_name>/<device_name>/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 <code>system/core/include/system/audio.h</code> and <code>system/core/include/system/audio_policy.h</code> |
| header files for a reference of the properties that you can define. |
| </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> |
| 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. See |
| <a href="media.html#expose"> Exposing Codecs to the Framework</a> for information on how to do this. |
| </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/<company_name>/<device_name>/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.<device_name> |
| </pre> |
| <p> |
| Notice your library must be named <code>audio_primary.<device_name>.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.<device_name></code> and |
| <code>audio.usb.<device_name></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_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/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/<company_name>/<device_name>/device.mk</code> |
| Makefile: |
| <pre> |
| PRODUCT_COPY_FILES := ... |
| |
| PRODUCT_COPY_FILES += \ |
| frameworks/native/data/etc/android.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/<company_name>/<device_name>/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/<company_name>/<device_name>/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 always 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 that are 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 any pre-processing effects that you need to turn on. For an example, |
| see the implementation for the Nexus 10 in <code>device/samsung/manta/audio_effects.conf</code></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="/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 following are the requirements for voice recognition:</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 < 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>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> |