Docs: Beginning to split Audio content
Bug: https://b.corp.google.com/issue?id=9391856
Staging location: http://claym.mtv.corp.google.com:8088/devices/audio.html
Change-Id: I37e2bfe7d2de5f02228bf23de65e1374a1117beb
diff --git a/src/devices/audio_implement.jd b/src/devices/audio_implement.jd
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+page.title=Audio
+@jd:body
+
+<!--
+ Copyright 2010 The Android Open Source Project
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+-->
+<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>
+</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>
+
+
+<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>
+<code><li>android.media.MediaRecorder.AudioSource.CAMCORDER</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.VOICE_COMMUNICATION</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.VOICE_CALL</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.VOICE_DOWNLINK</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.VOICE_UPLINK</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.VOICE_RECOGNITION</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.MIC</li></code>
+<code><li>android.media.MediaRecorder.AudioSource.DEFAULT</li></code>
+</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>