docs: Created content describing high-performance audio.

    path: /ndk/guides/audio/basics.html

  - title: OpenSL ES for Android

    path: /ndk/guides/audio/opensl-for-android.html

  - title: Audio Input Latency

    path: /ndk/guides/audio/input-latency.html

  - title: Audio Output Latency

    path: /ndk/guides/audio/output-latency.html

  - title: Floating-Point Audio

    path: /ndk/guides/audio/floating-point.html

  - title: Sample Rates

    path: /ndk/guides/audio/sample-rates.html

  - title: OpenSL ES Programming Notes

    path: /ndk/guides/audio/opensl-prog-notes.html

- title: Graphics

  path: /ndk/guides/graphics/index.html

page.title=OpenSL ES™ Basics

page.title=High-Performance Audio Basics

@jd:body

<div id="qv-wrapper">

      <h2>On this page</h2>

      <ol>

        <li><a href="#overview">Building Great Audio Apps</a></li>

        <li><a href="#adding">Adding OpenSL ES to Your App</a></li>

        <li><a href="#building">Building and Debugging</a></li>

        <li><a href="#power">Audio Power Consumption</a></li>

        <li><a href="#samples">Samples</a></li>

      </ol>

    </div>

  </div>

<a href="https://www.youtube.com/watch?v=d3kfEeMZ65c" class="notice-developers-video">

<div>

    <h3>Video</h3>

    <p>Google I/O 2013 - High Performance Audio</p>

</div>

</a>

<p>

The Khronos Group's OpenSL ES standard exposes audio features

The Khronos Group's OpenSL ES™ standard exposes audio features

similar to those in the {@link android.media.MediaPlayer} and {@link android.media.MediaRecorder}

APIs in the Android Java framework. OpenSL ES provides a C language interface as well as

C++ bindings, allowing you to call it from code written in either language.

</p>

<p>

This page describes how to add these audio APIs into your app's source code, and how to incorporate

them into the build process.

This page describes the typical use cases for these high-performance audio APIs, how to add them

into your app's source code, and how to incorporate them into the build process.

</p>

<h2 id="overview">Building Great Audio Apps</h2>

<p>

The OpenSL ES APIs are available to help you develop and improve your app's audio performance.

 Some typical use cases include the following:</p>

<ul>

  <li>Digital Audio Workstations (DAWs).</li>

  <li>Synthesizers.</li>

  <li>Drum machines.</li>

  <li>Music learning apps.</li>

  <li>Karaoke apps.</li>

  <li>DJ mixing.</li>

  <li>Audio effects.</li>

  <li>Video/audio conferencing.</li>

</ul>

<h2 id="adding">Adding OpenSL ES to your App</h2>

<p>

#include <SLES/OpenSLES_Android.h>

</pre>

<p>

When you include the {@code OpenSLES_Android.h} header file, the following headers are included

automatically:

</p>

<pre>

#include <SLES/OpenSLES_AndroidConfiguration.h>

#include <SLES/OpenSLES_AndroidMetadata.h>

</pre>

<p class="note"><strong>Note: </strong>

These headers are not required, but are shown as an aid in learning the API.

</p>

<h2 id="building">Building and Debugging</h2>

</p>

<p>

We use asserts in our <a href="https://github.com/googlesamples/android-ndk">examples</a>, because

they help catch unrealistic conditions that would indicate a coding error. We have used explicit

error handling for other conditions more likely to occur in production.

We use asserts in our <a class="external-link" href="https://github.com/googlesamples/android-ndk">

examples</a>, because they help catch unrealistic conditions that would indicate a coding error. We

have used explicit error handling for other conditions more likely to occur in production.

</p>

<p>

</pre>

<p>

To examine the log from Android Studio, either click the <em>Logcat</em> tab in the

<a href="{@docRoot}tools/debugging/debugging-studio.html#runDebug"><em>Debug</em></a>

window, or click the <em>Devices | logcat</em> tab in the

<a href="{@docRoot}tools/debugging/debugging-studio.html#systemLogView"><em>Android DDMS</em></a>

To examine the log from Android Studio, either click the <strong>Logcat</strong> tab in the

<a href="{@docRoot}tools/debugging/debugging-studio.html#runDebug">Debug</a>

window, or click the <strong>Devices | logcat</strong> tab in the

<a href="{@docRoot}tools/debugging/debugging-studio.html#systemLogView">Android DDMS</a>

window.

</p>

<h2 id="power">Audio Power Consumption</h2>

<p>Constantly outputting audio incurs significant power consumption. Ensure that you stop the

 output in the

 <a href="{@docRoot}reference/android/app/Activity.html#onPause()">onPause()</a> method.

 Also consider pausing the silent output after some period of user inactivity.

</p>

<h2 id="samples">Samples</h2>

<p>

Supported and tested example code that you can use as a model for your own code resides both locally

and on GitHub. The local examples are located in

and on

<a class="external-link" href="https://github.com/googlesamples/android-audio-high-performance/">

GitHub</a>. The local examples are located in

{@code platforms/android-9/samples/native-audio/}, under your NDK root installation directory.

On GitHub, they are available from the

<a class="external-link" href="https://github.com/googlesamples/android-ndk">{@code android-ndk}</a>

For more information on differences between the reference specification and the

Android implementation, see

<a href="{@docRoot}ndk/guides/audio/opensl-for-android.html">

OpenSL ES™ for Android</a>.

OpenSL ES for Android</a>.

page.title=Floating-Point Audio

@jd:body

<div id="qv-wrapper">

    <div id="qv">

      <h2>On this page</h2>

      <ol>

        <li><a href="#best">Best Practices for Floating-Point Audio</a></li>

        <li><a href="#support">Floating-Point Audio in Android SDK</a></li>

        <li><a href="#more">For More Information</a></li>

      </ol>

    </div>

  </div>

<a href="https://www.youtube.com/watch?v=sIcieUqMml8" class="notice-developers-video">

<div>

    <h3>Video</h3>

    <p>Will it Float? The Glory and Shame of Floating-Point Audio</p>

</div>

</a>

<p>Using floating-point numbers to represent audio data can significantly enhance audio

 quality in high-performance audio applications. Floating point offers the following

 advantages:</p>

<ul>

<li>Wider dynamic range.</li>

<li>Consistent accuracy across the dynamic range.</li>

<li>More headroom to avoid clipping during intermediate calculations and transients.</li>

</ul>

<p>While floating-point can enhance audio quality, it does present certain disadvantages:</p>

<ul>

<li>Floating-point numbers use more memory.</li>

<li>Floating-point operations employ unexpected properties, for example, addition is

 not associative.</li>

<li>Floating-point calculations can sometimes lose arithmetic precision due to rounding or

 numerically unstable algorithms.</li>

<li>Using floating-point effectively requires greater understanding to achieve accurate

 and reproducible results.</li>

</ul>

<p>

  Formerly, floating-point was notorious for being unavailable or slow. This is

  still true for low-end and embedded processors. But processors on modern

  mobile devices now have hardware floating-point with performance that is

  similar (or in some cases even faster) than integer. Modern CPUs also support

  <a href="http://en.wikipedia.org/wiki/SIMD" class="external-link">SIMD</a>

  (Single instruction, multiple data), which can improve performance further.

</p>

<h2 id="best">Best Practices for Floating-Point Audio</h2>

<p>The following best practices help you avoid problems with floating-point calculations:</p>

<ul>

<li>Use double precision floating-point for infrequent calculations,

such as computing filter coefficients.</li>

<li>Pay attention to the order of operations.</li>

<li>Declare explicit variables for intermediate values.</li>

<li>Use parentheses liberally.</li>

<li>If you get a NaN or infinity result, use binary search to discover

where it was introduced.</li>

</ul>

<h2 id="support">Floating-Point Audio in Android SDK</h2>

<p>For floating-point audio, the audio format encoding

 <code>AudioFormat.ENCODING_PCM_FLOAT</code> is used similarly to

 <code>ENCODING_PCM_16_BIT</code> or <code>ENCODING_PCM_8_BIT</code> for specifying

 AudioTrack data

formats. The corresponding overloaded method <code>AudioTrack.write()</code>

 takes in a float array to deliver data.</p>

<pre>

   public int write(float[] audioData,

        int offsetInFloats,

        int sizeInFloats,

        int writeMode)

</pre>

<h2 id="more">For More Information</h2>

<p>The following Wikipedia pages are helpful in understanding floating-point audio:</p>

<ul>

<li><a href="http://en.wikipedia.org/wiki/Audio_bit_depth" class="external-link" >Audio bit depth</a></li>

<li><a href="http://en.wikipedia.org/wiki/Floating_point" class="external-link" >Floating point</a></li>

<li><a href="http://en.wikipedia.org/wiki/IEEE_floating_point" class="external-link" >IEEE 754 floating-point</a></li>

<li><a href="http://en.wikipedia.org/wiki/Loss_of_significance" class="external-link" >Loss of significance</a>

 (catastrophic cancellation)</li>

<li><a href="https://en.wikipedia.org/wiki/Numerical_stability" class="external-link" >Numerical stability</a></li>

</ul>

<p>The following article provides information on those aspects of floating-point that have a

 direct impact on designers of computer systems:</p>

<ul>

<li><a href="http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html" class="external-link" >What every

 computer scientist should know about floating-point arithmetic</a>

by David Goldberg, Xerox PARC (edited reprint).</li>

</ul>

page.title=NDK Audio: OpenSL ES&#8482

page.title=NDK High-Performance Audio

@jd:body

<p>The NDK package includes an Android-specific implementation of the

<a href="https://www.khronos.org/opensles/">OpenSL ES</a> API

specification from the <a href="https://www.khronos.org">Khronos Group</a>. This library

allows you to use C or C++ to implement high-performance, low-latency audio in your game or other

demanding app.</p>

<a class="external-link" href="https://www.khronos.org/opensles/">OpenSL ES™</a> API

specification from the <a class="external-link" href="https://www.khronos.org">Khronos Group</a>.

This library allows you to use C or C++ to implement high-performance, low-latency audio, whether

you are writing a synthesizer, digital audio workstation, karaoke, game,

 or other real-time app.</p>

<p>This section begins by providing some

<a href="{@docRoot}ndk/guides/audio/basics.html">basic information</a> about the API, including how

to incorporate it into your app. It then explains what you need to know about the

<a href="{@docRoot}ndk/guides/audio/opensl-for-android.html">Android-specific implementation</a>

of OpenSL ES, focusing on differences between this implementation and the reference specification.

<a href="{@docRoot}ndk/guides/audio/basics.html">basic information</a> about the API, including

typical use cases and how to incorporate it into your app. It then explains what you need to know

about the <a href="{@docRoot}ndk/guides/audio/opensl-for-android.html">Android-specific

implementation</a> of OpenSL ES, focusing on the differences between this implementation and the

reference specification. Next, you'll learn how to minimze

 <a href="{@docRoot}ndk/guides/audio/input-latency.html">input latency</a>

 when using built-in or external microphones

and some actions that you can take to minimize

 <a href="{@docRoot}ndk/guides/audio/output-latency.html">output latency</a>.

 It describes the reasons that you should use

 <a href="{@docRoot}ndk/guides/audio/floating-point.html">floating-point</a>

 numbers to represent your audio data, and it provides information that will help you choose the

optimal <a href="{@docRoot}ndk/guides/audio/sample-rates.html">sample rate</a>. This section

 concludes with some supplemental <a href="{@docRoot}ndk/guides/audio/opensl-prog-notes.html">

 programming notes</a> to ensure proper implementation of OpenSL ES.

 </p>

page.title=Audio Input Latency

@jd:body

<div id="qv-wrapper">

    <div id="qv">

      <h2>On this page</h2>

      <ol>

        <li><a href="#check-list">Checklist</a></li>

        <li><a href="#ways">Ways to Reduce Audio Input Latency</a></li>

        <li><a href="#avoid">What to Avoid</a></li>

      </ol>

    </div>

  </div>

<p>This page provides guidelines to help you reduce audio input latency when recording with a

built-in microphone or an external headset microphone.</p>

<h2 id="check-list">Checklist</h2>

<p>Here are a few important prerequisites:</p>

<ul>

  <li>You must use the Android-specific implementation of the

  <a class="external-link" href="https://www.khronos.org/opensles/">OpenSL ES™</a> API.

  <li>If you haven't already done so, download and install the

  <a href="{@docRoot}tools/sdk/ndk/index.html">Android NDK</a>.</li>

  <li>Many of the same requirements for low-latency audio output also apply to low-latency input,

  so read the requirements for low-latency output in

  <a href="{@docRoot}ndk/guides/audio/output-latency.html">Audio Output Latency</a>.</li>

</ul>

<h2 id="ways">Ways to Reduce Audio Input Latency</h2>

<p>The following are some methods to help ensure low audio input latency:

<ul>

  <li>Suggest to your users, if your app relies on low-latency audio, that they use a headset

(for example, by displaying a <em>Best with headphones</em> screen on first run). Note

that just using the headset doesn’t guarantee the lowest possible latency. You may need to

perform other steps to remove any unwanted signal processing from the audio path, such as by

using the <a href="http://developer.android.com/reference/android/media/MediaRecorder.AudioSource.html#VOICE_RECOGNITION">

VOICE_RECOGNITION</a> preset when recording.</li>

  <li>It's difficult to test audio input and output latency in isolation. The best solution to

determine the lowest possible audio input latency is to measure round-trip audio and divide

by two.</li>

 <li> Be prepared to handle nominal sample rates of 44,100 and 48,000 Hz as reported by

<a href="{@docRoot}reference/android/media/AudioManager.html#getProperty(java.lang.String)">

getProperty(String)</a> for

<a href="{@docRoot}reference/android/media/AudioManager.html#PROPERTY_OUTPUT_SAMPLE_RATE">

PROPERTY_OUTPUT_SAMPLE_RATE</a>. Other sample rates are possible, but rare.</li>

  <li>Be prepared to handle the buffer size reported by

<a href="{@docRoot}reference/android/media/AudioManager.html#getProperty(java.lang.String)">

getProperty(String)</a> for

<a href="{@docRoot}reference/android/media/AudioManager.html#PROPERTY_OUTPUT_FRAMES_PER_BUFFER">

PROPERTY_OUTPUT_FRAMES_PER_BUFFER</a>. Typical buffer sizes include 96, 128, 160, 192, 240, 256,

or 512 frames, but other values are possible.</li>

</ul>

<h2 id="avoid">What to Avoid</h2>

<p>Be sure to take these things into account to help avoid latency issues:</p>

<ul>

  <li>Don’t assume that the speakers and microphones used in mobile devices generally have good

acoustics. Due to their small size, the acoustics are generally poor so signal processing is

added to improve the sound quality. This signal processing introduces latency.</li>

  <li>Don't assume that your input and output callbacks are synchronized. For simultaneous input

and output, separate buffer queue completion handlers are used for each side. There is no

guarantee of the relative order of these callbacks or the synchronization of the audio clocks,

even when both sides use the same sample rate. Your application should buffer the data with

proper buffer synchronization.</li>

  <li>Don't assume that the actual sample rate exactly matches the nominal sample rate. For

example, if the nominal sample rate is 48,000 Hz, it is normal for the audio clock to advance

at a slightly different rate than the operating system {@code CLOCK_MONOTONIC}. This is because

the audio and system clocks may derive from different crystals.</li>

  <li>Don't assume that the actual playback sample rate exactly matches the actual capture sample

rate, especially if the endpoints are on separate paths. For example, if you are capturing from

the on-device microphone at 48,000 Hz nominal sample rate, and playing on USB audio

at 48,000 Hz nominal sample rate, the actual sample rates are likely to be slightly different

from each other.</li>

</ul>

<p>A consequence of potentially independent audio clocks is the need for asynchronous sample rate

conversion. A simple (though not ideal for audio quality) technique for asynchronous sample rate

conversion is to duplicate or drop samples as needed near a zero-crossing point. More

sophisticated conversions are possible.</p>