118 lines
4.2 KiB
HTML
118 lines
4.2 KiB
HTML
<html devsite>
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<head>
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<title>Audio Warmup</title>
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<meta name="project_path" value="/_project.yaml" />
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<meta name="book_path" value="/_book.yaml" />
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</head>
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<body>
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<!--
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Copyright 2017 The Android Open Source Project
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<p>Audio warmup is the time it takes for the audio amplifier circuit in your device to
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be fully powered and reach its normal operation state. The major contributors
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to audio warmup time are power management and any "de-pop" logic to stabilize
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the circuit.
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</p>
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<p>This document describes how to measure audio warmup time and possible ways to decrease
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warmup time.</p>
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<h2 id="measuringOutput">Measuring Output Warmup</h2>
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<p>
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AudioFlinger's FastMixer thread automatically measures output warmup
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and reports it as part of the output of the <code>dumpsys media.audio_flinger</code> command.
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At warmup, FastMixer calls <code>write()</code>
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repeatedly until the time between two <code>write()</code>s is the amount expected.
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FastMixer determines audio warmup by seeing how long a Hardware Abstraction
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Layer (HAL) <code>write()</code> takes to stabilize.
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</p>
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<p>To measure audio warmup, follow these steps for the built-in speaker and wired headphones
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and at different times after booting. Warmup times are usually different for each output device
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and right after booting the device:</p>
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<ol>
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<li>Ensure that FastMixer is enabled.</li>
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<li>Enable touch sounds by selecting <b>Settings > Sound > Touch sounds</b> on the device.</li>
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<li>Ensure that audio has been off for at least three seconds. Five seconds or more is better, because
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the hardware itself might have its own power logic beyond the three seconds that AudioFlinger has.</li>
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<li>Press Home, and you should hear a click sound.</li>
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<li>Run the following command to receive the measured warmup:
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<pre class="devsite-terminal devsite-click-to-copy">
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adb shell dumpsys media.audio_flinger | grep measuredWarmup
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</pre>
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<p>
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You should see output like this:
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</p>
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<pre>
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sampleRate=44100 frameCount=256 measuredWarmup=X ms, warmupCycles=X
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</pre>
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<p>
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The <code>measuredWarmup=X</code> is X number of milliseconds
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it took for the first set of HAL <code>write()</code>s to complete.
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</p>
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<p>
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The <code>warmupCycles=X</code> is how many HAL write requests it took
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until the execution time of <code>write()</code> matches what is expected.
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</p>
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</li>
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<li>
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Take five measurements and record them all, as well as the mean.
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If they are not all approximately the same,
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then it's likely that a measurement is incorrect.
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For example, if you don't wait long enough after the audio has been off,
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you will see a lower warmup time than the mean value.
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</li>
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</ol>
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<h2 id="measuringInput">Measuring Input Warmup</h2>
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<p>
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There are currently no tools provided for measuring audio input warmup.
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However, input warmup time can be estimated by observing
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the time required for <a href="http://developer.android.com/reference/android/media/AudioRecord.html#startRecording()">startRecording()</a>
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to return.
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</p>
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<h2 id="reducing">Reducing Warmup Time</h2>
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<p>
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Warmup time can usually be reduced by a combination of:
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</p>
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<ul>
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<li>Good circuit design</li>
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<li>Accurate time delays in kernel device driver</li>
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<li>Performing independent warmup operations concurrently rather than sequentially</li>
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<li>Leaving circuits powered on or not reconfiguring clocks (increases idle power consumption)</li>
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<li>Caching computed parameters</li>
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</ul>
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<p>
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However, beware of excessive optimization. You may find that you
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need to tradeoff between low warmup time versus
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lack of popping at power transitions.
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</p>
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</body>
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</html>
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