s.a.c. redesign, first checkin
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+page.title=Audio Warmup
+@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>Audio warmup is the time for the audio amplifier circuit in your device to
+be fully powered and reach its normal operation state. The major contributors
+to audio warmup time are power management and any "de-pop" logic to stabilize
+the circuit.
+</p>
+
+<p>This document describes how to measure audio warmup time and possible ways to decrease
+warmup time.</p>
+
+<h2 id="measuringOutput">Measuring Output Warmup</h2>
+
+<p>
+ AudioFlinger's FastMixer thread automatically measures output warmup
+ and reports it as part of the output of the <code>dumpsys media.audio_flinger</code> command.
+ At warmup, FastMixer calls <code>write()</code>
+ repeatedly until the time between two <code>write()</code>s is the amount expected.
+ FastMixer determines audio warmup by seeing how long a HAL <code>write()</code> takes to stabilize.
+</p>
+
+<p>To measure audio warmup, do the following
+steps for the built-in speaker and wired headphones and at different times after booting.
+Warmup times are usually different for each output device and right after booting the device:</p>
+
+<ol>
+ <li>Ensure that FastMixer is enabled.</li>
+ <li>Enable touch sounds by selecting <b>Settings > Sound > Touch sounds</b> on the device.</li>
+ <li>Ensure that audio has been off for at least three seconds. Five seconds or more is better, because
+ the hardware itself might have its own power logic beyond the three seconds that AudioFlinger has.</li>
+ <li>Press Home, and you should hear a click sound.</li>
+ <li>Run the following command to receive the measured warmup:
+ <pre>adb shell dumpsys media.audio_flinger | grep measuredWarmup</code></pre>
+
+<p>
+You should see output like this:
+</p>
+
+<pre>
+sampleRate=44100 frameCount=256 measuredWarmup=X ms, warmupCycles=X
+</pre>
+
+<p>
+ The <code>measuredWarmup=X</code> is X number of milliseconds
+ it took for the first set of HAL <code>write()</code>s to complete.
+</p>
+
+<p>
+ The <code>warmupCycles=X</code> is how many HAL write requests it took
+ until the execution time of <code>write()</code> matches what is expected.
+</p>
+</li>
+<li>
+ Take five measurements and report them all, as well as the mean.
+ If they are not all approximately the same,
+ then it’s likely that a measurement is incorrect.
+ For example, if you don’t wait long enough after the audio has been off,
+ you will see a lower warmup time than the mean value.
+</li>
+</ol>
+
+
+<h2 id="measuringInput">Measuring Input Warmup</h2>
+
+<p>
+ There are currently no tools provided for measuring audio input warmup.
+ However, input warmup time can be estimated by observing
+ the time required for <a href="http://developer.android.com/reference/android/media/AudioRecord.html#startRecording()">startRecording()</a>
+ to return.
+</p>
+
+
+<h2 id="reducing">Reducing Warmup Time</h2>
+
+<p>
+ Warmup time can usually be reduced by a combination of:
+ <ul>
+ <li>Good circuit design</li>
+ <li>Accurate time delays in kernel device driver</li>
+ <li>Performing independent warmup operations concurrently rather than sequentially</li>
+ <li>Leaving circuits powered on or not reconfiguring clocks (increases idle power consumption).
+ <li>Caching computed parameters</li>
+ </ul>
+ However, beware of excessive optimization. You may find that you
+ need to tradeoff between low warmup time versus
+ lack of popping at power transitions.
+</p>