1 page.title=Sample Rate Conversion 2 @jd:body 3 4 <!-- 5 Copyright 2013 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18 --> 19 <div id="qv-wrapper"> 20 <div id="qv"> 21 <h2>In this document</h2> 22 <ol id="auto-toc"> 23 </ol> 24 </div> 25 </div> 26 27 <p> 28 See the Wikipedia article 29 <a class="external-link" href="http://en.wikipedia.org/wiki/Resampling_(audio)" target="_android">Resampling (audio)</a> 30 for a generic definition of sample rate conversion, also known as "resampling." 31 The remainder of this article describes resampling within Android. 32 </p> 33 34 <p> 35 Sample rate conversion is the process of changing a 36 stream of discrete samples at one sample rate to another stream at 37 another sample rate. A sample rate converter, or resampler, is a module 38 that implements sample rate conversion. With respect to the resampler, 39 the original stream is called the source signal, and the resampled stream is 40 the sink signal. 41 </p> 42 43 <p> 44 Resamplers are used in several places in Android. 45 For example, an MP3 file may be encoded at 44.1 kHz sample rate and 46 needs to be played back on an Android device supporting 48 kHz audio 47 internally. In that case, a resampler would be used to upsample the MP3 48 output audio from 44.1 kHz source sample rate to a 48 kHz sink sample rate 49 used within the Android device. 50 </p> 51 52 <p> 53 The characteristics of a resampler can be expressed using metrics, including: 54 </p> 55 56 <ul> 57 <li>degree of preservation of the overall amplitude of the signal</li> 58 <li>degree of preservation of the frequency bandwidth of the signal, 59 subject to limitations of the sink sample rate</li> 60 <li>overall latency through the resampler</li> 61 <li>consistent phase and group delay with respect to frequency</li> 62 <li>computational complexity, expressed in CPU cycles or power draw</li> 63 <li>permitted ratios of source and sink sample rates</li> 64 <li>ability to dynamically change sample rate ratios</li> 65 <li>which digital audio sample formats are supported</li> 66 </ul> 67 68 <p> 69 The ideal resampler would exactly preserve the source signal's amplitude 70 and frequency bandwidth (subject to limitations of the sink 71 sample rate), have minimal and consistent delay, have minimal 72 computational complexity, permit arbitrary and dynamic conversion ratios, 73 and support all common digital audio sample formats. 74 In practice, ideal resamplers do not exist, and actual resamplers are 75 a compromise among these characteristics. 76 For example, the goals of ideal quality conflict with short delay and low complexity. 77 </p> 78 79 <p> 80 Android includes a variety of audio resamplers, so that appropriate 81 compromises can be made depending on the application use case and load. 82 Section <a href="#srcResamplers">Resampler implementations</a> 83 below lists the available resamplers, summarizes their characteristics, 84 and identifies where they should typically be used. 85 </p> 86 87 <h2 id="srcTerms">Terminology</h2> 88 89 <dl> 90 91 <dt>downsample</dt> 92 <dd>to resample, where sink sample rate < source sample rate</dd> 93 94 <dt>Nyquist frequency</dt> 95 <dd> 96 The Nyquist frequency, equal to 1/2 of a given sample rate, is the 97 maximum frequency component that can be represented by a discretized 98 signal at that sample rate. For example, the human hearing range is 99 typically assumed to extend up to approximately 20 kHz, and so a digital 100 audio signal must have a sample rate of at least 40 kHz to represent that 101 range. In practice, sample rates of 44.1 kHz and 48 kHz are commonly 102 used, with Nyquist frequencies of 22.05 kHz and 24 kHz respectively. 103 See the Wikipedia articles 104 <a class="external-link" href="http://en.wikipedia.org/wiki/Nyquist_frequency" target="_android">Nyquist frequency</a> 105 and 106 <a class="external-link" href="http://en.wikipedia.org/wiki/Hearing_range" target="_android">Hearing range</a> 107 for more information. 108 </dd> 109 110 <dt>resampler</dt> 111 <dd>synonym for sample rate converter</dd> 112 113 <dt>resampling</dt> 114 <dd>the process of converting sample rate</dd> 115 116 <dt>sample rate converter</dt> 117 <dd>a module that resamples</dd> 118 119 <dt>sink</dt> 120 <dd>the output of a resampler</dd> 121 122 <dt>source</dt> 123 <dd>the input to a resampler</dd> 124 125 <dt>upsample</dt> 126 <dd>to resample, where sink sample rate > source sample rate</dd> 127 128 </dl> 129 130 <h2 id="srcResamplers">Resampler implementations</h2> 131 132 <p> 133 Available resampler implementations change frequently, 134 and may be customized by OEMs. 135 As of Android 4.4, the default resamplers 136 in descending order of signal distortion, and ascending order of 137 computational complexity include: 138 </p> 139 140 <ul> 141 <li>linear</li> 142 <li>cubic</li> 143 <li>sinc with original coefficients</li> 144 <li>sinc with revised coefficients</li> 145 </ul> 146 147 <p> 148 In general, the sinc resamplers are more appropriate for higher-quality 149 music playback, and the other resamplers should be reserved for cases 150 where quality is less important (an example might be "key clicks" or similar). 151 </p> 152 153 <p> 154 The specific resampler implementation selected depends on 155 the use case, load, and the value of system property 156 <code>af.resampler.quality</code>. For details, 157 consult the audio resampler source code in AudioFlinger. 158 </p> 159
