Glenn Kasten | 46ac61c | 2014-01-24 08:59:11 -0800 | [diff] [blame] | 1 | page.title=Sample Rate Conversion |
| 2 | @jd:body |
| 3 | |
Clay Murphy | bc92aea | 2014-10-16 10:13:18 -0700 | [diff] [blame] | 4 | <!-- |
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Glenn Kasten | 46ac61c | 2014-01-24 08:59:11 -0800 | [diff] [blame] | 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 | |
Clay Murphy | 5d83ab4 | 2014-09-09 17:29:09 -0700 | [diff] [blame] | 27 | <h2 id="srcIntro">Introduction</h2> |
| 28 | |
Glenn Kasten | 46ac61c | 2014-01-24 08:59:11 -0800 | [diff] [blame] | 29 | <p> |
| 30 | See the Wikipedia article |
Clay Murphy | 460c688 | 2014-09-10 15:10:03 -0700 | [diff] [blame] | 31 | <a href="http://en.wikipedia.org/wiki/Resampling_(audio)">Resampling (audio)</a> |
Glenn Kasten | 46ac61c | 2014-01-24 08:59:11 -0800 | [diff] [blame] | 32 | for a generic definition of sample rate conversion, also known as "resampling." |
| 33 | The remainder of this article describes resampling within Android. |
| 34 | </p> |
| 35 | |
| 36 | <p> |
| 37 | Sample rate conversion is the process of changing a |
| 38 | stream of discrete samples at one sample rate to another stream at |
| 39 | another sample rate. A sample rate converter, or resampler, is a module |
| 40 | that implements sample rate conversion. With respect to the resampler, |
| 41 | the original stream is called the source signal, and the resampled stream is |
| 42 | the sink signal. |
| 43 | </p> |
| 44 | |
| 45 | <p> |
| 46 | Resamplers are used in several places in Android. |
| 47 | For example, an MP3 file may be encoded at 44.1 kHz sample rate and |
| 48 | needs to be played back on an Android device supporting 48 kHz audio |
| 49 | internally. In that case, a resampler would be used to upsample the MP3 |
| 50 | output audio from 44.1 kHz source sample rate to a 48 kHz sink sample rate |
| 51 | used within the Android device. |
| 52 | </p> |
| 53 | |
| 54 | <p> |
| 55 | The characteristics of a resampler can be expressed using metrics, including: |
| 56 | </p> |
| 57 | |
| 58 | <ul> |
| 59 | <li>degree of preservation of the overall amplitude of the signal</li> |
| 60 | <li>degree of preservation of the frequency bandwidth of the signal, |
| 61 | subject to limitations of the sink sample rate</li> |
| 62 | <li>overall latency through the resampler</li> |
| 63 | <li>consistent phase and group delay with respect to frequency</li> |
| 64 | <li>computational complexity, expressed in CPU cycles or power draw</li> |
| 65 | <li>permitted ratios of source and sink sample rates</li> |
| 66 | <li>ability to dynamically change sample rate ratios</li> |
| 67 | <li>which digital audio sample formats are supported</li> |
| 68 | </ul> |
| 69 | |
| 70 | <p> |
| 71 | The ideal resampler would exactly preserve the source signal's amplitude |
| 72 | and frequency bandwidth (subject to limitations of the sink |
| 73 | sample rate), have minimal and consistent delay, have minimal |
| 74 | computational complexity, permit arbitrary and dynamic conversion ratios, |
| 75 | and support all common digital audio sample formats. |
| 76 | In practice, ideal resamplers do not exist, and actual resamplers are |
| 77 | a compromise among these characteristics. |
| 78 | For example, the goals of ideal quality conflict with short delay and low complexity. |
| 79 | </p> |
| 80 | |
| 81 | <p> |
| 82 | Android includes a variety of audio resamplers, so that appropriate |
| 83 | compromises can be made depending on the application use case and load. |
| 84 | Section <a href="#srcResamplers">Resampler implementations</a> |
| 85 | below lists the available resamplers, summarizes their characteristics, |
| 86 | and identifies where they should typically be used. |
| 87 | </p> |
| 88 | |
Glenn Kasten | 46ac61c | 2014-01-24 08:59:11 -0800 | [diff] [blame] | 89 | <h2 id="srcResamplers">Resampler implementations</h2> |
| 90 | |
| 91 | <p> |
| 92 | Available resampler implementations change frequently, |
| 93 | and may be customized by OEMs. |
| 94 | As of Android 4.4, the default resamplers |
| 95 | in descending order of signal distortion, and ascending order of |
| 96 | computational complexity include: |
| 97 | </p> |
| 98 | |
| 99 | <ul> |
| 100 | <li>linear</li> |
| 101 | <li>cubic</li> |
| 102 | <li>sinc with original coefficients</li> |
| 103 | <li>sinc with revised coefficients</li> |
| 104 | </ul> |
| 105 | |
| 106 | <p> |
| 107 | In general, the sinc resamplers are more appropriate for higher-quality |
| 108 | music playback, and the other resamplers should be reserved for cases |
| 109 | where quality is less important (an example might be "key clicks" or similar). |
| 110 | </p> |
| 111 | |
| 112 | <p> |
| 113 | The specific resampler implementation selected depends on |
| 114 | the use case, load, and the value of system property |
| 115 | <code>af.resampler.quality</code>. For details, |
| 116 | consult the audio resampler source code in AudioFlinger. |
| 117 | </p> |