1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 2 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4 <head> 5 <meta name="generator" content="PSPad editor, www.pspad.com" /> 6 <meta http-equiv="content-type" content="text/html; charset=UTF-8" /> 7 <link rel="stylesheet" type="text/css" href="styles/lame.css" /> 8 <title>Introduction to encoding</title> 9 </head> 10 <body> 11<div id="menu"> 12 <ul> 13 <li><a href="index.html">Index page</a></li> 14 <li><a href="about.html">About LAME</a></li> 15 <li><a style="border: 0" href="introduction.html">Intro to encoding</a> 16 <div id="submenu"> 17 <ul> 18 <li><a href="cbr.html">CBR </a></li> 19 <li><a href="abr.html">ABR </a></li> 20 <li><a href="vbr.html">VBR </a></li> 21 <li><a style="border: 0" href="ms_stereo.html">M/S Stereo</a></li> 22 </ul> 23 </div> 24 </li> 25 <li><a href="usage.html">Usage of LAME</a></li> 26 <li><a href="history.html">Version history</a></li> 27 <li><a href="contributors.html">Coders of LAME</a></li> 28 <li><a href="contact.html">Contact LAME</a></li> 29 <li><a style="border: 0" href="links.html">LAME links</a></li> 30 </ul> 31</div> 32<div id="container"> 33<div id="content"> 34 35<div align="center"> 36 <img src="images/logo.gif" width="358" height="231" alt="LAME Official Logo" /> 37 <h2 class="hilight">Introduction to encoding</h2> 38</div> 39<h3> Introduction</h3> 40<p> 41 There is a lot of confusion surrounding the terms audio compression<a href="#note1"> 42 <sup>1</sup></a>, audio encoding, and audio decoding. This section will give you 43 an overview what audio coding (another one of these terms...) is all about. 44</p> 45<h3>The purpose of audio compression</h3> 46<p>Up to the advent of audio compression, high-quality digital audio data took 47 a lot of hard disk space to store. Let us go through a short example. 48</p> 49<p> 50 You want to sample 1 minute of your favourite song and store it on your harddisk. 51 Because you want CD quality, you sample at 44.1 kHz, stereo, with 16 bits per sample. 52</p> 53<p> 54 44100 Hz means that you have 44100 values per second coming in from your sound card 55 (or input file). Multiply that by two because you have two channels. Multiply by 56 another factor of two because you have two bytes per value (that's what 16 bit 57 means). The song will take up 44100 <sup>samples</sup>/<sub>s</sub> · 2 channels 58 · 2 <sup>bytes</sup>/<sub>sample</sub> · 59 60 <sup>s</sup>/<sub>min</sub> ~ 10 MBytes of storage space on your harddisk. 60</p> 61<p> 62 In order to stream this over internet, a speed of at least 1.41<sup>Mbits</sup>/ 63 <sub>s</sub> is needed, 64 which wasn't a common speed at all at the time MP3 was invented. 65 If you wanted to download that, given an average 56k modem connected at 44k, it 66 would take 1.41Mbits · 1000 <sup>kbits</sup>/<sub>Mbit</sub> / 44 kbits ~ 32 67 times as much. 68 <br/>This means 32 minutes just to download one minute of music! 69</p> 70<p> 71 Digital audio coding, which - in this context - is synonymously called digital 72 audio compression as well, is the art of minimizing storage space (or channel 73 bandwidth) requirements for audio data. Modern perceptual audio coding techniques 74 (like MPEG Layer III) exploit the properties of the human ear (the perception of 75 sound) to achieve a size reduction by a factor of 11 with little or no perceptible 76 loss of quality. 77</p> 78<p> 79 Therefore, such schemes are the key technology for high quality low bit-rate 80 applications, like soundtracks for CD-ROM games, solid-state sound memories, 81 Internet audio, digital audio broadcasting systems, and the like. 82</p> 83<h3>The two parts of audio compression</h3> 84<p> 85 Audio compression really consists of two parts. The first part, called encoding, 86 transforms the digital audio data that resides, say, in a WAVE file, into a highly 87 compressed form called bitstream. To play the bitstream on your soundcard, you 88 need the second part, called decoding. Decoding takes the bitstream and re-expands 89 it to a WAVE file. 90</p> 91<p> 92 The program that effects the first part is called an audio encoder. LAME is such 93 an encoder . The program that does the second part is called an audio decoder. 94 Nowadays there are lots of players that decode MP3 95</p> 96<h3>Compression ratios, bitrate and quality</h3> 97<p> 98 It has not been explicitly mentioned up to now: What you end up with after 99 encoding and decoding is not the same sound file anymore: All superfluous 100 information has been squeezed out, so to say. It is not the same file, but it 101 will sound the same - more or less, depending on how much compression has been 102 performed on it. 103</p> 104<p> 105 Generally speaking, the lower the compression ratio achieved, the better the 106 sound quality will be in the end - and vice versa.<br/> 107 Table 1.1 gives you a rough estimate about the quality you can expect. 108</p> 109<p> 110 Because compression ratio is a somewhat unwieldy measure, experts use the term 111 bitrate when speaking of the strength of compression. Bitrate denotes the average 112 number of bits that one second of audio data will take up in your compressed 113 bitstream. Usually the units used will be kbps, which is kbits/s, or 1000 bits/s 114 (not 1024).<br/> 115 To calculate the number of bytes per second of audio data, simply divide the 116 number of bits per second by eight. 117</p> 118<table align="center" cellpadding="5"> 119 <caption><strong>table 1.1:</strong> bitrate versus sound quality</caption> 120 <tr> 121 <th>Bitrate</th> 122 <th>Bandwidth</th> 123 <th>Quality comparable to</th> 124 </tr> 125 <tr> 126 <td>16 kbps mono</td> 127 <td>5.5 khz</td> 128 <td>above shortwave radio / telephone</td> 129 </tr> 130 <tr> 131 <td>32 kbps mono</td> 132 <td>8.5 khz</td> 133 <td>near AM (medium wave) radio</td> 134 </tr> 135 <tr> 136 <td>64kbps mono, 128 kbps stereo</td> 137 <td>16 khz</td> 138 <td>FM radio</td> 139 </tr> 140 <tr> 141 <td style="border-bottom:0px">-V 3~-V 0 (160~200 kbps) <br/> 142 (variable bitrate) 143 </td> 144 <td style="border-bottom:0px">18~20 khz</td> 145 <td style="border-bottom:0px">perceptual transparency versus CD<a href="#transparency"><sup>2</sup> 146 </a> 147 </td> 148 </tr> 149</table> 150<div id="notes"> 151 <ol> 152 <li><a name="note1"></a>Audio compression (also called coding) 153 means reduce the size (bytes) that the original source requires to be stored. 154 This is not the same than compressors in DSP (or audio effects). The latter 155 reduces the dynamic range of the audio so that there is less difference in 156 perceived loudness between its strong and subtle parts. 157 </li> 158 <li><a name="note2"></a>Lossy encoding (as opposed to lossless) cannot guarantee 159 transparency all of the time. This is the value accepted as the <i>sweet spot</i>. 160 </li> 161 </ol> 162</div> 163</div> 164<div id="footer"> 165 166<a href="http://sourceforge.net/projects/lame"><img src="images/sflogo.gif" 167 alt="Get LAME (Lame Aint an MP3 Encoder) at SourceForge.net. 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