186 lines
7.4 KiB
HTML
186 lines
7.4 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<link rel="stylesheet" type="text/css" href="styles/lame.css" />
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<title>Introduction to encoding</title>
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</head>
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<body>
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<div id="menu">
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<ul>
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<li><a href="index.html">Index page</a></li>
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<li><a href="about.html">About LAME</a></li>
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<li><a style="border: 0" href="introduction.html">Intro to encoding</a>
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<li><a href="cbr.html">CBR </a></li>
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<li><a href="usage.html">Usage of LAME</a></li>
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<li><a href="history.html">Version history</a></li>
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<li><a href="contributors.html">Coders of LAME</a></li>
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<div align="center">
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<img src="images/logo.gif" width="358" height="231" alt="LAME Official Logo" />
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<h2 class="hilight">Introduction to encoding</h2>
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</div>
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<h3> Introduction</h3>
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<p>
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There is a lot of confusion surrounding the terms audio compression<a href="#note1">
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<sup>1</sup></a>, audio encoding, and audio decoding. This section will give you
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an overview what audio coding (another one of these terms...) is all about.
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</p>
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<h3>The purpose of audio compression</h3>
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<p>Up to the advent of audio compression, high-quality digital audio data took
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a lot of hard disk space to store. Let us go through a short example.
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</p>
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<p>
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You want to sample 1 minute of your favourite song and store it on your harddisk.
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Because you want CD quality, you sample at 44.1 kHz, stereo, with 16 bits per sample.
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</p>
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<p>
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44100 Hz means that you have 44100 values per second coming in from your sound card
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(or input file). Multiply that by two because you have two channels. Multiply by
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another factor of two because you have two bytes per value (that's what 16 bit
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means). The song will take up 44100 <sup>samples</sup>/<sub>s</sub> · 2 channels
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· 2 <sup>bytes</sup>/<sub>sample</sub> ·
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60 <sup>s</sup>/<sub>min</sub> ~ 10 MBytes of storage space on your harddisk.
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</p>
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<p>
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In order to stream this over internet, a speed of at least 1.41<sup>Mbits</sup>/
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<sub>s</sub> is needed,
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which wasn't a common speed at all at the time MP3 was invented.
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If you wanted to download that, given an average 56k modem connected at 44k, it
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would take 1.41Mbits · 1000 <sup>kbits</sup>/<sub>Mbit</sub> / 44 kbits ~ 32
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times as much.
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<br/>This means 32 minutes just to download one minute of music!
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</p>
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<p>
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Digital audio coding, which - in this context - is synonymously called digital
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audio compression as well, is the art of minimizing storage space (or channel
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bandwidth) requirements for audio data. Modern perceptual audio coding techniques
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(like MPEG Layer III) exploit the properties of the human ear (the perception of
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sound) to achieve a size reduction by a factor of 11 with little or no perceptible
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loss of quality.
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</p>
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<p>
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Therefore, such schemes are the key technology for high quality low bit-rate
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applications, like soundtracks for CD-ROM games, solid-state sound memories,
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Internet audio, digital audio broadcasting systems, and the like.
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</p>
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<h3>The two parts of audio compression</h3>
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<p>
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Audio compression really consists of two parts. The first part, called encoding,
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transforms the digital audio data that resides, say, in a WAVE file, into a highly
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compressed form called bitstream. To play the bitstream on your soundcard, you
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need the second part, called decoding. Decoding takes the bitstream and re-expands
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it to a WAVE file.
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</p>
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<p>
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The program that effects the first part is called an audio encoder. LAME is such
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an encoder . The program that does the second part is called an audio decoder.
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Nowadays there are lots of players that decode MP3
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</p>
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<h3>Compression ratios, bitrate and quality</h3>
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<p>
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It has not been explicitly mentioned up to now: What you end up with after
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encoding and decoding is not the same sound file anymore: All superfluous
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information has been squeezed out, so to say. It is not the same file, but it
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will sound the same - more or less, depending on how much compression has been
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performed on it.
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</p>
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<p>
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Generally speaking, the lower the compression ratio achieved, the better the
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sound quality will be in the end - and vice versa.<br/>
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Table 1.1 gives you a rough estimate about the quality you can expect.
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</p>
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<p>
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Because compression ratio is a somewhat unwieldy measure, experts use the term
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bitrate when speaking of the strength of compression. Bitrate denotes the average
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number of bits that one second of audio data will take up in your compressed
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bitstream. Usually the units used will be kbps, which is kbits/s, or 1000 bits/s
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(not 1024).<br/>
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To calculate the number of bytes per second of audio data, simply divide the
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number of bits per second by eight.
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</p>
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<table align="center" cellpadding="5">
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<caption><strong>table 1.1:</strong> bitrate versus sound quality</caption>
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<tr>
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<th>Bitrate</th>
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<th>Bandwidth</th>
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<th>Quality comparable to</th>
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</tr>
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<tr>
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<td>16 kbps mono</td>
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<td>5.5 khz</td>
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<td>above shortwave radio / telephone</td>
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</tr>
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<tr>
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<td>32 kbps mono</td>
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<td>8.5 khz</td>
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<td>near AM (medium wave) radio</td>
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</tr>
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<tr>
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<td>64kbps mono, 128 kbps stereo</td>
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<td>16 khz</td>
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<td>FM radio</td>
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</tr>
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<tr>
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<td style="border-bottom:0px">-V 3~-V 0 (160~200 kbps) <br/>
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(variable bitrate)</td>
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<td style="border-bottom:0px">18~20 khz</td>
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<td style="border-bottom:0px">perceptual transparency versus CD<a href="#transparency"><sup>2</sup>
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</a></td>
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</tr>
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</table>
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<div id="notes">
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<ol>
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<li><a name="note1"></a>Audio compression (also called coding)
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means reduce the size (bytes) that the original source requires to be stored.
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This is not the same than compressors in DSP (or audio effects). The latter
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reduces the dynamic range of the audio so that there is less difference in
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perceived loudness between its strong and subtle parts.
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</li>
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<li><a name="note2"></a>Lossy encoding (as opposed to lossless) cannot guarantee
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transparency all of the time. This is the value accepted as the <i>sweet spot</i>.
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</li>
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</ol>
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</div>
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</div>
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<div id="footer">
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<a href="http://sourceforge.net/projects/lame"><img src="http://sflogo.sourceforge.net/sflogo.php?group_id=290&type=12"
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alt="Get LAME (Lame Aint an MP3 Encoder) at SourceForge.net. Fast, secure and Free Open Source software downloads"
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<p>HTML markup and design by <a
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href="http://www.rjamorim.com" target="_blank">Roberto Amorim</a> and <a
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href="http://www.maresweb.de" target="_blank">Sebastian Mares</a>. Logo by <a
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href="http://www.brightercreative.co.uk">Sam Fisher</a>.</p>
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