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| 4 | <TITLE>Introduction</TITLE>
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| 14 | ALINK=#995500>
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| 15 | <FONT FACE = Helvetica ><strong>Subsections</strong>
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| 16 | <UL>
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| 17 | <LI><A
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| 18 | HREF="node6.html#SECTION00311000000000000000"> The purpose of audio compression</A>
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| 19 | <LI><A
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| 20 | HREF="node6.html#SECTION00312000000000000000"> The two parts of audio compression</A>
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| 21 | <LI><A
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| 22 | HREF="node6.html#SECTION00313000000000000000"> Compression ratios, bitrate and
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| 23 | quality</A>
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| 24 | </UL>
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| 25 |
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| 26 | <HR>
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| 27 | <H1><A NAME="SECTION00310000000000000000"> Introduction</A> </H1>
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| 28 | There is a lot of confusion surrounding the terms <I>audio compression</I>, <I>audio
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| 29 | encoding</I>, and <I>audio decoding</I>. This section will give you an overview
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| 30 | what audio coding (another one of these terms...) is all about.
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| 31 | <P>
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| 32 | <H2><A NAME="SECTION00311000000000000000"> The purpose of audio compression</A>
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| 33 | </H2>
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| 34 | <P> Up to the advent of audio compression, high-quality digital audio data took
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| 35 | a lot of hard disk space to store. Let us go through a short example.
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| 36 | <P> You want to, say, sample your favorite 1-minute song and store it on your
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| 37 | harddisk. Because you want CD quality, you sample at 44.1 kHz, stereo,
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| 38 | with 16 bits per sample.
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| 39 | <P> 44100 Hz means that you have 44100 values per second coming in from your sound
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| 40 | card (or input file). Multiply that by two because you have two channels. Multiply
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| 41 | by another factor of two because you have two bytes per value (that's what 16
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| 42 | bit means). The song will take up <NOBR>44100 samples/s · 2 channels
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| 43 | · 2 bytes/sample · 60 s/min ~ 10 MBytes</NOBR> of storage space
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| 44 | on your harddisk.
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| 45 | <P> If you wanted to download that over the internet, given an average 56k modem
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| 46 | connected at 44k (which is a typical case), it would take you (at least) <NOBR>10000000
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| 47 | bytes · 8 bits/byte / (44000 bits/s) · / (60 s/min) ~ 30 minutes</NOBR>
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| 48 | <DIV ALIGN="CENTER"> <B>Just to download one minute of music!</B> </DIV>
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| 49 | <P> Digital audio coding, which - in this context - is synonymously called digital
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| 50 | audio compression as well, is the art of minimizing storage space (or channel
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| 51 | bandwidth) requirements for audio data. Modern perceptual audio coding techniques
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| 52 | (like MPEG Layer III) exploit the properties of the human ear (the perception
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| 53 | of sound) to achieve a size reduction by a factor of 11 with little or no perceptible
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| 54 | loss of quality.
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| 55 | <P> Therefore, such schemes are the key technology for high quality low bit-rate
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| 56 | applications, like soundtracks for CD-ROM games, solid-state sound memories,
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| 57 | Internet audio, digital audio broadcasting systems, and the like.
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| 58 | <P>
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| 59 | <H2><A NAME="SECTION00312000000000000000"> The two parts of audio compression</A>
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| 60 | </H2>
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| 61 | <P> Audio compression really consists of two parts. The first part, called <I>encoding</I>,
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| 62 | transforms the digital audio data that resides, say, in a WAVE file, into a
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| 63 | highly compressed form called <I>bitstream</I>. To play the bitstream on your
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| 64 | soundcard, you need the second part, called <I>decoding</I>. Decoding takes
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| 65 | the bitstream and re-expands it to a WAVE file.
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| 66 | <P> The program that effects the first part is called an audio <I>encoder</I>.
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| 67 | <i>LAME</i> is such an encoder . The program that does the second part is called
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| 68 | an audio <I>decoder</I>. One well-known MPEG Layer III decoder is <tt>Xmms</tt>,
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| 69 | another <TT>mpg123</TT>. Both can be found on <A NAME="tex2html1"
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| 70 | HREF="http://www.mp3-tech.org">ww.mp3-tech.org</A> .
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| 71 | <H2><A NAME="SECTION00313000000000000000"> Compression ratios, bitrate and quality</A>
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| 72 | </H2>
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| 73 | <P> It has not been explicitly mentioned up to now: What you end up with after
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| 74 | encoding and decoding is not the same sound file anymore: All superflous information
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| 75 | has been squeezed out, so to say. It is not the same <I>file</I>, but it will
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| 76 | <I>sound</I> the same - more or less, depending on how much compression had
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| 77 | been performed on it.
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| 78 | <P> Generally speaking, the lower the compression ratio achieved, the better the
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| 79 | sound quality will be in the end - and <I>vice versa</I>. Table <A HREF="node6.html#table-soundq">1.1</A>
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| 80 | gives you an overview about quality achievable.
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| 81 | <P> Because compression ratio is a somewhat unwieldy measure, experts use the
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| 82 | term <I>bitrate</I> when speaking of the strength of compression. Bitrate denotes
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| 83 | the average number of bits that one second of audio data will take up in your
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| 84 | compressed bitstream. Usually the units used will be kbps, which is <SUP>kbits</SUP>/<SUB>s</SUB>,
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| 85 | or 1000 <SUP>bits</SUP>/<SUB>s</SUB>. To calculate the number of bytes
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| 86 | per second of audio data, simply divide the number of bits per second by eight.
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| 87 | <P> <BR>
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| 88 | <DIV ALIGN="CENTER"><A NAME="table-soundq"> </A> <A NAME="95"> </A>
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| 89 | <TABLE CELLPADDING=3 BORDER="1" width="512" height="225">
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| 90 | <CAPTION><STRONG>Table 1.1:</STRONG> Bitrate versus sound quality</CAPTION>
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| 91 | <TR VALIGN="TOP">
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| 92 | <TD ALIGN="RIGHT" nowrap width="115">Bitrate</TD>
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| 93 | <TD ALIGN="RIGHT" nowrap width="67">Bandwidth</TD>
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| 94 | <TD ALIGN="LEFT" nowrap width="246">Quality comparable to or better than</TD>
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| 95 | </TR>
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| 96 | <TR VALIGN="TOP">
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| 97 | <TD ALIGN="RIGHT" nowrap width="115">16 kbps</TD>
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| 98 | <TD ALIGN="RIGHT" nowrap width="67">4.5 kHz</TD>
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| 99 | <TD ALIGN="LEFT" nowrap width="246">shortwave radio</TD>
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| 100 | </TR>
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| 101 | <TR VALIGN="TOP">
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| 102 | <TD ALIGN="RIGHT" nowrap width="115">32 kbps</TD>
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| 103 | <TD ALIGN="RIGHT" nowrap width="67">7.5 kHz</TD>
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| 104 | <TD ALIGN="LEFT" nowrap width="246">AM radio</TD>
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| 105 | </TR>
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| 106 | <TR VALIGN="TOP">
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| 107 | <TD ALIGN="RIGHT" nowrap width="115">96 kbps</TD>
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| 108 | <TD ALIGN="RIGHT" nowrap width="67">11 kHz</TD>
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| 109 | <TD ALIGN="LEFT" nowrap width="246">FM radio</TD>
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| 110 | </TR>
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| 111 | <tr valign="TOP">
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| 112 | <td align="RIGHT" nowrap width="115">128 kbps</td>
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| 113 | <td align="RIGHT" nowrap width="67">16 kHz</td>
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| 114 | <td align="LEFT" nowrap width="246">near CD</td>
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| 115 | </tr>
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| 116 | <tr valign="TOP">
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| 117 | <td align="RIGHT" nowrap width="115">160-180 kbps <br>
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| 118 | (variable bitrate)</td>
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| 119 | <td align="RIGHT" nowrap width="67">20 kHz</td>
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| 120 | <td align="LEFT" nowrap width="246">perceptual transparency</td>
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| 121 | </tr>
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| 122 | <TR VALIGN="TOP">
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| 123 | <TD ALIGN="RIGHT" nowrap width="115">256 kbps</TD>
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| 124 | <TD ALIGN="RIGHT" nowrap width="67">22 kHz</TD>
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| 125 | <TD ALIGN="LEFT" nowrap width="246">studio</TD>
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| 126 | </TR>
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| 127 | </TABLE>
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