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1 <html>
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2
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3 <head>
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4 <title>libvorbisenc - API Overview</title>
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5 <link rel=stylesheet href="style.css" type="text/css">
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6 </head>
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7
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8 <body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff">
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9 <table border=0 width=100%>
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10 <tr>
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11 <td><p class=tiny>libvorbisenc documentation</p></td>
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12 <td align=right><p class=tiny>libvorbisenc version 1.3.2 - 20101101</p></td>
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13 </tr>
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14 </table>
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15
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16 <h1>Libvorbisenc API Overview</h1>
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17
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18 <p>Libvorbisenc is an encoding convenience library intended to
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19 encapsulate the elaborate setup that libvorbis requires for encoding.
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20 Libvorbisenc gives easy access to all high-level adjustments an
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21 application may require when encoding and also exposes some low-level
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22 tuning parameters to allow applications to make detailed adjustments
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23 to the encoding process. <p>
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24
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25 All the <b>libvorbisenc</b> routines are declared in "vorbis/vorbisenc.h".
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26
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27 <em>Note: libvorbis and libvorbisenc always
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28 encode in a single pass. Thus, all possible encoding setups will work
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29 properly with live input and produce streams that decode properly when
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30 streamed. See the subsection titled <a href="#BBR">"managed bitrate
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31 modes"</a> for details on setting limits on bitrate usage when Vorbis
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32 streams are used in a limited-bandwidth environment.</em>
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33
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34 <h2>workflow</h2>
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35
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36 <p>Libvorbisenc is used only during encoder setup; its function
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37 is to automate initialization of a multitude of settings in a
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38 <tt>vorbis_info</tt> structure which libvorbis then uses as a reference
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39 during the encoding process. Libvorbisenc plays no part in the
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40 encoding process after setup.
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41
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42 <p>Encode setup using libvorbisenc consists of three steps:
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43
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44 <ol>
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45 <li>high-level initialization of a <tt>vorbis_info</tt> structure by
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46 calling one of <a
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47 href="vorbis_encode_setup_vbr.html">vorbis_encode_setup_vbr()</a> or <a
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48 href="vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()</a>
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49 with the basic input audio parameters (rate and channels) and the
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50 basic desired encoded audio output parameters (VBR quality or ABR/CBR
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51 bitrate)<p>
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52
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53 <li>optional adjustment of the basic setup defaults using <a
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54 href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a><p>
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55
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56 <li>calling <a
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57 href="vorbis_encode_setup_init.html">vorbis_encode_setup_init()</a> to
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58 finalize the high-level setup into the detailed low-level reference
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59 values needed by libvorbis to encode audio. The <tt>vorbis_info</tt>
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60 structure is then ready to use for encoding by libvorbis.<p>
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61
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62 </ol>
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63
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64 These three steps can be collapsed into a single call by using <a
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65 href="vorbis_encode_init_vbr.html">vorbis_encode_init_vbr</a> to set up a
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66 quality-based VBR stream or <a
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67 href="vorbis_encode_init.html">vorbis_encode_init</a> to set up a managed
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68 bitrate (ABR or CBR) stream.<p>
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69
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70 <h2>adjustable encoding parameters</h2>
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71
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72 <h3>input audio parameters</h3>
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73
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74 <p>
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75 <table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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76 <tr bgcolor=#cccccc>
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77 <td><b>parameter</b></td>
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78 <td><b>description</b></td>
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79 </tr>
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80 <tr valign=top>
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81 <td>sampling rate</td>
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82 <td>
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83 The sampling rate (in samples per second) of the input audio. Common examples are 8000 for telephony, 44100 for CD audio and 48000 for DAT. Note that a mono sample (one center value) and a stereo sample (one left value and one right value) both are a single sample.
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84
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85 </td>
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86 </tr>
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87 <tr valign=top>
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88 <td>channels</td>
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89 <td>
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90
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91 The number of channels encoded in each input sample. By default,
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92 stereo input modes (two channels) are 'coupled' by Vorbis 1.1 such
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93 that the stereo relationship between the samples is taken into account
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94 when encoding. Stereo coupling my be disabled by using <a
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95 href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> with <a
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96 href="vorbis_encode_ctl.html#OV_ECTL_COUPLE_SET">OV_ECTL_COUPLE_SET</a>.
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97
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98 </td>
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99 </tr>
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100 </table>
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101
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102 <h3>quality and VBR modes</h3>
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103
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104 Vorbis is natively a VBR codec; a user requests a given constant
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105 <em>quality</em> and the encoder keeps the encoding quality constant
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106 while allowing the bitrate to vary. 'Quality' modes (Variable BitRate)
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107 will always produce the most consistent encoding results as well as
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108 the highest quality for the amount of bits used.
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109
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110 <p>
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111 <table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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112 <tr bgcolor=#cccccc>
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113 <td><b>parameter</b></td>
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114 <td><b>description</b></td>
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115 </tr>
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116 <tr valign=top>
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117 <td>quality</td>
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118 <td>
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119 A decimal float value requesting a desired quality. Libvorbisenc 1.1 allows quality requests in the range of -0.1 (lowest quality, smallest files) through +1.0 (highest-quality, largest files). Quality -0.1 is intended as an ultra-low setting in which low bitrate is much more important than quality consistency. Quality settings 0.0 and above are intended to produce consistent results at all times.
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120
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121 </td>
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122 </tr>
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123 </table>
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124
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125 <a name="BBR">
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126 <h3>managed bitrate modes</h3>
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127
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128 Although the Vorbis codec is natively VBR, libvorbis includes
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129 infrastructure for 'managing' the bitrate of streams by setting
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130 minimum and maximum usage constraints, as well as functionality for
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131 nudging a stream toward a desired average value. These features
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132 should <em>only</em> be used when there is a requirement to limit
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133 bitrate in some way. Although the difference is usually slight,
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134 managed bitrate modes will always produce output inferior to VBR
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135 (given equal bitrate usage). Setting overly or impossibly tight
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136 bitrate management requirements can affect output quality dramatically
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137 for the worse.<p>
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138
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139 Beginning in libvorbis 1.1, bitrate management is implemented using a
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140 <em>bit-reservoir</em> algorithm. The encoder has a fixed-size
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141 reservoir used as a 'savings account' in encoding. When a frame is
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142 smaller than the target rate, the unused bits go into the reservoir so
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143 that they may be used by future frames. When a frame is larger than
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144 target bitrate, it draws 'banked' bits out of the reservoir. Encoding
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145 is managed so that the reservoir never goes negative (when a maximum
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146 bitrate is specified) or fills beyond a fixed limit (when a minimum
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147 bitrate is specified). An 'average bitrate' request is used as the
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148 set-point in a long-range bitrate tracker which adjusts the encoder's
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149 aggressiveness up or down depending on whether or not frames are coming
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150 in larger or smaller than the requested average point.
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151
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152 <p>
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153 <table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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154 <tr bgcolor=#cccccc>
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155 <td><b>parameter</b></td>
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156 <td><b>description</b></td>
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157 </tr>
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158 <tr valign=top>
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159 <td>maximum bitrate</td> <td> The maximum allowed bitrate, set in bits
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160 per second. If the bitrate would otherwise rise such that oversized
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161 frames would underflow the bit-reservoir by consuming banked bits,
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162 bitrate management will force the encoder to use fewer bits per frame
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163 by encoding with a more aggressive psychoacoustic model.<p> This
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164 setting is a hard limit; the bitstream will never be allowed, under
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165 any circumstances, to increase above the specified bitrate over the
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166 average period set by the reservoir; it may momentarily rise over if
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167 inspected on a granularity much finer than the average period across
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168 the reservoir. Normally, the encoder will conserve bits gracefully by
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169 using more aggressive psychoacoustics to shrink a frame when forced
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170 to. However, if the encoder runs out of means of gracefully shrinking
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171 a frame, it will simply take the smallest frame it can otherwise
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172 generate and truncate it to the maximum allowed length. Note that
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173 this is not an error and although it will obviously adversely affect
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174 audio quality, a Vorbis decoder will be able to decode a truncated
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175 frame into audio.
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176
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177 </td>
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178 </tr>
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179
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180 <tr valign=top>
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181 <td>average bitrate</td>
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182
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183 <td>
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184
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185 The average desired bitrate of a stream, set
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186 in bits per second. Average bitrate is tracked via a reservoir like
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187 minimum and maximum bitrate, however the averaging reservior does not
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188 impose a hard limit; it is used to nudge the bitrate toward the
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189 desired average by slowly adjusting the psychoacoustic aggressiveness.
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190 As such, the reservoir size does not affect the average bitrate
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191 behavior. Because this setting alone is not used to impose hard
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192 bitrate limits, the bitrate of a stream produced using only the
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193 <tt>average bitrate</tt> constraint will track the average over time
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194 but not necessarily adhere strictly to that average for any given
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195 period. Should a strict localized average be required, <tt>average
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196 bitrate</tt> should be used along with <tt>minimum bitrate</tt> and
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197 <tt>maximum bitrate</tt>.
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198 </td>
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199
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200 </tr>
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201
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202 <tr valign=top>
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203 <td>minimum bitrate</td>
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204 <td>
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205 The minimum allowed bitrate, set in bits per second. If
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206 the bitrate would otherwise fall such that undersized frames would
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207 overflow the bit-reservoir with unused bits, bitrate management will
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208 force the encoder to use more bits per frame by encoding with a less
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209 aggressive psychoacoustic model.<p> This setting is a hard limit; the
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210 bitstream will never be allowed, under any circumstances, to drop
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211 below the specified bitrate over the average period set by the
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212 reservoir; it may momentarily fall under if inspected on a granularity
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213 much finer than the average period across the reservoir. Normally,
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214 the encoder will fill out undersided frames with additional useful
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215 coding information by increasing the perceived quality of the stream.
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216 If the encoder runs out of useful ways to consume more bits, it will
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217 pad frames out with zeroes.
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218 </td>
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219 </tr>
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220
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221 <tr valign=top>
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222 <td>reservoir size</td> <td> The size of the minimum/maximum bitrate
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223 tracking reservoir, set in bits. The reservoir is used as a 'bit
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224 bank' to average out localized surges and dips in bitrate while
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225 providing predictable, guaranteed buffering behavior for streams to be
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226 used in situations with constrained transport bandwidth. The default
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227 setting is two seconds of average bitrate.<p>
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228
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229 When a single frame is larger than the maximum allowed overall
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230 bitrate, the bits are 'borrowed' from the bitrate reservoir; if the
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231 reservoir contains insufficient bits to cover the defecit, the encoder
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232 must find some way to reduce the frame size. <p>
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233
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234 When a frame is under the minimum limit, the surplus bits are placed
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235 into the reservoir, banking them for future use. If the reservoir is
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236 already full of banked bits, the encoder is forced to find some way to
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237 make the frame larger.<p>
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238
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239 If the frame size is between the minimum and maximum rates (thus
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240 implying the minimum and maximum allowed rates are different), the
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241 reservoir gravitates toward a fill point configured by the
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242 <tt>reservoir bias</tt> setting described next. If the reservoir is
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243 fuller than the fill point (a 'surplus of surplus'), the encoder will
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244 consume a number bits from the reservoir equal to the number of the
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245 bits by which the frame exceeds minimum size. If the reservoir is
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246 emptier than the fillpoint (a 'surplus of defecit'), bits are returned
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247 to the reservoir equaling the current frame's number of bits under the
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248 maximum frame size. The idea of the fill point is to buffer against
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249 both underruns and overruns, by trying to hold the reservoir to a
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250 middle course.
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251 </td>
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252 </tr>
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253
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254 <tr valign=top>
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255 <td>reservoir bias</td>
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256
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257 <td>
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258
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259 Reservoir bias is a setting between 0.0 and 1.0 that biases bitrate
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260 management toward smoothing bitrate spikes (0.0) or bitrate peaks
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261 (1.0); the default setting is 0.1.<p>
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262
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263 Using settings toward 0.0 causes the bitrate manager to hoard bits in
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264 the bit reservoir such that there is a large pool of banked surplus to
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265 draw upon during short spikes in bitrate. As a result, the encoder
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266 will react less aggressively and less drastically to curtail framesize
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267 during brief surges in bitrate.<p>
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268
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269 Using settings toward 1.0 causes the bitrate manager to empty the bit
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270 reservoir such that there is a large buffer available to store surplus
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271 bits during sudden drops in bitrate. As a result, the encoder will
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272 react less aggressively and less drastically to support minimum frame
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273 sizes during drops in bitrate and will tend not to store any extra
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274 bits in the reservoir for future bitrate spikes.<p>
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275
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276 </td>
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277 </tr>
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278
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279 <tr valign=top>
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280 <td>average track damping</td>
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281 <td>
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282
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283 A decimal value, in seconds, that controls how quickly the average
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284 bitrate tracker is allowed to slew from enforcing minimum frame sizes
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285 to maximum framesizes and vice versa. Default value is 1.5
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286 seconds.<p>
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287
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288 When the 'average bitrate' setting is in use, the average bitrate
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289 tracker uses an unbounded reservoir to track overall bitrate-to-date
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290 in the stream. When bitrates are too low, the tracker will try to
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291 nudge bitrates up and when the bitrate is too high, nudge it down.
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292 The damping value regulates the maximum strength of the nudge; it
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293 describes, in seconds, how quickly the tracker may transition from an
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294 extreme nudge in one direction to an extreme nudge in the other.<p>
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295
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296 </td>
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297 </tr>
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298
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299 </table>
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300
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301 <h3>encoding model adjustments</h3>
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302
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303 The <a href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> call provides
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304 a generalized interface for making encoding setup adjustments to the
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305 basic high-level setup provided by <a
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306 href="vorbis_encode_setup_vbr.html">vorbis_encode_setup_vbr()</a> or <a
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307 href="vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()</a>.
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308 In reality, these two calls use <a
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309 href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> internally, and <a
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310 href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> can be used to adjust
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311 most of the parameters set by other calls.<p>
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312
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313 In Vorbis 1.1, <a href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> can
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314 adjust the following additional parameters not described elsewhere:
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315
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316 <p>
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317 <table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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318 <tr bgcolor=#cccccc>
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319 <td><b>parameter</b></td>
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320 <td><b>description</b></td>
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321 </tr>
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322 <tr valign=top>
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323 <td>management mode</td> <td> Configures whether or not bitrate
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324 management is in use or not. Normally, this value is set implicitly
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325 during encoding setup; however, the supported means of selecting a
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326 quality mode by bitrate (that is, requesting a true VBR stream, but
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327 doing so by asking for an approximate bitrate) is to use <a
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328 href="vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()</a>
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329 and then to explicitly turn off bitrate management by calling <a
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330 href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> with <a
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331 href="vorbis_encode_ctl.html#OV_ECTL_RATEMANAGE2_SET">OV_ECTL_RATEMANAGE2_SET</a>
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332 </td>
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333 </tr>
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334
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335 <tr valign=top>
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336 <td>coupling</td> <td> Stereo encoding (and in the future, surround
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337 encodings) are normally encoded assuming the channels form a stereo
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338 image and that lossy-stereo modelling is appropriate; this is called
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339 'coupling'. Stereo coupling may be explicitly enabled or disabled.
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340 </td>
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341 </tr>
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342 <tr valign=top>
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343 <td>lowpass</td> <td> Sets the hard lowpass of a given encoding mode;
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344 this may be used to conserve a few bits in high-rate audio that has
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345 limited bandwidth, or in testing of the encoder's acoustic model. The
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346 encoder is generally already configured with ideal lowpasses (if any
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347 at all) for given modes; use of this parameter is strongly discouraged
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348 if the point is to try to 'improve' a given encoding mode for general
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349 encoding.
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350 </td>
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351 </tr>
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352
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353 <tr valign=top>
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354 <td>impulse coding aggressiveness</td> <td>By default, libvorbis
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355 attempts to compromise between preventing wide bitrate swings and
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356 high-resolution impulse coding (which is required for the crispest
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357 possible attacks, but also requires a relatively large momentary
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358 bitrate increase). This parameter allows an application to tune the
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359 compromise or eliminate it; A value of 0.0 indicates normal behavior
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360 while a value of -15.0 requests maximum possible impulse
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361 resolution.</td>
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362 </tr>
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363
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364 </table>
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365
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366
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367 <br><br>
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368 <hr noshade>
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369 <table border=0 width=100%>
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370 <tr valign=top>
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371 <td><p class=tiny>copyright © 2000-2010 Xiph.Org</p></td>
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372 <td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/index.html">Ogg Vorbis</a></p></td>
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373 </tr><tr>
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374 <td><p class=tiny>libvorbisenc documentation</p></td>
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375 <td align=right><p class=tiny>libvorbisenc version 1.3.2 - 20101101</p></td>
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376 </tr>
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377 </table>
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378
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379 </body>
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380
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381 </html>
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382
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