383 lines
15 KiB
HTML
383 lines
15 KiB
HTML
<html>
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<head>
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<title>libvorbisenc - API Overview</title>
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<link rel=stylesheet href="style.css" type="text/css">
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</head>
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<body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff">
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<table border=0 width=100%>
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<tr>
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<td><p class=tiny>libvorbisenc documentation</p></td>
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<td align=right><p class=tiny>libvorbisenc version 1.3.2 - 20101101</p></td>
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</tr>
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</table>
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<h1>Libvorbisenc API Overview</h1>
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<p>Libvorbisenc is an encoding convenience library intended to
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encapsulate the elaborate setup that libvorbis requires for encoding.
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Libvorbisenc gives easy access to all high-level adjustments an
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application may require when encoding and also exposes some low-level
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tuning parameters to allow applications to make detailed adjustments
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to the encoding process. <p>
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All the <b>libvorbisenc</b> routines are declared in "vorbis/vorbisenc.h".
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<em>Note: libvorbis and libvorbisenc always
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encode in a single pass. Thus, all possible encoding setups will work
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properly with live input and produce streams that decode properly when
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streamed. See the subsection titled <a href="#BBR">"managed bitrate
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modes"</a> for details on setting limits on bitrate usage when Vorbis
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streams are used in a limited-bandwidth environment.</em>
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<h2>workflow</h2>
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<p>Libvorbisenc is used only during encoder setup; its function
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is to automate initialization of a multitude of settings in a
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<tt>vorbis_info</tt> structure which libvorbis then uses as a reference
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during the encoding process. Libvorbisenc plays no part in the
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encoding process after setup.
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<p>Encode setup using libvorbisenc consists of three steps:
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<ol>
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<li>high-level initialization of a <tt>vorbis_info</tt> structure by
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calling one of <a
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href="vorbis_encode_setup_vbr.html">vorbis_encode_setup_vbr()</a> or <a
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href="vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()</a>
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with the basic input audio parameters (rate and channels) and the
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basic desired encoded audio output parameters (VBR quality or ABR/CBR
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bitrate)<p>
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<li>optional adjustment of the basic setup defaults using <a
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href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a><p>
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<li>calling <a
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href="vorbis_encode_setup_init.html">vorbis_encode_setup_init()</a> to
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finalize the high-level setup into the detailed low-level reference
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values needed by libvorbis to encode audio. The <tt>vorbis_info</tt>
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structure is then ready to use for encoding by libvorbis.<p>
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</ol>
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These three steps can be collapsed into a single call by using <a
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href="vorbis_encode_init_vbr.html">vorbis_encode_init_vbr</a> to set up a
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quality-based VBR stream or <a
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href="vorbis_encode_init.html">vorbis_encode_init</a> to set up a managed
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bitrate (ABR or CBR) stream.<p>
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<h2>adjustable encoding parameters</h2>
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<h3>input audio parameters</h3>
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<p>
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<table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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<tr bgcolor=#cccccc>
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<td><b>parameter</b></td>
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<td><b>description</b></td>
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</tr>
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<tr valign=top>
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<td>sampling rate</td>
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<td>
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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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</td>
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</tr>
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<tr valign=top>
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<td>channels</td>
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<td>
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The number of channels encoded in each input sample. By default,
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stereo input modes (two channels) are 'coupled' by Vorbis 1.1 such
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that the stereo relationship between the samples is taken into account
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when encoding. Stereo coupling my be disabled by using <a
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href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> with <a
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href="vorbis_encode_ctl.html#OV_ECTL_COUPLE_SET">OV_ECTL_COUPLE_SET</a>.
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</td>
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</tr>
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</table>
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<h3>quality and VBR modes</h3>
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Vorbis is natively a VBR codec; a user requests a given constant
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<em>quality</em> and the encoder keeps the encoding quality constant
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while allowing the bitrate to vary. 'Quality' modes (Variable BitRate)
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will always produce the most consistent encoding results as well as
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the highest quality for the amount of bits used.
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<p>
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<table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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<tr bgcolor=#cccccc>
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<td><b>parameter</b></td>
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<td><b>description</b></td>
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</tr>
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<tr valign=top>
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<td>quality</td>
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<td>
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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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</td>
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</tr>
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</table>
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<a name="BBR">
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<h3>managed bitrate modes</h3>
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Although the Vorbis codec is natively VBR, libvorbis includes
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infrastructure for 'managing' the bitrate of streams by setting
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minimum and maximum usage constraints, as well as functionality for
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nudging a stream toward a desired average value. These features
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should <em>only</em> be used when there is a requirement to limit
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bitrate in some way. Although the difference is usually slight,
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managed bitrate modes will always produce output inferior to VBR
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(given equal bitrate usage). Setting overly or impossibly tight
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bitrate management requirements can affect output quality dramatically
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for the worse.<p>
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Beginning in libvorbis 1.1, bitrate management is implemented using a
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<em>bit-reservoir</em> algorithm. The encoder has a fixed-size
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reservoir used as a 'savings account' in encoding. When a frame is
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smaller than the target rate, the unused bits go into the reservoir so
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that they may be used by future frames. When a frame is larger than
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target bitrate, it draws 'banked' bits out of the reservoir. Encoding
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is managed so that the reservoir never goes negative (when a maximum
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bitrate is specified) or fills beyond a fixed limit (when a minimum
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bitrate is specified). An 'average bitrate' request is used as the
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set-point in a long-range bitrate tracker which adjusts the encoder's
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aggressiveness up or down depending on whether or not frames are coming
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in larger or smaller than the requested average point.
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<p>
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<table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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<tr bgcolor=#cccccc>
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<td><b>parameter</b></td>
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<td><b>description</b></td>
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</tr>
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<tr valign=top>
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<td>maximum bitrate</td> <td> The maximum allowed bitrate, set in bits
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per second. If the bitrate would otherwise rise such that oversized
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frames would underflow the bit-reservoir by consuming banked bits,
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bitrate management will force the encoder to use fewer bits per frame
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by encoding with a more aggressive psychoacoustic model.<p> This
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setting is a hard limit; the bitstream will never be allowed, under
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any circumstances, to increase above the specified bitrate over the
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average period set by the reservoir; it may momentarily rise over if
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inspected on a granularity much finer than the average period across
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the reservoir. Normally, the encoder will conserve bits gracefully by
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using more aggressive psychoacoustics to shrink a frame when forced
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to. However, if the encoder runs out of means of gracefully shrinking
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a frame, it will simply take the smallest frame it can otherwise
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generate and truncate it to the maximum allowed length. Note that
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this is not an error and although it will obviously adversely affect
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audio quality, a Vorbis decoder will be able to decode a truncated
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frame into audio.
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</td>
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</tr>
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<tr valign=top>
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<td>average bitrate</td>
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<td>
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The average desired bitrate of a stream, set
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in bits per second. Average bitrate is tracked via a reservoir like
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minimum and maximum bitrate, however the averaging reservior does not
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impose a hard limit; it is used to nudge the bitrate toward the
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desired average by slowly adjusting the psychoacoustic aggressiveness.
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As such, the reservoir size does not affect the average bitrate
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behavior. Because this setting alone is not used to impose hard
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bitrate limits, the bitrate of a stream produced using only the
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<tt>average bitrate</tt> constraint will track the average over time
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but not necessarily adhere strictly to that average for any given
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period. Should a strict localized average be required, <tt>average
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bitrate</tt> should be used along with <tt>minimum bitrate</tt> and
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<tt>maximum bitrate</tt>.
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</td>
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</tr>
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<tr valign=top>
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<td>minimum bitrate</td>
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<td>
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The minimum allowed bitrate, set in bits per second. If
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the bitrate would otherwise fall such that undersized frames would
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overflow the bit-reservoir with unused bits, bitrate management will
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force the encoder to use more bits per frame by encoding with a less
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aggressive psychoacoustic model.<p> This setting is a hard limit; the
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bitstream will never be allowed, under any circumstances, to drop
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below the specified bitrate over the average period set by the
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reservoir; it may momentarily fall under if inspected on a granularity
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much finer than the average period across the reservoir. Normally,
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the encoder will fill out undersided frames with additional useful
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coding information by increasing the perceived quality of the stream.
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If the encoder runs out of useful ways to consume more bits, it will
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pad frames out with zeroes.
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</td>
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</tr>
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<tr valign=top>
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<td>reservoir size</td> <td> The size of the minimum/maximum bitrate
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tracking reservoir, set in bits. The reservoir is used as a 'bit
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bank' to average out localized surges and dips in bitrate while
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providing predictable, guaranteed buffering behavior for streams to be
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used in situations with constrained transport bandwidth. The default
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setting is two seconds of average bitrate.<p>
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When a single frame is larger than the maximum allowed overall
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bitrate, the bits are 'borrowed' from the bitrate reservoir; if the
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reservoir contains insufficient bits to cover the defecit, the encoder
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must find some way to reduce the frame size. <p>
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When a frame is under the minimum limit, the surplus bits are placed
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into the reservoir, banking them for future use. If the reservoir is
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already full of banked bits, the encoder is forced to find some way to
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make the frame larger.<p>
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If the frame size is between the minimum and maximum rates (thus
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implying the minimum and maximum allowed rates are different), the
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reservoir gravitates toward a fill point configured by the
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<tt>reservoir bias</tt> setting described next. If the reservoir is
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fuller than the fill point (a 'surplus of surplus'), the encoder will
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consume a number bits from the reservoir equal to the number of the
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bits by which the frame exceeds minimum size. If the reservoir is
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emptier than the fillpoint (a 'surplus of defecit'), bits are returned
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to the reservoir equaling the current frame's number of bits under the
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maximum frame size. The idea of the fill point is to buffer against
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both underruns and overruns, by trying to hold the reservoir to a
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middle course.
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</td>
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</tr>
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<tr valign=top>
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<td>reservoir bias</td>
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<td>
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Reservoir bias is a setting between 0.0 and 1.0 that biases bitrate
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management toward smoothing bitrate spikes (0.0) or bitrate peaks
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(1.0); the default setting is 0.1.<p>
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Using settings toward 0.0 causes the bitrate manager to hoard bits in
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the bit reservoir such that there is a large pool of banked surplus to
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draw upon during short spikes in bitrate. As a result, the encoder
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will react less aggressively and less drastically to curtail framesize
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during brief surges in bitrate.<p>
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Using settings toward 1.0 causes the bitrate manager to empty the bit
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reservoir such that there is a large buffer available to store surplus
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bits during sudden drops in bitrate. As a result, the encoder will
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react less aggressively and less drastically to support minimum frame
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sizes during drops in bitrate and will tend not to store any extra
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bits in the reservoir for future bitrate spikes.<p>
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</td>
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</tr>
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<tr valign=top>
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<td>average track damping</td>
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<td>
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A decimal value, in seconds, that controls how quickly the average
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bitrate tracker is allowed to slew from enforcing minimum frame sizes
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to maximum framesizes and vice versa. Default value is 1.5
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seconds.<p>
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When the 'average bitrate' setting is in use, the average bitrate
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tracker uses an unbounded reservoir to track overall bitrate-to-date
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in the stream. When bitrates are too low, the tracker will try to
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nudge bitrates up and when the bitrate is too high, nudge it down.
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The damping value regulates the maximum strength of the nudge; it
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describes, in seconds, how quickly the tracker may transition from an
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extreme nudge in one direction to an extreme nudge in the other.<p>
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</td>
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</tr>
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</table>
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<h3>encoding model adjustments</h3>
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The <a href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> call provides
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a generalized interface for making encoding setup adjustments to the
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basic high-level setup provided by <a
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href="vorbis_encode_setup_vbr.html">vorbis_encode_setup_vbr()</a> or <a
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href="vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()</a>.
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In reality, these two calls use <a
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href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> internally, and <a
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href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> can be used to adjust
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most of the parameters set by other calls.<p>
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In Vorbis 1.1, <a href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> can
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adjust the following additional parameters not described elsewhere:
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<p>
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<table border=1 color=black width=50% cellspacing=0 cellpadding=7>
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<tr bgcolor=#cccccc>
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<td><b>parameter</b></td>
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<td><b>description</b></td>
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</tr>
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<tr valign=top>
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<td>management mode</td> <td> Configures whether or not bitrate
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management is in use or not. Normally, this value is set implicitly
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during encoding setup; however, the supported means of selecting a
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quality mode by bitrate (that is, requesting a true VBR stream, but
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doing so by asking for an approximate bitrate) is to use <a
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href="vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()</a>
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and then to explicitly turn off bitrate management by calling <a
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href="vorbis_encode_ctl.html">vorbis_encode_ctl()</a> with <a
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href="vorbis_encode_ctl.html#OV_ECTL_RATEMANAGE2_SET">OV_ECTL_RATEMANAGE2_SET</a>
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</td>
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</tr>
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<tr valign=top>
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<td>coupling</td> <td> Stereo encoding (and in the future, surround
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encodings) are normally encoded assuming the channels form a stereo
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image and that lossy-stereo modelling is appropriate; this is called
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'coupling'. Stereo coupling may be explicitly enabled or disabled.
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</td>
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</tr>
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<tr valign=top>
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<td>lowpass</td> <td> Sets the hard lowpass of a given encoding mode;
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this may be used to conserve a few bits in high-rate audio that has
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limited bandwidth, or in testing of the encoder's acoustic model. The
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encoder is generally already configured with ideal lowpasses (if any
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at all) for given modes; use of this parameter is strongly discouraged
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if the point is to try to 'improve' a given encoding mode for general
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encoding.
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</td>
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</tr>
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<tr valign=top>
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<td>impulse coding aggressiveness</td> <td>By default, libvorbis
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attempts to compromise between preventing wide bitrate swings and
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high-resolution impulse coding (which is required for the crispest
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possible attacks, but also requires a relatively large momentary
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bitrate increase). This parameter allows an application to tune the
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compromise or eliminate it; A value of 0.0 indicates normal behavior
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while a value of -15.0 requests maximum possible impulse
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resolution.</td>
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</tr>
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</table>
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<br><br>
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<hr noshade>
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<table border=0 width=100%>
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<tr valign=top>
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<td><p class=tiny>copyright © 2000-2010 Xiph.Org</p></td>
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<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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</tr><tr>
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<td><p class=tiny>libvorbisenc documentation</p></td>
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<td align=right><p class=tiny>libvorbisenc version 1.3.2 - 20101101</p></td>
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</tr>
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</table>
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</body>
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</html>
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