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