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diff --git a/vorbis/doc/vorbisenc/overview.html b/vorbis/doc/vorbisenc/overview.html deleted file mode 100644 index 51af7b5..0000000 --- a/vorbis/doc/vorbisenc/overview.html +++ /dev/null @@ -1,382 +0,0 @@ -<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 © 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> - |