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