4 <title>libvorbisenc - API Overview
</title>
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11 <td><p class=tiny
>libvorbisenc documentation
</p></td>
12 <td align=right
><p class=tiny
>libvorbisenc release
1.1 -
20040709</p></td>
16 <h1>Libvorbisenc API Overview
</h1>
18 <p>Libvorbisenc is an encoding convenience library intended to
19 encapsulate the elaborate setup that libvorbis requires for encoding.
20 Libvorbisenc gives easy access to all high-level adjustments an
21 application may require when encoding and also exposes some low-level
22 tuning parameters to allow applications to make detailed adjustments
23 to the encoding process.
<p>
25 All the
<b>libvorbisenc
</b> routines are declared in
"vorbis/vorbisenc.h".
27 <em>Note: libvorbis and libvorbisenc always
28 encode in a single pass. Thus, all possible encoding setups will work
29 properly with live input and produce streams that decode properly when
30 streamed. See the subsection titled
<a href=
"#BBR">"managed bitrate
31 modes"</a> for details on setting limits on bitrate usage when Vorbis
32 streams are used in a limited-bandwidth environment.
</em>
36 <p>Libvorbisenc is used only during encoder setup; its function
37 is to automate initialization of a multitude of settings in a
38 <tt>vorbis_info
</tt> structure which libvorbis then uses as a reference
39 during the encoding process. Libvorbisenc plays no part in the
40 encoding process after setup.
42 <p>Encode setup using libvorbisenc consists of three steps:
45 <li>high-level initialization of a
<tt>vorbis_info
</tt> structure by
47 href=
"vorbis_encode_setup_vbr.html">vorbis_encode_setup_vbr()
</a> or
<a
48 href=
"vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()
</a>
49 with the basic input audio parameters (rate and channels) and the
50 basic desired encoded audio output parameters (VBR quality or ABR/CBR
53 <li>optional adjustment of the basic setup defaults using
<a
54 href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a><p>
57 href=
"vorbis_encode_setup_init.html">vorbis_encode_setup_init()
</a> to
58 finalize the high-level setup into the detailed low-level reference
59 values needed by libvorbis to encode audio. The
<tt>vorbis_info
</tt>
60 structure is then ready to use for encoding by libvorbis.
<p>
64 These three steps can be collapsed into a single call by using
<a
65 href=
"vorbis_encode_init_vbr.html">vorbis_encode_init_vbr
</a> to set up a
66 quality-based VBR stream or
<a
67 href=
"vorbis_encode_init.html">vorbis_encode_init
</a> to set up a managed
68 bitrate (ABR or CBR) stream.
<p>
70 <h2>adjustable encoding parameters
</h2>
72 <h3>input audio parameters
</h3>
75 <table border=
1 color=black width=
50% cellspacing=
0 cellpadding=
7>
77 <td><b>parameter
</b></td>
78 <td><b>description
</b></td>
81 <td>sampling rate
</td>
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.
91 The number of channels encoded in each input sample. By default,
92 stereo input modes (two channels) are 'coupled' by Vorbis
1.1 such
93 that the stereo relationship between the samples is taken into account
94 when encoding. Stereo coupling my be disabled by using
<a
95 href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a> with
<a
96 href=
"vorbis_encode_ctl.html#OV_ECTL_COUPLE_SET">OV_ECTL_COUPLE_SET
</a>.
102 <h3>quality and VBR modes
</h3>
104 Vorbis is natively a VBR codec; a user requests a given constant
105 <em>quality
</em> and the encoder keeps the encoding quality constant
106 while allowing the bitrate to vary. 'Quality' modes (Variable BitRate)
107 will always produce the most consistent encoding results as well as
108 the highest quality for the amount of bits used.
111 <table border=
1 color=black width=
50% cellspacing=
0 cellpadding=
7>
113 <td><b>parameter
</b></td>
114 <td><b>description
</b></td>
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.
126 <h3>managed bitrate modes
</h3>
128 Although the Vorbis codec is natively VBR, libvorbis includes
129 infrastructure for 'managing' the bitrate of streams by setting
130 minimum and maximum usage constraints, as well as functionality for
131 nudging a stream toward a desired average value. These features
132 should
<em>only
</em> be used when there is a requirement to limit
133 bitrate in some way. Although the difference is usually slight,
134 managed bitrate modes will always produce output inferior to VBR
135 (given equal bitrate usage). Setting overly or impossibly tight
136 bitrate management requirements can affect output quality dramatically
139 Beginning in libvorbis
1.1, bitrate management is implemented using a
140 <em>bit-reservoir
</em> algorithm. The encoder has a fixed-size
141 reservoir used as a 'savings account' in encoding. When a frame is
142 smaller than the target rate, the unused bits go into the reservoir so
143 that they may be used by future frames. When a frame is larger than
144 target bitrate, it draws 'banked' bits out of the reservoir. Encoding
145 is managed so that the reservoir never goes negative (when a maximum
146 bitrate is specified) or fills beyond a fixed limit (when a minimum
147 bitrate is specified). An 'average bitrate' request is used as the
148 set-point in a long-range bitrate tracker which adjusts the encoder's
149 aggressiveness up or down depending on whether or not frames are coming
150 in larger or smaller than the requested average point.
153 <table border=
1 color=black width=
50% cellspacing=
0 cellpadding=
7>
155 <td><b>parameter
</b></td>
156 <td><b>description
</b></td>
159 <td>maximum bitrate
</td> <td> The maximum allowed bitrate, set in bits
160 per second. If the bitrate would otherwise rise such that oversized
161 frames would underflow the bit-reservoir by consuming banked bits,
162 bitrate management will force the encoder to use fewer bits per frame
163 by encoding with a more aggressive psychoacoustic model.
<p> This
164 setting is a hard limit; the bitstream will never be allowed, under
165 any circumstances, to increase above the specified bitrate over the
166 average period set by the reservoir; it may momentarily rise over if
167 inspected on a granularity much finer than the average period across
168 the reservoir. Normally, the encoder will conserve bits gracefully by
169 using more aggressive psychoacoustics to shrink a frame when forced
170 to. However, if the encoder runs out of means of gracefully shrinking
171 a frame, it will simply take the smallest frame it can otherwise
172 generate and truncate it to the maximum allowed length. Note that
173 this is not an error and although it will obviously adversely affect
174 audio quality, a Vorbis decoder will be able to decode a truncated
181 <td>average bitrate
</td>
185 The average desired bitrate of a stream, set
186 in bits per second. Average bitrate is tracked via a reservoir like
187 minimum and maximum bitrate, however the averaging reservior does not
188 impose a hard limit; it is used to nudge the bitrate toward the
189 desired average by slowly adjusting the psychoacoustic aggressiveness.
190 As such, the reservoir size does not affect the average bitrate
191 behavior. Because this setting alone is not used to impose hard
192 bitrate limits, the bitrate of a stream produced using only the
193 <tt>average bitrate
</tt> constraint will track the average over time
194 but not necessarily adhere strictly to that average for any given
195 period. Should a strict localized average be required,
<tt>average
196 bitrate
</tt> should be used along with
<tt>minimum bitrate
</tt> and
197 <tt>maximum bitrate
</tt>.
203 <td>minimum bitrate
</td>
205 The minimum allowed bitrate, set in bits per second. If
206 the bitrate would otherwise fall such that undersized frames would
207 overflow the bit-reservoir with unused bits, bitrate management will
208 force the encoder to use more bits per frame by encoding with a less
209 aggressive psychoacoustic model.
<p> This setting is a hard limit; the
210 bitstream will never be allowed, under any circumstances, to drop
211 below the specified bitrate over the average period set by the
212 reservoir; it may momentarily fall under if inspected on a granularity
213 much finer than the average period across the reservoir. Normally,
214 the encoder will fill out undersided frames with additional useful
215 coding information by increasing the perceived quality of the stream.
216 If the encoder runs out of useful ways to consume more bits, it will
217 pad frames out with zeroes.
222 <td>reservoir size
</td> <td> The size of the minimum/maximum bitrate
223 tracking reservoir, set in bits. The reservoir is used as a 'bit
224 bank' to average out localized surges and dips in bitrate while
225 providing predictable, guaranteed buffering behavior for streams to be
226 used in situations with constrained transport bandwidth. The default
227 setting is two seconds of average bitrate.
<p>
229 When a single frame is larger than the maximum allowed overall
230 bitrate, the bits are 'borrowed' from the bitrate reservoir; if the
231 reservoir contains insufficient bits to cover the defecit, the encoder
232 must find some way to reduce the frame size.
<p>
234 When a frame is under the minimum limit, the surplus bits are placed
235 into the reservoir, banking them for future use. If the reservoir is
236 already full of banked bits, the encoder is forced to find some way to
237 make the frame larger.
<p>
239 If the frame size is between the minimum and maximum rates (thus
240 implying the minimum and maximum allowed rates are different), the
241 reservoir gravitates toward a fill point configured by the
242 <tt>reservoir bias
</tt> setting described next. If the reservoir is
243 fuller than the fill point (a 'surplus of surplus'), the encoder will
244 consume a number bits from the reservoir equal to the number of the
245 bits by which the frame exceeds minimum size. If the reservoir is
246 emptier than the fillpoint (a 'surplus of defecit'), bits are returned
247 to the reservoir equaling the current frame's number of bits under the
248 maximum frame size. The idea of the fill point is to buffer against
249 both underruns and overruns, by trying to hold the reservoir to a
255 <td>reservoir bias
</td>
259 Reservoir bias is a setting between
0.0 and
1.0 that biases bitrate
260 management toward smoothing bitrate spikes (
0.0) or bitrate peaks
261 (
1.0); the default setting is
0.1.
<p>
263 Using settings toward
0.0 causes the bitrate manager to hoard bits in
264 the bit reservoir such that there is a large pool of banked surplus to
265 draw upon during short spikes in bitrate. As a result, the encoder
266 will react less aggressively and less drastically to curtail framesize
267 during brief surges in bitrate.
<p>
269 Using settings toward
1.0 causes the bitrate manager to empty the bit
270 reservoir such that there is a large buffer available to store surplus
271 bits during sudden drops in bitrate. As a result, the encoder will
272 react less aggressively and less drastically to support minimum frame
273 sizes during drops in bitrate and will tend not to store any extra
274 bits in the reservoir for future bitrate spikes.
<p>
280 <td>average track damping
</td>
283 A decimal value, in seconds, that controls how quickly the average
284 bitrate tracker is allowed to slew from enforcing minimum frame sizes
285 to maximum framesizes and vice versa. Default value is
1.5
288 When the 'average bitrate' setting is in use, the average bitrate
289 tracker uses an unbounded reservoir to track overall bitrate-to-date
290 in the stream. When bitrates are too low, the tracker will try to
291 nudge bitrates up and when the bitrate is too high, nudge it down.
292 The damping value regulates the maximum strength of the nudge; it
293 describes, in seconds, how quickly the tracker may transition from an
294 extreme nudge in one direction to an extreme nudge in the other.
<p>
301 <h3>encoding model adjustments
</h3>
303 The
<a href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a> call provides
304 a generalized interface for making encoding setup adjustments to the
305 basic high-level setup provided by
<a
306 href=
"vorbis_encode_setup_vbr.html">vorbis_encode_setup_vbr()
</a> or
<a
307 href=
"vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()
</a>.
308 In reality, these two calls use
<a
309 href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a> internally, and
<a
310 href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a> can be used to adjust
311 most of the parameters set by other calls.
<p>
313 In Vorbis
1.1,
<a href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a> can
314 adjust the following additional parameters not described elsewhere:
317 <table border=
1 color=black width=
50% cellspacing=
0 cellpadding=
7>
319 <td><b>parameter
</b></td>
320 <td><b>description
</b></td>
323 <td>management mode
</td> <td> Configures whether or not bitrate
324 management is in use or not. Normally, this value is set implicitly
325 during encoding setup; however, the supported means of selecting a
326 quality mode by bitrate (that is, requesting a true VBR stream, but
327 doing so by asking for an approximate bitrate) is to use
<a
328 href=
"vorbis_encode_setup_managed.html">vorbis_encode_setup_managed()
</a>
329 and then to explicitly turn off bitrate management by calling
<a
330 href=
"vorbis_encode_ctl.html">vorbis_encode_ctl()
</a> with
<a
331 href=
"vorbis_encode_ctl.html#OV_ECTL_RATEMANAGE2_SET">OV_ECTL_RATEMANAGE2_SET
</a>
336 <td>coupling
</td> <td> Stereo encoding (and in the future, surround
337 encodings) are normally encoded assuming the channels form a stereo
338 image and that lossy-stereo modelling is appropriate; this is called
339 'coupling'. Stereo coupling may be explicitly enabled or disabled.
343 <td>lowpass
</td> <td> Sets the hard lowpass of a given encoding mode;
344 this may be used to conserve a few bits in high-rate audio that has
345 limited bandwidth, or in testing of the encoder's acoustic model. The
346 encoder is generally already configured with ideal lowpasses (if any
347 at all) for given modes; use of this parameter is strongly discouraged
348 if the point is to try to 'improve' a given encoding mode for general
354 <td>impulse coding aggressiveness
</td> <td>By default, libvorbis
355 attempts to compromise between preventing wide bitrate swings and
356 high-resolution impulse coding (which is required for the crispest
357 possible attacks, but also requires a relatively large momentary
358 bitrate increase). This parameter allows an application to tune the
359 compromise or eliminate it; A value of
0.0 indicates normal behavior
360 while a value of -
15.0 requests maximum possible impulse
369 <table border=
0 width=
100%
>
371 <td><p class=tiny
>copyright
© 2004 Vorbis team
</p></td>
372 <td align=right
><p class=tiny
><a href=
"http://www.xiph.org/ogg/vorbis/index.html">Ogg Vorbis
</a><br><a href=
"mailto:team@vorbis.org">team@vorbis.org
</a></p></td>
374 <td><p class=tiny
>libvorbisenc documentation
</p></td>
375 <td align=right
><p class=tiny
>libvorbisenc release
1.1 -
20040709</p></td>