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-Stereo Channel Coupling in the Vorbis CODEC
-</font></h1>
-
-<em>Last update to this document: July 2, 2002</em><br>
-
-<h2>Abstract</h2> The Vorbis audio CODEC provides a channel coupling
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+
+<h1>Ogg Vorbis stereo-specific channel coupling discussion</h1>
+
+<h2>Abstract</h2>
+
+<p>The Vorbis audio CODEC provides a channel coupling
mechanisms designed to reduce effective bitrate by both eliminating
interchannel redundancy and eliminating stereo image information
labeled inaudible or undesirable according to spatial psychoacoustic
-models. This document describes both the mechanical coupling
+models. This document describes both the mechanical coupling
mechanisms available within the Vorbis specification, as well as the
specific stereo coupling models used by the reference
-<tt>libvorbis</tt> CODEC provided by xiph.org.
-
-<h2>Terminology</h2> Terminology as used in this document is based on
-common terminology associated with contemporary CODECs such as MPEG I
-audio layer 3 (mp3). However, some differences in terminology are
-useful in the context of Vorbis as Vorbis functions somewhat
-differently than most current formats. For clarity, a few terms are
-defined beforehand here, and others will be defined where they first
-appear in context.<p>
-
-<h3>Subjective and Objective</h3>
-
-<em>Objective</em> fidelity is a measure, based on a computable,
-mechanical metric, of how carefully an output matches an input. For
-example, a stereo amplifier may claim to introduce less that .01%
-total harmonic distortion when amplifying an input signal; this claim
-is easy to verify given proper equipment, and any number of testers are
-likely to arrive at the same, exact results. One need not listen to
-the equipment to make this measurement.<p>
-
-However, given two amplifiers with identical, verifiable objective
-specifications, listeners may strongly prefer the sound quality of one
-over the other. This is actually the case in the decades old debate
-[some would say jihad] among audiophiles involving vacuum tube versus
-solid state amplifiers. There are people who can tell the difference,
-and strongly prefer one over the other despite seemingly identical,
-measurable quality. This preference is <em>subjective</em> and
-difficult to measure but nonetheless real.
-
-Individual elements of subjective differences often can be qualified,
-but overall subjective quality generally is not measurable. Different
-observers are likely to disagree on the exact results of a subjective
-test as each observer's perspective differs. When measuring
-subjective qualities, the best one can hope for is average, empirical
-results that show statistical significance across a group.<p>
-
-Perceptual codecs are most concerned with subjective, not objective,
-quality. This is why evaluating a perceptual codec via distortion
-measures and sonograms alone is useless; these objective measures may
-provide insight into the quality or functioning of a codec, but cannot
-answer the much squishier subjective question, "Does it sound
-good?". The tube amplifier example is perhaps not the best as very few
-people can hear, or care to hear, the minute differences between tubes
-and transistors, whereas the subjective differences in perceptual
-codecs tend to be quite large even when objective differences are
-not.<p>
-
-<h3>Fidelity, Artifacts and Differences</h3> Audio <em>artifacts</em>
-and loss of fidelity or more simply put, audio <em>differences</em>
-are not the same thing.<p>
-
-A loss of fidelity implies differences between the perceived input and
-output signal; it does not necessarily imply that the differences in
-output are displeasing or that the output sounds poor (although this
-is often the case). Tube amplifiers are <em>not</em> higher fidelity
-than modern solid state and digital systems. They simply produce a
-form of distortion and coloring that is either unnoticeable or actually
-pleasing to many ears.<p>
-
-As compared to an original signal using hard metrics, all perceptual
-codecs [ASPEC, ATRAC, MP3, WMA, AAC, TwinVQ, AC3 and Vorbis included]
-lose objective fidelity in order to reduce bitrate. This is fact. The
-idea is to lose fidelity in ways that cannot be perceived. However,
-most current streaming applications demand bitrates lower than what
-can be achieved by sacrificing only objective fidelity; this is also
-fact, despite whatever various company press releases might claim.
-Subjective fidelity eventually must suffer in one way or another.<p>
-
-The goal is to choose the best possible tradeoff such that the
-fidelity loss is graceful and not obviously noticeable. Most listeners
-of FM radio do not realize how much lower fidelity that medium is as
-compared to compact discs or DAT. However, when compared directly to
-source material, the difference is obvious. A cassette tape is lower
-fidelity still, and yet the degredation, relatively speaking, is
-graceful and generally easy not to notice. Compare this graceful loss
-of quality to an average 44.1kHz stereo mp3 encoded at 80 or 96kbps.
-The mp3 might actually be higher objective fidelity but subjectively
-sounds much worse.<p>
-
-Thus, when a CODEC <em>must</em> sacrifice subjective quality in order
-to satisfy a user's requirements, the result should be a
-<em>difference</em> that is generally either difficult to notice
-without comparison, or easy to ignore. An <em>artifact</em>, on the
-other hand, is an element introduced into the output that is
-immediately noticeable, obviously foreign, and undesired. The famous
-'underwater' or 'twinkling' effect synonymous with low bitrate (or
-poorly encoded) mp3 is an example of an <em>artifact</em>. This
-working definition differs slightly from common usage, but the coined
-distinction between differences and artifacts is useful for our
-discussion.<p>
-
-The goal, when it is absolutely necessary to sacrifice subjective
-fidelity, is obviously to strive for differences and not artifacts.
-The vast majority of CODECs today fail at this task miserably,
-predictably, and regularly in one way or another. Avoiding such
-failures when it is necessary to sacrifice subjective quality is a
-fundamental design objective of Vorbis and that objective is reflected
-in Vorbis's channel coupling design.<p>
+<tt>libvorbis</tt> codec provided by xiph.org.</p>
<h2>Mechanisms</h2>
-In encoder release beta 4 and earlier, Vorbis supported multiple
+<p>In encoder release beta 4 and earlier, Vorbis supported multiple
channel encoding, but the channels were encoded entirely separately
with no cross-analysis or redundancy elimination between channels.
This multichannel strategy is very similar to the mp3's <em>dual
stereo</em> mode and Vorbis uses the same name for its analogous
-uncoupled multichannel modes.
+uncoupled multichannel modes.</p>
-However, the Vorbis spec provides for, and Vorbis release 1.0 rc1 and
-later implement a coupled channel strategy. Vorbis has two specific
+<p>However, the Vorbis spec provides for, and Vorbis release 1.0 rc1 and
+later implement a coupled channel strategy. Vorbis has two specific
mechanisms that may be used alone or in conjunction to implement
-channel coupling. The first is <em>channel interleaving</em> via
-residue backend #2, and the second is <em>square polar mapping</em>.
-These two general mechanisms are particularly well suited to coupling
-due to the structure of Vorbis encoding, as we'll explore below, and
-using both we can implement both totally <em>lossless stereo image
-coupling</em> [bit-for-bit decode-identical to uncoupled modes], as
-well as various lossy models that seek to eliminate inaudible or
-unimportant aspects of the stereo image in order to enhance
-bitrate. The exact coupling implementation is generalized to allow the
-encoder a great deal of flexibility in implementation of a stereo
-model without requiring any significant complexity increase over the
-combinatorically simpler mid/side joint stereo of mp3 and other
-current audio codecs.<p>
-
-An encoder may apply channel coupling directly to more than a single
-channel and polar mapping is hierarchical such that polar coupling may be
-extrapolated to an arbitrary number of channels and is not restricted
-to only stereo, quadriphonics, ambisonics or 5.1 surround. However,
-the scope of this document restricts itself to the stereo coupling
-case.<p>
-
+channel coupling. The first is <em>channel interleaving</em> via
+residue backend type 2, and the second is <em>square polar
+mapping</em>. These two general mechanisms are particularly well
+suited to coupling due to the structure of Vorbis encoding, as we'll
+explore below, and using both we can implement both totally
+<em>lossless stereo image coupling</em> [bit-for-bit decode-identical
+to uncoupled modes], as well as various lossy models that seek to
+eliminate inaudible or unimportant aspects of the stereo image in
+order to enhance bitrate. The exact coupling implementation is
+generalized to allow the encoder a great deal of flexibility in
+implementation of a stereo or surround model without requiring any
+significant complexity increase over the combinatorially simpler
+mid/side joint stereo of mp3 and other current audio codecs.</p>
+
+<p>A particular Vorbis bitstream may apply channel coupling directly to
+more than a pair of channels; polar mapping is hierarchical such that
+polar coupling may be extrapolated to an arbitrary number of channels
+and is not restricted to only stereo, quadraphonics, ambisonics or 5.1
+surround. However, the scope of this document restricts itself to the
+stereo coupling case.</p>
+
+<a name="sqpm"></a>
<h3>Square Polar Mapping</h3>
<h4>maximal correlation</h4>
-Recall that the basic structure of a a Vorbis I stream first generates
+<p>Recall that the basic structure of a a Vorbis I stream first generates
from input audio a spectral 'floor' function that serves as an
-MDCT-domain whitening filter. This floor is meant to represent the
+MDCT-domain whitening filter. This floor is meant to represent the
rough envelope of the frequency spectrum, using whatever metric the
-encoder cares to define. This floor is subtracted from the log
+encoder cares to define. This floor is subtracted from the log
frequency spectrum, effectively normalizing the spectrum by frequency.
-Each input channel is associated with a unique floor function.<p>
+Each input channel is associated with a unique floor function.</p>
-The basic idea behind any stereo coupling is that the left and right
-channels usually correlate. This correlation is even stronger if one
+<p>The basic idea behind any stereo coupling is that the left and right
+channels usually correlate. This correlation is even stronger if one
first accounts for energy differences in any given frequency band
across left and right; think for example of individual instruments
mixed into different portions of the stereo image, or a stereo
-recording with a dominant feature not perfectly in the center. The
+recording with a dominant feature not perfectly in the center. The
floor functions, each specific to a channel, provide the perfect means
of normalizing left and right energies across the spectrum to maximize
correlation before coupling. This feature of the Vorbis format is not
-a convenient accident.<p>
+a convenient accident.</p>
-Because we strive to maximally correlate the left and right channels
+<p>Because we strive to maximally correlate the left and right channels
and generally succeed in doing so, left and right residue is typically
-nearly identical. We could use channel interleaving (discussed below)
+nearly identical. We could use channel interleaving (discussed below)
alone to efficiently remove the redundancy between the left and right
channels as a side effect of entropy encoding, but a polar
representation gives benefits when left/right correlation is
-strong. <p>
+strong.</p>
<h4>point and diffuse imaging</h4>
-The first advantage of a polar representation is that it effectively
-seperates the spatial audio information into a 'point image'
+<p>The first advantage of a polar representation is that it effectively
+separates the spatial audio information into a 'point image'
(magnitude) at a given frequency and located somewhere in the sound
field, and a 'diffuse image' (angle) that fills a large amount of
-space simultaneously. Even if we preserve only the magnitude (point)
+space simultaneously. Even if we preserve only the magnitude (point)
data, a detailed and carefully chosen floor function in each channel
provides us with a free, fine-grained, frequency relative intensity
-stereo*. Angle information represents diffuse sound fields, such as
-reverberation that fills the entire space simultaneously.<p>
+stereo*. Angle information represents diffuse sound fields, such as
+reverberation that fills the entire space simultaneously.</p>
-*<em>Because the Vorbis model supports a number of different possible
+<p>*<em>Because the Vorbis model supports a number of different possible
stereo models and these models may be mixed, we do not use the term
'intensity stereo' talking about Vorbis; instead we use the terms
-'point stereo', 'phase stereo' and subcategories of each.</em><p>
+'point stereo', 'phase stereo' and subcategories of each.</em></p>
-The majority of a stereo image is representable by polar magnitude
+<p>The majority of a stereo image is representable by polar magnitude
alone, as strong sounds tend to be produced at near-point sources;
even non-diffuse, fast, sharp echoes track very accurately using
magnitude representation almost alone (for those experimenting with
Vorbis tuning, this strategy works much better with the precise,
piecewise control of floor 1; the continuous approximation of floor 0
-results in unstable imaging). Reverberation and diffuse sounds tend
+results in unstable imaging). Reverberation and diffuse sounds tend
to contain less energy and be psychoacoustically dominated by the
-point sources embedded in them. Thus, we again tend to concentrate
+point sources embedded in them. Thus, we again tend to concentrate
more represented energy into a predictably smaller number of numbers.
Separating representation of point and diffuse imaging also allows us
-to model and manipulate point and diffuse qualities separately.<p>
+to model and manipulate point and diffuse qualities separately.</p>
-<h4>controlling bit leakage and symbol crosstalk</h4> Because polar
+<h4>controlling bit leakage and symbol crosstalk</h4>
+
+<p>Because polar
representation concentrates represented energy into fewer large
values, we reduce bit 'leakage' during cascading (multistage VQ
-encoding) as a secondary benefit. A single large, monolithic VQ
+encoding) as a secondary benefit. A single large, monolithic VQ
codebook is more efficient than a cascaded book due to entropy
'crosstalk' among symbols between different stages of a multistage cascade.
Polar representation is a way of further concentrating entropy into
predictable locations so that codebook design can take steps to
-improve multistage codebook efficiency. It also allows us to cascade
-various elements of the stereo image independently.<p>
+improve multistage codebook efficiency. It also allows us to cascade
+various elements of the stereo image independently.</p>
<h4>eliminating trigonometry and rounding</h4>
-Rounding and computational complexity are potential problems with a
+<p>Rounding and computational complexity are potential problems with a
polar representation. As our encoding process involves quantization,
mixing a polar representation and quantization makes it potentially
impossible, depending on implementation, to construct a coupled stereo
mechanism that results in bit-identical decompressed output compared
-to an uncoupled encoding should the encoder desire it.<p>
+to an uncoupled encoding should the encoder desire it.</p>
-Vorbis uses a mapping that preserves the most useful qualities of
+<p>Vorbis uses a mapping that preserves the most useful qualities of
polar representation, relies only on addition/subtraction (during
decode; high quality encoding still requires some trig), and makes it
trivial before or after quantization to represent an angle/magnitude
through a one-to-one mapping from possible left/right value
-permutations. We do this by basing our polar representation on the
-unit square rather than the unit-circle.<p>
+permutations. We do this by basing our polar representation on the
+unit square rather than the unit-circle.</p>
-Given a magnitude and angle, we recover left and right using the
+<p>Given a magnitude and angle, we recover left and right using the
following function (note that A/B may be left/right or right/left
-depending on the coupling definition used by the encoder):<p>
+depending on the coupling definition used by the encoder):</p>
<pre>
if(magnitude>0)
}
</pre>
-The function is antisymmetric for positive and negative magnitudes in
-order to eliminate a redundant value when quantizing. For example, if
+<p>The function is antisymmetric for positive and negative magnitudes in
+order to eliminate a redundant value when quantizing. For example, if
we're quantizing to integer values, we can visualize a magnitude of 5
-and an angle of -2 as follows:<p>
+and an angle of -2 as follows:</p>
-<img src="squarepolar.png">
+<p><img src="squarepolar.png" alt="square polar"/></p>
-<p>
-This representation loses or replicates no values; if the range of A
+<p>This representation loses or replicates no values; if the range of A
and B are integral -5 through 5, the number of possible Cartesian
-permutations is 121. Represented in square polar notation, the
-possible values are:
+permutations is 121. Represented in square polar notation, the
+possible values are:</p>
<pre>
0, 0
2,-4 2,-3 ... following the pattern ...
- ... 5, 1 5, 2 5, 3 5, 4 5, 5 5, 6 5, 7 5, 8 5, 9
+ ... 5, 1 5, 2 5, 3 5, 4 5, 5 5, 6 5, 7 5, 8 5, 9
</pre>
-...for a grand total of 121 possible values, the same number as in
+<p>...for a grand total of 121 possible values, the same number as in
Cartesian representation (note that, for example, <tt>5,-10</tt> is
the same as <tt>-5,10</tt>, so there's no reason to represent
both. 2,10 cannot happen, and there's no reason to account for it.)
-It's also obvious that this mapping is exactly reversible.<p>
+It's also obvious that this mapping is exactly reversible.</p>
<h3>Channel interleaving</h3>
-We can remap and A/B vector using polar mapping into a magnitude/angle
+<p>We can remap and A/B vector using polar mapping into a magnitude/angle
vector, and it's clear that, in general, this concentrates energy in
the magnitude vector and reduces the amount of information to encode
-in the angle vector. Encoding these vectors independently with
+in the angle vector. Encoding these vectors independently with
residue backend #0 or residue backend #1 will result in bitrate
-savings. However, there are still implicit correlations between the
-magnitude and angle vectors. The most obvious is that the amplitude
-of the angle is bounded by its corresponding magnitude value.<p>
+savings. However, there are still implicit correlations between the
+magnitude and angle vectors. The most obvious is that the amplitude
+of the angle is bounded by its corresponding magnitude value.</p>
-Entropy coding the results, then, further benefits from the entropy
-model being able to compress magnitude and angle simultaneously. For
-this reason, Vorbis implements residuebackend #2 which preinterleaves
+<p>Entropy coding the results, then, further benefits from the entropy
+model being able to compress magnitude and angle simultaneously. For
+this reason, Vorbis implements residue backend #2 which pre-interleaves
a number of input vectors (in the stereo case, two, A and B) into a
single output vector (with the elements in the order of
-A_0, B_0, A_1, B_1, A_2 ... A_n-1, B_n-1) before entropy encoding. Thus
+A_0, B_0, A_1, B_1, A_2 ... A_n-1, B_n-1) before entropy encoding. Thus
each vector to be coded by the vector quantization backend consists of
-matching magnitude and angle values.<p>
+matching magnitude and angle values.</p>
-The astute reader, at this point, will notice that in the theoretical
+<p>The astute reader, at this point, will notice that in the theoretical
case in which we can use monolithic codebooks of arbitrarily large
size, we can directly interleave and encode left and right without
polar mapping; in fact, the polar mapping does not appear to lend any
-benefit whatsoever to the efficiency of the entropy coding. In fact,
+benefit whatsoever to the efficiency of the entropy coding. In fact,
it is perfectly possible and reasonable to build a Vorbis encoder that
dispenses with polar mapping entirely and merely interleaves the
-channel. Libvorbis based encoders may configure such an encoding and
-it will work as intended.<p>
+channel. Libvorbis based encoders may configure such an encoding and
+it will work as intended.</p>
-However, when we leave the ideal/theoretical domain, we notice that
+<p>However, when we leave the ideal/theoretical domain, we notice that
polar mapping does give additional practical benefits, as discussed in
-the above section on polar mapping and summarized again here:<p>
+the above section on polar mapping and summarized again here:</p>
+
<ul>
<li>Polar mapping aids in controlling entropy 'leakage' between stages
-of a cascaded codebook. <li>Polar mapping separates the stereo image
+of a cascaded codebook.</li>
+<li>Polar mapping separates the stereo image
into point and diffuse components which may be analyzed and handled
-differently.
+differently.</li>
</ul>
<h2>Stereo Models</h2>
<h3>Dual Stereo</h3>
-Dual stereo refers to stereo encoding where the channels are entirely
+<p>Dual stereo refers to stereo encoding where the channels are entirely
separate; they are analyzed and encoded as entirely distinct entities.
-This terminology is familiar from mp3.<p>
+This terminology is familiar from mp3.</p>
<h3>Lossless Stereo</h3>
-Using polar mapping and/or channel interleaving, it's possible to
+<p>Using polar mapping and/or channel interleaving, it's possible to
couple Vorbis channels losslessly, that is, construct a stereo
coupling encoding that both saves space but also decodes
-bit-identically to dual stereo. OggEnc 1.0 and later uses this
-mode in all high-bitrate encoding.<p>
+bit-identically to dual stereo. OggEnc 1.0 and later uses this
+mode in all high-bitrate encoding.</p>
-Overall, this stereo mode is overkill; however, it offers a safe
+<p>Overall, this stereo mode is overkill; however, it offers a safe
alternative to users concerned about the slightest possible
-degredation to the stereo image or archival quality audio.<p>
+degradation to the stereo image or archival quality audio.</p>
<h3>Phase Stereo</h3>
-Phase stereo is the least aggressive means of gracefully dropping
-resolution from the stereo image; it affects only diffuse imaging.<p>
+<p>Phase stereo is the least aggressive means of gracefully dropping
+resolution from the stereo image; it affects only diffuse imaging.</p>
-It's often quoted that the human ear is deaf to signal phase above
+<p>It's often quoted that the human ear is deaf to signal phase above
about 4kHz; this is nearly true and a passable rule of thumb, but it
can be demonstrated that even an average user can tell the difference
-between high frequency in-phase and out-of-phase noise. Obviously
-then, the statement is not entirely true. However, it's also the case
-that one must resort to nearly such an extreme demostration before
-finding the counterexample.<p>
+between high frequency in-phase and out-of-phase noise. Obviously
+then, the statement is not entirely true. However, it's also the case
+that one must resort to nearly such an extreme demonstration before
+finding the counterexample.</p>
-'Phase stereo' is simply a more aggressive quantization of the polar
+<p>'Phase stereo' is simply a more aggressive quantization of the polar
angle vector; above 4kHz it's generally quite safe to quantize noise
and noisy elements to only a handful of allowed phases, or to thin the
-phase with respect to the magnitude. The phases of high amplitude
+phase with respect to the magnitude. The phases of high amplitude
pure tones may or may not be preserved more carefully (they are
relatively rare and L/R tend to be in phase, so there is generally
-little reason not to spend a few more bits on them) <p>
+little reason not to spend a few more bits on them)</p>
<h4>example: eight phase stereo</h4>
-Vorbis may implement phase stereo coupling by preserving the entirety
+<p>Vorbis may implement phase stereo coupling by preserving the entirety
of the magnitude vector (essential to fine amplitude and energy
resolution overall) and quantizing the angle vector to one of only
four possible values. Given that the magnitude vector may be positive
or negative, this results in left and right phase having eight
-possible permutation, thus 'eight phase stereo':<p>
+possible permutation, thus 'eight phase stereo':</p>
-<img src="eightphase.png"><p>
+<p><img src="eightphase.png" alt="eight phase"/></p>
-Left and right may be in phase (positive or negative), the most common
-case by far, or out of phase by 90 or 180 degrees.<p>
+<p>Left and right may be in phase (positive or negative), the most common
+case by far, or out of phase by 90 or 180 degrees.</p>
<h4>example: four phase stereo</h4>
-Similarly, four phase stereo takes the quantization one step further;
-it allows only in-phase and 180 degree out-out-phase signals:<p>
+<p>Similarly, four phase stereo takes the quantization one step further;
+it allows only in-phase and 180 degree out-out-phase signals:</p>
-<img src="fourphase.png"><p>
+<p><img src="fourphase.png" alt="four phase"/></p>
<h3>example: point stereo</h3>
-Point stereo eliminates the possibility of out-of-phase signal
-entirely. Any diffuse quality to a sound source tends to collapse
-inward to a point somewhere within the stereo image. A practical
+<p>Point stereo eliminates the possibility of out-of-phase signal
+entirely. Any diffuse quality to a sound source tends to collapse
+inward to a point somewhere within the stereo image. A practical
example would be balanced reverberations within a large, live space;
normally the sound is diffuse and soft, giving a sonic impression of
-volume. In point-stereo, the reverberations would still exist, but
+volume. In point-stereo, the reverberations would still exist, but
sound fairly firmly centered within the image (assuming the
reverberation was centered overall; if the reverberation is stronger
to the left, then the point of localization in point stereo would be
-to the left). This effect is most noticeable at low and mid
+to the left). This effect is most noticeable at low and mid
frequencies and using headphones (which grant perfect stereo
separation). Point stereo is is a graceful but generally easy to
-detect degrdation to the sound quality and is thus used in frequency
-ranges where it is least noticeable.<p>
+detect degradation to the sound quality and is thus used in frequency
+ranges where it is least noticeable.</p>
<h3>Mixed Stereo</h3>
-Mixed stereo is the simultaneous use of more than one of the above
+<p>Mixed stereo is the simultaneous use of more than one of the above
stereo encoding models, generally using more aggressive modes in
-higher frequencies, lower amplitudes or 'nearly' in-phase sound.<p>
+higher frequencies, lower amplitudes or 'nearly' in-phase sound.</p>
-It is also the case that near-DC frequencies should be encoded using
-lossless coupling to avoid frame blocking artifacts.<p>
+<p>It is also the case that near-DC frequencies should be encoded using
+lossless coupling to avoid frame blocking artifacts.</p>
<h3>Vorbis Stereo Modes</h3>
-Vorbis, as of 1.0, uses lossless stereo and a number of mixed modes
-constructed out of lossless and point stereo. Phase stereo was used
-in the rc2 encoder, but is not currently used for simplicity's sake. It
-will likely be readded to the stereo model in the future.
-
-<p>
-<hr>
-<a href="http://www.xiph.org/">
-<img src="white-xifish.png" align=left border=0>
-</a>
-<font size=-2 color=#505050>
-
-Ogg is a <a href="http://www.xiph.org">Xiphophorus</a> effort to
-protect essential tenets of Internet multimedia from corporate
-hostage-taking; Open Source is the net's greatest tool to keep
-everyone honest. See <a href="http://www.xiph.org/about.html">About
-Xiphophorus</a> for details.
-<p>
-
-Ogg Vorbis is the first Ogg audio CODEC. Anyone may
-freely use and distribute the Ogg and Vorbis specification,
-whether in a private, public or corporate capacity. However,
-Xiphophorus and the Ogg project (xiph.org) reserve the right to set
-the Ogg/Vorbis specification and certify specification compliance.<p>
-
-Xiphophorus's Vorbis software CODEC implementation is distributed
-under a BSD-like License. This does not restrict third parties from
-distributing independent implementations of Vorbis software under
-other licenses.<p>
-
-OggSquish, Vorbis, Xiphophorus and their logos are trademarks (tm) of
-<a href="http://www.xiph.org/">Xiphophorus</a>. These pages are
-copyright (C) 1994-2001 Xiphophorus. All rights reserved.<p>
+<p>Vorbis, as of 1.0, uses lossless stereo and a number of mixed modes
+constructed out of lossless and point stereo. Phase stereo was used
+in the rc2 encoder, but is not currently used for simplicity's sake. It
+will likely be re-added to the stereo model in the future.</p>
+
+<div id="copyright">
+ The Xiph Fish Logo is a
+ trademark (™) of Xiph.Org.<br/>
+
+ These pages © 1994 - 2005 Xiph.Org. All rights reserved.
+</div>
</body>
+</html>
projects / platform / upstream / libvorbis.git / blobdiff