181 lines
6.5 KiB
HTML
Executable File
181 lines
6.5 KiB
HTML
Executable File
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
|
<html>
|
|
<head>
|
|
|
|
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
|
|
<title>Ogg Vorbis Documentation</title>
|
|
|
|
<style type="text/css">
|
|
body {
|
|
margin: 0 18px 0 18px;
|
|
padding-bottom: 30px;
|
|
font-family: Verdana, Arial, Helvetica, sans-serif;
|
|
color: #333333;
|
|
font-size: .8em;
|
|
}
|
|
|
|
a {
|
|
color: #3366cc;
|
|
}
|
|
|
|
img {
|
|
border: 0;
|
|
}
|
|
|
|
#xiphlogo {
|
|
margin: 30px 0 16px 0;
|
|
}
|
|
|
|
#content p {
|
|
line-height: 1.4;
|
|
}
|
|
|
|
h1, h1 a, h2, h2 a, h3, h3 a {
|
|
font-weight: bold;
|
|
color: #ff9900;
|
|
margin: 1.3em 0 8px 0;
|
|
}
|
|
|
|
h1 {
|
|
font-size: 1.3em;
|
|
}
|
|
|
|
h2 {
|
|
font-size: 1.2em;
|
|
}
|
|
|
|
h3 {
|
|
font-size: 1.1em;
|
|
}
|
|
|
|
li {
|
|
line-height: 1.4;
|
|
}
|
|
|
|
#copyright {
|
|
margin-top: 30px;
|
|
line-height: 1.5em;
|
|
text-align: center;
|
|
font-size: .8em;
|
|
color: #888888;
|
|
clear: both;
|
|
}
|
|
</style>
|
|
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<div id="xiphlogo">
|
|
<a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
|
|
</div>
|
|
|
|
<h1>Ogg Vorbis: Fidelity measurement and terminology discussion</h1>
|
|
|
|
<p>Terminology discussed 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, then, we describe a common terminology
|
|
for discussion of Vorbis's and other formats' audio quality.</p>
|
|
|
|
<h2>Subjective and Objective</h2>
|
|
|
|
<p><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>
|
|
|
|
<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.</p>
|
|
|
|
<p>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>
|
|
|
|
<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>
|
|
|
|
<h2>Fidelity, Artifacts and Differences</h2>
|
|
|
|
<p>Audio <em>artifacts</em> and loss of fidelity or more simply
|
|
put, audio <em>differences</em> are not the same thing.</p>
|
|
|
|
<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>
|
|
|
|
<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>
|
|
|
|
<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 degradation, 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>
|
|
|
|
<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>
|
|
|
|
<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 design and tuning.</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>
|