-<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">\r
-<html>\r
-<head>\r
-\r
-<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>\r
-<title>Ogg Vorbis Documentation</title>\r
-\r
-<style type="text/css">\r
-body {\r
- margin: 0 18px 0 18px;\r
- padding-bottom: 30px;\r
- font-family: Verdana, Arial, Helvetica, sans-serif;\r
- color: #333333;\r
- font-size: .8em;\r
-}\r
-\r
-a {\r
- color: #3366cc;\r
-}\r
-\r
-img {\r
- border: 0;\r
-}\r
-\r
-#xiphlogo {\r
- margin: 30px 0 16px 0;\r
-}\r
-\r
-#content p {\r
- line-height: 1.4;\r
-}\r
-\r
-h1, h1 a, h2, h2 a, h3, h3 a {\r
- font-weight: bold;\r
- color: #ff9900;\r
- margin: 1.3em 0 8px 0;\r
-}\r
-\r
-h1 {\r
- font-size: 1.3em;\r
-}\r
-\r
-h2 {\r
- font-size: 1.2em;\r
-}\r
-\r
-h3 {\r
- font-size: 1.1em;\r
-}\r
-\r
-li {\r
- line-height: 1.4;\r
-}\r
-\r
-#copyright {\r
- margin-top: 30px;\r
- line-height: 1.5em;\r
- text-align: center;\r
- font-size: .8em;\r
- color: #888888;\r
- clear: both;\r
-}\r
-</style>\r
-\r
-</head>\r
-\r
-<body>\r
-\r
-<div id="xiphlogo">\r
- <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.org"/></a>\r
-</div>\r
-\r
-<h1>Ogg Vorbis: Fidelity measurement and terminology discussion</h1>\r
-\r
-<p>Terminology discussed in this document is based on common terminology\r
-associated with contemporary codecs such as MPEG I audio layer 3\r
-(mp3). However, some differences in terminology are useful in the\r
-context of Vorbis as Vorbis functions somewhat differently than most\r
-current formats. For clarity, then, we describe a common terminology\r
-for discussion of Vorbis's and other formats' audio quality.</p>\r
-\r
-<h2>Subjective and Objective</h2>\r
-\r
-<p><em>Objective</em> fidelity is a measure, based on a computable,\r
-mechanical metric, of how carefully an output matches an input. For\r
-example, a stereo amplifier may claim to introduce less that .01%\r
-total harmonic distortion when amplifying an input signal; this claim\r
-is easy to verify given proper equipment, and any number of testers are\r
-likely to arrive at the same, exact results. One need not listen to\r
-the equipment to make this measurement.</p>\r
-\r
-<p>However, given two amplifiers with identical, verifiable objective\r
-specifications, listeners may strongly prefer the sound quality of one\r
-over the other. This is actually the case in the decades old debate\r
-[some would say jihad] among audiophiles involving vacuum tube versus\r
-solid state amplifiers. There are people who can tell the difference,\r
-and strongly prefer one over the other despite seemingly identical,\r
-measurable quality. This preference is <em>subjective</em> and\r
-difficult to measure but nonetheless real.</p>\r
-\r
-<p>Individual elements of subjective differences often can be qualified,\r
-but overall subjective quality generally is not measurable. Different\r
-observers are likely to disagree on the exact results of a subjective\r
-test as each observer's perspective differs. When measuring\r
-subjective qualities, the best one can hope for is average, empirical\r
-results that show statistical significance across a group.</p>\r
-\r
-<p>Perceptual codecs are most concerned with subjective, not objective,\r
-quality. This is why evaluating a perceptual codec via distortion\r
-measures and sonograms alone is useless; these objective measures may\r
-provide insight into the quality or functioning of a codec, but cannot\r
-answer the much squishier subjective question, "Does it sound\r
-good?". The tube amplifier example is perhaps not the best as very few\r
-people can hear, or care to hear, the minute differences between tubes\r
-and transistors, whereas the subjective differences in perceptual\r
-codecs tend to be quite large even when objective differences are\r
-not.</p>\r
-\r
-<h2>Fidelity, Artifacts and Differences</h2>\r
-\r
-<p>Audio <em>artifacts</em> and loss of fidelity or more simply\r
-put, audio <em>differences</em> are not the same thing.</p>\r
-\r
-<p>A loss of fidelity implies differences between the perceived input and\r
-output signal; it does not necessarily imply that the differences in\r
-output are displeasing or that the output sounds poor (although this\r
-is often the case). Tube amplifiers are <em>not</em> higher fidelity\r
-than modern solid state and digital systems. They simply produce a\r
-form of distortion and coloring that is either unnoticeable or actually\r
-pleasing to many ears.</p>\r
-\r
-<p>As compared to an original signal using hard metrics, all perceptual\r
-codecs [ASPEC, ATRAC, MP3, WMA, AAC, TwinVQ, AC3 and Vorbis included]\r
-lose objective fidelity in order to reduce bitrate. This is fact. The\r
-idea is to lose fidelity in ways that cannot be perceived. However,\r
-most current streaming applications demand bitrates lower than what\r
-can be achieved by sacrificing only objective fidelity; this is also\r
-fact, despite whatever various company press releases might claim.\r
-Subjective fidelity eventually must suffer in one way or another.</p>\r
-\r
-<p>The goal is to choose the best possible tradeoff such that the\r
-fidelity loss is graceful and not obviously noticeable. Most listeners\r
-of FM radio do not realize how much lower fidelity that medium is as\r
-compared to compact discs or DAT. However, when compared directly to\r
-source material, the difference is obvious. A cassette tape is lower\r
-fidelity still, and yet the degradation, relatively speaking, is\r
-graceful and generally easy not to notice. Compare this graceful loss\r
-of quality to an average 44.1kHz stereo mp3 encoded at 80 or 96kbps.\r
-The mp3 might actually be higher objective fidelity but subjectively\r
-sounds much worse.</p>\r
-\r
-<p>Thus, when a CODEC <em>must</em> sacrifice subjective quality in order\r
-to satisfy a user's requirements, the result should be a\r
-<em>difference</em> that is generally either difficult to notice\r
-without comparison, or easy to ignore. An <em>artifact</em>, on the\r
-other hand, is an element introduced into the output that is\r
-immediately noticeable, obviously foreign, and undesired. The famous\r
-'underwater' or 'twinkling' effect synonymous with low bitrate (or\r
-poorly encoded) mp3 is an example of an <em>artifact</em>. This\r
-working definition differs slightly from common usage, but the coined\r
-distinction between differences and artifacts is useful for our\r
-discussion.</p>\r
-\r
-<p>The goal, when it is absolutely necessary to sacrifice subjective\r
-fidelity, is obviously to strive for differences and not artifacts.\r
-The vast majority of codecs today fail at this task miserably,\r
-predictably, and regularly in one way or another. Avoiding such\r
-failures when it is necessary to sacrifice subjective quality is a\r
-fundamental design objective of Vorbis and that objective is reflected\r
-in Vorbis's design and tuning.</p>\r
-\r
-<div id="copyright">\r
- The Xiph Fish Logo is a\r
- trademark (™) of Xiph.Org.<br/>\r
-\r
- These pages © 1994 - 2005 Xiph.Org. All rights reserved.\r
-</div>\r
-\r
-</body>\r
-</html>\r
+<!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>
projects / platform / upstream / libvorbis.git / blobdiff