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diff --git a/vorbis/doc/vorbis-fidelity.html b/vorbis/doc/vorbis-fidelity.html deleted file mode 100644 index 2321d67..0000000 --- a/vorbis/doc/vorbis-fidelity.html +++ /dev/null @@ -1,180 +0,0 @@ -<!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> |