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