doc/vorbis-fidelity.html - platform/external/libvorbis - Git at Google

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 <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>

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	<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>