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-</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>
-
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