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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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