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<title>Ogg Vorbis Documentation</title>
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</head>
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<body>
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Ogg Vorbis: Fidelity measurement and terminology discussion
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Terminology discussed in this document is based on common terminology
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associated with contemporary codecs such as MPEG I audio layer 3
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(mp3). However, some differences in terminology are useful in the
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context of Vorbis as Vorbis functions somewhat differently than most
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current formats. For clarity, then, we describe a common terminology
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for discussion of Vorbis's and other formats' audio quality.
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Subjective and Objective
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Objective fidelity is a measure, based on a computable,
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mechanical metric, of how carefully an output matches an input. For
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example, a stereo amplifier may claim to introduce less that .01%
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total harmonic distortion when amplifying an input signal; this claim
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is easy to verify given proper equipment, and any number of testers are
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likely to arrive at the same, exact results. One need not listen to
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the equipment to make this measurement.
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However, given two amplifiers with identical, verifiable objective
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specifications, listeners may strongly prefer the sound quality of one
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over the other. This is actually the case in the decades old debate
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[some would say jihad] among audiophiles involving vacuum tube versus
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solid state amplifiers. There are people who can tell the difference,
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and strongly prefer one over the other despite seemingly identical,
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measurable quality. This preference is subjective and
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difficult to measure but nonetheless real.
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Individual elements of subjective differences often can be qualified,
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but overall subjective quality generally is not measurable. Different
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observers are likely to disagree on the exact results of a subjective
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test as each observer's perspective differs. When measuring
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subjective qualities, the best one can hope for is average, empirical
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results that show statistical significance across a group.
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Perceptual codecs are most concerned with subjective, not objective,
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quality. This is why evaluating a perceptual codec via distortion
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measures and sonograms alone is useless; these objective measures may
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provide insight into the quality or functioning of a codec, but cannot
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answer the much squishier subjective question, "Does it sound
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good?". The tube amplifier example is perhaps not the best as very few
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people can hear, or care to hear, the minute differences between tubes
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and transistors, whereas the subjective differences in perceptual
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codecs tend to be quite large even when objective differences are
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not.
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Fidelity, Artifacts and Differences
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Audio artifacts and loss of fidelity or more simply
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put, audio differences are not the same thing.
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A loss of fidelity implies differences between the perceived input and
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output signal; it does not necessarily imply that the differences in
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output are displeasing or that the output sounds poor (although this
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is often the case). Tube amplifiers are not higher fidelity
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than modern solid state and digital systems. They simply produce a
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form of distortion and coloring that is either unnoticeable or actually
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pleasing to many ears.
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As compared to an original signal using hard metrics, all perceptual
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codecs [ASPEC, ATRAC, MP3, WMA, AAC, TwinVQ, AC3 and Vorbis included]
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lose objective fidelity in order to reduce bitrate. This is fact. The
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idea is to lose fidelity in ways that cannot be perceived. However,
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most current streaming applications demand bitrates lower than what
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can be achieved by sacrificing only objective fidelity; this is also
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fact, despite whatever various company press releases might claim.
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Subjective fidelity eventually must suffer in one way or another.
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The goal is to choose the best possible tradeoff such that the
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fidelity loss is graceful and not obviously noticeable. Most listeners
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of FM radio do not realize how much lower fidelity that medium is as
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compared to compact discs or DAT. However, when compared directly to
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source material, the difference is obvious. A cassette tape is lower
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fidelity still, and yet the degradation, relatively speaking, is
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graceful and generally easy not to notice. Compare this graceful loss
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of quality to an average 44.1kHz stereo mp3 encoded at 80 or 96kbps.
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The mp3 might actually be higher objective fidelity but subjectively
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sounds much worse.
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Thus, when a CODEC must sacrifice subjective quality in order
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to satisfy a user's requirements, the result should be a
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difference that is generally either difficult to notice
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without comparison, or easy to ignore. An artifact, on the
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other hand, is an element introduced into the output that is
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immediately noticeable, obviously foreign, and undesired. The famous
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'underwater' or 'twinkling' effect synonymous with low bitrate (or
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poorly encoded) mp3 is an example of an artifact. This
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working definition differs slightly from common usage, but the coined
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distinction between differences and artifacts is useful for our
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discussion.
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The goal, when it is absolutely necessary to sacrifice subjective
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fidelity, is obviously to strive for differences and not artifacts.
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The vast majority of codecs today fail at this task miserably,
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predictably, and regularly in one way or another. Avoiding such
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failures when it is necessary to sacrifice subjective quality is a
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fundamental design objective of Vorbis and that objective is reflected
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in Vorbis's design and tuning.
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The Xiph Fish Logo is a
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trademark (™) of Xiph.Org.
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These pages © 1994 - 2005 Xiph.Org. All rights reserved.
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