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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 -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.

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Subjective and Objective

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Objective 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.

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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 subjective and -difficult to measure but nonetheless real.

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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.

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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.

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Fidelity, Artifacts and Differences

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Audio artifacts and loss of fidelity or more simply -put, audio differences are not the same thing.

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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 not 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.

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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.

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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.

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Thus, when a CODEC must sacrifice subjective quality in order -to satisfy a user's requirements, the result should be a -difference that is generally either difficult to notice -without comparison, or easy to ignore. An artifact, 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 artifact. This -working definition differs slightly from common usage, but the coined -distinction between differences and artifacts is useful for our -discussion.

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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.

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