LosaDev source snapshot 2019

etc/Library/OggVobisSDK/doc/vorbis/vorbis-fidelity.html

@@ -0,0 +1,142 @@
+<HTML><HEAD><TITLE>xiph.org: Ogg Vorbis documentation</TITLE>
+<BODY bgcolor="#ffffff" text="#202020" link="#006666" vlink="#000000">
+<nobr><img src="white-ogg.png"><img src="vorbisword2.png"></nobr><p>
+
+
+<h1><font color=#000070>
+Ogg Vorbis: Fidelity measurement and terminology discussion
+</font></h1>
+
+<em>Last update to this document: July 16, 2002</em><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>
+
+<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>
+
+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.
+
+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>
+
+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> Audio <em>artifacts</em>
+and loss of fidelity or more simply put, audio <em>differences</em>
+are not the same thing.<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>
+
+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>
+
+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>
+
+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>
+
+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>
+
+<hr>
+<a href="http://www.xiph.org/">
+<img src="white-xifish.png" align=left border=0>
+</a>
+<font size=-2 color=#505050>
+
+Ogg is a <a href="http://www.xiph.org">Xiph.org Foundation</a> effort
+to protect essential tenets of Internet multimedia from corporate
+hostage-taking; Open Source is the net's greatest tool to keep
+everyone honest. See <a href="http://www.xiph.org/about.html">About
+the Xiph.org Foundation</a> for details.
+<p>
+
+Ogg Vorbis is the first Ogg audio CODEC.  Anyone may freely use and
+distribute the Ogg and Vorbis specification, whether in a private,
+public or corporate capacity.  However, the Xiph.org Foundation and
+the Ogg project (xiph.org) reserve the right to set the Ogg Vorbis
+specification and certify specification compliance.<p>
+
+Xiph.org's Vorbis software CODEC implementation is distributed under a
+BSD-like license.  This does not restrict third parties from
+distributing independent implementations of Vorbis software under
+other licenses.<p>
+
+Ogg, Vorbis, Xiph.org Foundation and their logos are trademarks (tm)
+of the <a href="http://www.xiph.org/">Xiph.org Foundation</a>.  These
+pages are copyright (C) 1994-2002 Xiph.org Foundation. All rights
+reserved.<p>
+</body>
+
+
+
+
+
+