Transparency (data compression)
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In data compression or psychoacoustics, transparency is the ideal result of lossy data compression. If a lossily compressed result is perceptually indistinguishable from the uncompressed input, then the compression can be declared to be transparent. In other words, transparency is the situation where compression artifacts are nonexistent or imperceptible.
Transparency, like sound quality, is subjective. It depends most on the listener's familiarity with artifacts, and to a lesser extent, the compression method, bit-rate used, input characteristics, listening conditions, and listening equipment. Despite this, sometimes general consensus is formed around roughly what "should" be transparent for most people on most equipment. Using MP3 audio, it is popular to assume that files with 192 kbit/s bitrate (compressed from a 44.1 kHz sample rate, 16 bit sample size, 2 channel source) should be either transparent or close to transparent[citation needed]. Using Ogg Vorbis, a quality setting of 5 (or a nominal bitrate of ~160 kbit/s) is considered equivalent[citation needed]. Due to the aforementioned subjectivity and the changing nature of both software encoders and audio technology, such opinions should be considered only as rough estimates rather than established fact.
All lossless data compression methods are transparent. Current lossless audio codecs generate a compressed file typically about half the size of the uncompressed input audio file.
Judging transparency can be difficult due to observer bias, in which subjective like/dislike of a certain compression methodology emotionally influences his/her judgment. This bias is commonly referred to as placebo, although this use is slightly different from the medical use of the term.
There is no way to prove whether a certain lossy compression methodology is transparent. To scientifically prove that a compression method is not transparent, double-blind tests may be useful. The ABX method is normally used, with a null hypothesis that the samples tested are the same and with an alternative hypothesis that the samples are in fact different. Failure of an ABX test or any other comparison does not prove that there is no difference; it can only be said that the difference could not be proved.
[edit] See also
[edit] References
- Bosi, Marina; Richard E. Goldberg. Introduction to digital audio coding and standards. Springer, 2003. ISBN 1402073577
- Cvejic, Nedeljko; Tapio Seppänen. Digital audio watermarking techniques and technologies: applications and benchmarks. Idea Group Inc (IGI), 2007. ISBN 1599045133
- Pohlmann, Ken C. Principles of digital audio. McGraw-Hill Professional, 2005. ISBN 0071441565
- Spanias, Andreas; Ted Painter; Venkatraman Atti. Audio signal processing and coding. Wiley-Interscience, 2007. ISBN 0471791474
- Syed, Mahbubur Rahman. Multimedia technologies: concepts, methodologies, tools, and applications, Volume 3. Idea Group Inc (IGI), 2008. ISBN 1599049538
[edit] External links
- "Transparency", Hydrogen Audio Wiki
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