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. Using Ogg Vorbis, a quality setting of 5 (or a nominal bitrate of ~160 kbit/s) is considered equivalent. 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.

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 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.

[edit] See also

• Codec listening test
• Ascertaining high fidelity: double-blind tests