Transparency (data compression)
Encyclopedia
In data compression
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....

 or psychoacoustics
Psychoacoustics
Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...

, transparency is the ideal result of lossy data compression
Lossy data compression
In information technology, "lossy" compression is a data encoding method that compresses data by discarding some of it. The procedure aims to minimize the amount of data that need to be held, handled, and/or transmitted by a computer...

. If a lossy compressed result is perceptually
Perception
Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...

 indistinguishable from the uncompressed input, then the compression can be declared to be transparent. In other words, transparency is the situation where compression artifact
Compression artifact
A compression artifact is a noticeable distortion of media caused by the application of lossy data compression....

s are nonexistent or imperceptible.

A transparency threshold is a term used to describe a given value at which transparency is reached. It is commonly used to describe compressed data bitrates. For example, the transparency threshold for .MP3 to .WAV audio is said to be around 192kbs 44.1 kHz MP3. This means that when MP3 is playing back at 192kbs 44.1 kHz it is indistinguishable from the original WAV, and transparent to compression.

Transparency, like sound or video 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, and the listening/viewing conditions & equipment. Despite this, sometimes general consensus is formed for what compression options "should" provide transparent results for most people on most equipment. Due to the subjectivity and the changing nature of compression, recording, and playback technology, such opinions should be considered only as rough estimates rather than established fact.

Judging transparency can be difficult due to observer bias
Observer effect
Observer effect may refer to:* Observer effect , the impact of observing a process while it is running* Observer effect , the impact of observing a physical system...

, in which subjective like/dislike of a certain compression methodology emotionally influences his or her judgment. This bias is commonly referred to as placebo
Placebo
A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...

,
although this use is slightly different from the medical use of the term.

To scientifically prove that a compression method is not transparent, double-blind
Double-blind
A blind or blinded experiment is a scientific experiment where some of the people involved are prevented from knowing certain information that might lead to conscious or subconscious bias on their part, invalidating the results....

 tests may be useful. The ABX method
ABX test
An ABX test is a method of comparing two kinds of sensory stimuli to identify detectable differences. A subject is presented with two known samples , and one unknown sample X, for three samples total. X is randomly selected from A and B, and the subject identifies X as being either A or B...

 is normally used, with a null hypothesis
Null hypothesis
The practice of science involves formulating and testing hypotheses, assertions that are capable of being proven false using a test of observed data. The null hypothesis typically corresponds to a general or default position...

 that the samples tested are the same and with an alternative hypothesis that the samples are in fact different. If two different versions of a sound are given the same score in a double blind test, the two sounds are proven "transparent"—to have no subjective difference.

All lossless data compression methods are transparent, by nature. However, a double-blind comparison could still yield claims of perceived differences and thus lack of transparency, even though such claims would be in error.

See also

  • Codec listening test
    Codec listening test
    A codec listening test is a scientific study designed to compare two or more lossy audio codecs, usually with respect to perceived fidelity or compression efficiency....

  • Ascertaining high fidelity: double-blind tests

External links

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