Transparency (data compression)
In data compression and psychoacoustics, transparency is the ideal result of lossy data compression. If a lossy 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.
A transparency threshold is 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 Linear PCM audio is said to be between 175 to 245 kbit/s, at 44.1 kHz, when encoded as VBR MP3 (corresponding to the -V3 and -V0 settings of the highly popular LAME MP3 encoder). This means that when an MP3 that was encoded at those bitrates is being played back, it is indistinguishable from the original PCM, and transparent to compression.
Transparency, like sound or video quality, is subjective. It depends most on the listener's familiarity with digital artifacts, their awareness that artifacts may in fact be present, and to a lesser extent, the compression method, bit-rate used, input characteristics, and the listening/viewing conditions and 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, in which subjective like/dislike of a certain compression methodology emotionally influences his or her judgment. This bias is commonly referred to as placebo, 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 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.
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.
- LAME Recommended Encoder Settings, hydrogenaudio
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- "Transparency", Hydrogen Audio Wiki
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