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 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, 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. 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. [1]
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.