In
data compression
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compressi ...
and
psychoacoustics
Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, ...
, transparency is the result of
lossy data compression accurate enough that the compressed result is
perceptually indistinguishable from the uncompressed input, i.e. perceptually lossless.
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 and 245 kbit/s, at
44.1 kHz, when encoded as
VBR MP3 (corresponding to the -V3 and -V0 settings of the highly popular
LAME
LAME is a software encoder that converts digital audio into the MP3 audio coding format. LAME is a free software project that was first released in 1998 and has incorporated many improvements since then, including an improved psychoacoustic ...
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 the compression is transparent to the listener.
The term ''transparent compression'' can also refer to a
filesystem feature that allows compressed files to be read and written just like regular ones. In this case, the compressor is typically a general-purpose lossless compressor.
Determination
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
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...
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 their judgment. This bias is commonly referred to as ''
placebo
A placebo ( ) can be roughly defined as a sham medical treatment. Common placebos include inert tablets (like sugar pills), inert injections (like saline), sham surgery, and other procedures.
Placebos are used in randomized clinical trials ...
'', 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
In a blind or blinded experiment, information which may influence the participants of the experiment is withheld until after the experiment is complete. Good blinding can reduce or eliminate experimental biases that arise from a participants' expec ...
tests may be useful. The
ABX method is normally used, with a
null hypothesis
The null hypothesis (often denoted ''H''0) is the claim in scientific research that the effect being studied does not exist. The null hypothesis can also be described as the hypothesis in which no relationship exists between two sets of data o ...
that the samples tested are the same and with an
alternative hypothesis
In statistical hypothesis testing, the alternative hypothesis is one of the proposed propositions in the hypothesis test. In general the goal of hypothesis test is to demonstrate that in the given condition, there is sufficient evidence supporting ...
that the samples are in fact different.
All
lossless data compression
Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits Redundanc ...
methods are transparent, by nature.
In image compression
Both the DSC in
DisplayPort
DisplayPort (DP) is a digital interface used to connect a video source, such as a Personal computer, computer, to a display device like a Computer monitor, monitor. Developed by the Video Electronics Standards Association (VESA), it can also car ...
and the default settings of
JPEG XL
The JPEG XL Image Coding System is a royalty-free open standard for a image compression, compressed Raster graphics, raster image format. It defines a graphics file format and the abstract device for coding JPEG XL bitstreams. It is developed by t ...
are regarded as ''visually lossless''. The losslessness is usually determined by a ''flicker'' test: the display initially shows the compressed and the original side-by-side, switches them around for a tiny fraction of a second and then goes back to the original. This test is more sensitive than a side-by-side comparison ("visually almost lossless"), as the human eye is highly sensitive to temporal changes in light. There is also a ''panning'' test that is purportedly more representative of sensitivity in the case of moving images than the ''flicker'' test.
Difference from a lack of artifacts
A perceptually lossless compression is always free of
compression artifacts, but the inverse is not true: it is possible for a compressor to produce a signal that appears natural but with altered contents. Such a confusion is widely present in the field of
radiology
Radiology ( ) is the medical specialty that uses medical imaging to diagnose diseases and guide treatment within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiation), but tod ...
(specifically for the study of
diagnostically acceptable irreversible compression), where ''visually lossless'' is taken to mean anywhere from artifact-free to being indistinguishable on a side-to-side view,
neither being as stringent as the ''flicker'' test.
See also
*
Codec listening test
A codec listening test is a scientific Experiment, study designed to compare two or more lossy sound reproduction, audio codecs, usually with respect to perceived fidelity or compression efficiency.
Most tests take the form of a double-blind comp ...
*
References
* Bosi, Marina; Richard E. Goldberg. ''Introduction to digital audio coding and standards''. Springer, 2003.
* Cvejic, Nedeljko; Tapio Seppänen. ''Digital audio watermarking techniques and technologies: applications and benchmarks''. Idea Group Inc (IGI), 2007.
* Pohlmann, Ken C. ''Principles of digital audio''. McGraw-Hill Professional, 2005.
* Spanias, Andreas; Ted Painter; Venkatraman Atti. ''Audio signal processing and coding''. Wiley-Interscience, 2007.
* Syed, Mahbubur Rahman. ''Multimedia technologies: concepts, methodologies, tools, and applications, Volume 3''. Idea Group Inc (IGI), 2008.
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External links
"Transparency" Hydrogen Audio Wiki
Data compression
Audio codecs