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 compression ...
and
psychoacoustics
Psychoacoustics is the branch of psychophysics involving the scientific study of sound perception and audiology—how humans perceive various sounds. More specifically, it is the branch of science studying the psychological responses associated wit ...
, transparency is the result of
lossy data compression
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
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
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the a ...
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 or LAME may refer to:
Music
* "Lame" (song) by Unwritten Law
* ''Lame'' (album) by Iame
People
* Ibrahim Lame (born 1953), Nigerian educator and politician
* Jennifer Lame (), American film editor
* Quintín Lame (1880–1967), Colombi ...
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
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
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 Observer bias is one of the types of detection bias and is defined as any kind of systematic divergence from accurate facts during observation and the recording of data and information in studies. The definition can be further expanded upon to incl ...
, in which subjective like/dislike of a certain compression methodology emotionally influences their judgment. This bias is commonly referred to as ''
placebo
A placebo ( ) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like saline), sham surgery, and other procedures.
In general, placebos can aff ...
'', 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
In scientific research, the null hypothesis (often denoted ''H''0) is the claim that no difference or relationship exists between two sets of data or variables being analyzed. The null hypothesis is that any experimentally observed difference is d ...
that the samples tested are the same and with an
alternative hypothesis
In statistical hypothesis testing, the alternative hypothesis is one of the proposed proposition 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 statistic ...
methods are transparent, by nature.
In image compression
Both the DSC in
DisplayPort
DisplayPort (DP) is a digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association (VESA). It is primarily used to connect a video source to a display device su ...
and the default settings of
JPEG XL
JPEG XL is a royalty-free raster-graphics file format that supports both lossy and lossless compression. It is designed to outperform existing raster formats and thus become their universal replacement.
Name
The name consists of ''JPEG'' (for 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 sensitive 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 discipline that uses medical imaging to diagnose diseases and guide their treatment, within the bodies of humans and other animals. It began with radiography (which is why its name has a root referring to radiatio ...
(specifically for the study of
diagnostically acceptable irreversible compression Diagnostically acceptable irreversible compression (DAIC) is the amount of lossy compression which can be used on a medical image to produce a result that does not prevent the reader from using the image to make a medical diagnosis.
The term was f ...
), 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
*
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.
{{refend
External links
"Transparency" Hydrogen Audio Wiki
Data compression
Audio codecs