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 (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 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. External links "Transparency", Hydrogen Audio Wiki Data compression Audio codecs