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==Introduction==
In the field of [[time-frequency signal processing]] (including [[time-frequency analysis]]), the goal is to define signal formulations that are used for representing the signal in a joint time-frequency ___domain (see also [[time-frequency representationsrepresentation]]s<ref>B. Boashash, “Time-Frequency Concepts”, Chapter 1, pp. 3–28, in B. Boashash, ed,, Time-Frequency Signal Analysis & Processing: A Comprehensive Reference, Elsevier Science, Oxford, 2003; ISBN 008044335.</ref>). thereThere are several methods and transforms called Time"time-Frequencyfrequency Distributionsdistributions" (TFDs).<ref>B. Boashash, “Heuristic Formulation of Time-Frequency Distributions”, Chapter 2, pp. 29–58, in B. Boashash, editor, Time-Frequency Signal Analysis and Processing: A Comprehensive Reference, Elsevier Science, Oxford, 2003; ISBN 008044335.</ref> . The most useful and used methods form a class referred to as [["quadratic time-frequency distributions]]" or [[bilinear time-frequency distributionsdistribution]]s. A core member of this class is the [[Wigner-Ville Distributiondistribution]] (WVD),<ref>B. Boashash, "Note on the Use of the Wigner Distribution for Time Frequency Signal Analysis", IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. 36, No. 9, pp. 1518–1521, Sept. 1988</ref> as all other TFDs can be written as a smoothed version of the WVD. Another popular member of this class is the spectrogram (square magnitude of the short-time Fourier transform) which has the advantage of being positive and therefore easy to interpret (but has other disadvantages). The theory and methodology for defining a TFD that verifies certain desirable properties is given in <ref>B. Boashash, “Theory of Quadratic TFDs”, Chapter 3, pp. 59–82, in B. Boashash, editor, Time-Frequency Signal Analysis & Processing: A Comprehensive Reference, Elsevier, Oxford, 2003; ISBN 0080443354.</ref>. This article outlines some elements of the procedure to transform one distribution into another, using an approach and terminology that is borrowed from Quantum[[quantum Mechanicsmechanics]] although the subject matter of the article is "signal processing". Noting that a signal can recovered from a particular distribution under certain conditions, given a certain TFD ρ1(t,f) representing the signal in a joint time-frequency ___domain, another different TFD ρ2(t,f) of the same signal can be obtained to calculate any other distribution, by simple smoothing or filtering; some of these relationships are shown below. A full treatment of the question iscan be given from a signal processing perspective in .<ref>B. Boashash, editor, “Time-Frequency Signal Analysis and Processing – A Comprehensive Reference”, Elsevier Science, Oxford, 2003; ISBN 0080443354</ref>.
==General class==
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