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A time–frequency representation is a view of a signal represented over both time and frequency. Time–frequency analysis means analysis into the time–frequency domain provided by a TFR. This is achieved by using a formulation often called "Time–Frequency Distribution", abbreviated as TFD.
TFRs are often complex-valued fields over time and frequency, where the modulus of the field represents either amplitude or "energy density" , and the argument of the field represents phase.