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. 2013 Nov 4;61(3):736–744. doi: 10.1109/TBME.2013.2288497

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Fig. 2. — (Modeling the respiration signal) Panel (a) plots a cosine function, an overly simplified model of the respiration; moving to the right shows enrichment of this model by allowing nonconstant instantaneous frequency (b) amplitude modulation (c) and a more complex wave shape function (d). The (nonconstant) instantaneous frequency and the amplitude modulation function should capture how the oscillations vary in time, independent of whether the basic shape function is a simple trigonometric function or not.