Figure 1.

Time–frequency resolution of the wavelet approach. (a) Examples of wavelets and their time–frequency boxes representing the corresponding variance (energy) distribution. When the scale a decreases, the time resolution improves but the frequency resolution gets poorer and is shifted towards high frequencies. Conversely, if a increases the boxes shift towards the region of low frequencies and the height of the boxes becomes shorter (with a better frequency resolution) but their widths are longer (with a poor time resolution). (b) In contrast with wavelets, all the boxes of the windowed Fourier transform are obtained by a time- or frequency shift of a unique function, which yields the same variance spreads over the entire time–frequency plane.