An immediate observation for those who deal with the processing of signals is the

omnipresence of the concepts of frequency, which are called “spectra” here. This

presence is important in the fields of theoretical research as well as in most applied

sectors of engineering. We can, with good reason, wonder on the pertinence of the

importance given to spectral approaches. On the fundamental plane, they relate to

the Fourier transformation, projection of signals, signal descr iptors, on the basis of

special periodic functions, which include complex exponential functions. By

generalizing this concept, the basis of functions can be wider (Hadamard, Walsh, etc.),

while maintaining the essential characteristics of the Fourier basis. By projections, the

spectral representations measure the “similarity” between the projected quantity and a

particular base: they have henceforth no more – and no less – legitimacy than this.