and operates on a corresponding subset Si of the input spectra Xj(f), j=1, 2, . . . , NM. (To say that A is a subset of B does not exclude the possibility that subset A may equal set B.) The processor 207 may window each of the spectra of the subset Si with a window function Wi(f) corresponding to the frequency range R(i) to obtain windowed spectra, and, operate on the windowed spectra with the be