The subband received signal xk (m) can also be generated by such a method as shown in FIG. 21, in which the received signal and the echo signal are multiplied by N signals Wk =ej2??k/M (where k=0, . . . ,N-1) by multipliers 23k to shift their frequencies toward the low-frequency band in steps of k/M of the bandwidth, then the N signals are band-limited by low-pass filters 24k of the same frequency