Abstract: Precalculated samples of a power-density spectrum, taken at equispaced frequencies, are converted into modified samples with a nonlinear (e.g. logarithmic) distribution by being selectively weighted with the aid of several groups of coefficients defining a series of overlapping windows of mutually different widths. The system determines the weighting coefficient whose position in the frequency band of any window is closest to that of a precalculated sample appearing within that window; for each window, any sample appearing therein is multiplied by its closest coefficient and the sum of the resulting products is then divided by a factor proportional to the width of the window.