% ddiff.m - ideal lowpass differentiator FIR filter%% h = ddiff(wc, N) = row vector%% N = 2M+1 = filter length (odd)% wc in rads/samplefunction h = ddiff(wc, N)M = (N-1)/2;for k = -M:M,  if k == 0,    h(k+M+1) = 0;  else    h(k+M+1) = wc * cos(wc * k) / (pi * k) - sin(wc * k) / (pi * k^2);  endend