freqs Compute the s-plane frequency response of the IIR filter. h = freqs (b, a, w) b: vector containing the coefficients of the numerator of the filter. a: vector containing the coefficients of the denominator of the filter. w: vector containing frequencies This is an Octave function. It computes the s-plane frequency response of the IIR filter B(s)/A(s) as H = polyval(B,j*W)./polyval(A,j*W). If called with no output argument, a plot of magnitude and phase are displayed.