Usage: H = freqs(B,A,W);

Compute 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.

Example:
B = [1 2]; A = [1 1];
w = linspace(0,4,128);
freqs(B,A,w);