function s = freq(b,a,n)

% FREQ	Frequency response of a digital filter.

%	FREQ(B,A,N) returns the n-point frequency response of filter B/A:

%

%		                      -1              -(nb-1) 

%		   B(z)   b(1) + b(2)z + .... + b(nb)z

%		   ---- = ---------------------------

%		                      -1              -(na-1)

%		   A(z)    1   + a(2)z + .... + a(na)z

%

%	given the numerator and denominator coefficients in vectors B and A. 

%	If N is not a power of two it will be rounded up to the next power.

%	The frequency response is evaluated at N points equally spaced around

%	the unit circle.  The magnitude and phase can be plotted with the

%	commands:

%			s = freq(b,a,n);

%			mag = abs(s);

%			phase = imag(log(s));

%			f = pi*(0:n-1)/n;

%			semilogy(f,mag), plot(f,phase)

%

%	See also FDESIGN, FILTER, and FFT.

na = max(size(a));

nb = max(size(b));

s  = fft([b zeros(1,n-nb)]) ./ fft([a zeros(1,n-na)]);