Re: freqz behavior

[Augustin Lefèvre]

    Hello Tony,
    
    The reason is that you are feeding complex coefficients in the
    second case, whereas in the first case all coefficients are real.
    
    cf.  "octave/4.2.2/m/signal/freqz.m"

On 18/06/2020 21:57, Tony Richardson wrote:

% Poles 0.9 exp(+-j pi/4), Zero at -1
subplot(2, 1, 1)
theta_r = [-1 1]*pi/4;
a = poly(0.9*exp(j*theta_r));
b = poly(-1);
[H1 W1] = freqz(b, a);
plot(W1/pi, abs(H1))

% No poles.  Zeros at angles that are multiples of pi/4 on unit circle except at angle 0
subplot(2, 1, 2)
theta_r = [-3:-1 1:4]*pi/4;
a = 1;
b = poly(exp(j*theta_r));
[H2 W2] = freqz(b, a);
plot(W2/pi, abs(H2))