How to convert filter object to state space?

I'm creating a filter object using

%B1(f) is my target transfer function
d_1a = fdesign.arbmagnphase('n,f,h',8,(0:0.1:30),double(B1(0:0.1:30)),60);
filtObj = design(d_1a,'iirls','SystemObject',true)

Then, I can use

freqz(filtObj)

to confirm it matches the target function. However, when I convert it to state space using

[A,B,C,D] = ss(filtObj);
sys_ss1 = ss(A,B,C,D);

Then I try to confirm the behavior of this system using

[H,wout] = freqresp(sys_ss1);

this output does not match my target transfer function. What am I missing here?

Answer 1

[Robert U]

Hi tfg250,

You missed to supply the sample frequency for your state space model. Converting the filter object for a time discrete system to state space requires to use a time-discrete state space model as well.

[A,B,C,D] = ss(filtObj);
sys_ss1 = ss(A,B,C,D,1/60);

Kind regards,

Robert

[tfg250]

that's is, thank you so much!