Why do I get different frequency response using state-space representation and transfer function

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After using butter function to design a filter (as follow), I wanted to check the frequency response. However I suddenly find that the frequency response seems to be different using different representation. That doesn't make sense to me. Thanks a lot for answering me.
[Input, Fs]=audioread('input.wav');
Ws=[10 3800]/(Fs/2);
Wp=[30 3500]/(Fs/2);
[N, Wn]=buttord(Wp,Ws,0.1,30); %Design band pass filter.
[A,B,C,D]=butter(N,Wn);
sos=ss2sos(A,B,C,D);
fvtool(sos,'Fs',Fs); % Plot Freq Response using state space representation.
[b,a]=ss2tf(A,B,C,D);
fvt=fvtool(b,a,'Fs',Fs); % Plot Freq Response using Transfer Function.
Output=filter(b,a,Input);
audiowrite('output.wav',Output,Fs);

Respuestas (1)

Christoph F.
Christoph F. el 17 de En. de 2018
Editada: Christoph F. el 17 de En. de 2018
124th order recursive filters are iffy, numerically. At this point, the limited precision of even the double datatype can lead to erroneous behavior.
Generally: max(abs(a)) is on the order of 10^31 min(abs(a)) is on the order of 10^-9

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