Question

I went over and over again with MATLAB Help and google but couldn’t find any solution.

First of all, designing the bandstop filter, the help says :

type:’stop’ for an order 2*n bandstop digital filter if Wn is a two-element vector, Wn = [w1 w2]. The stopband is w1 < w < w2.

Does that mean when i design my bandstop filter, the code should be :

[B,A] = butter(2*N,Wn);  OR  [B,A] = butter(N,Wn);`

where the order N is given by user during runtime and Wn is calculated using buttrod function with known parameters.

Second of all, according to the help, only 3 types can be used to design a butterworth filter (low,high,and stop) , How do i design a bandpass filter ?

Answer 1

1. [B,A] = butter(N,Wn) means you will have 2*N poles/roots/…. and usually 2*N+1 terms in both B and A. Note that the poles are complex conjugates, so if you want 10 poles, N = 5….

2. The documentation is confusing… you can have all 4 filer types:

   [B,A] = butter(N,Wn, 'type');
   

where type = ‘stop’, ‘bandpass’, ‘low’, ‘high’