Re: Butter Filters

[Doug Stewart]

I found that Matlab gives a result that I think is wrong!
here it is:


EDU» [z,p,g]=butter(2,1,'s')
z =
         0
         0
    0.2500
p =
  -0.7071+ 0.7071i
  -0.7071- 0.7071i
g =
     1

EDU»
EDU» [n d]=zp2tf(z,p,g)
n =
    1.0000   -0.2500         0         0
d =
    1.0000    1.4142    1.0000
EDU»

Now this is my Octave results


>> [z,p,g]=butter1(2,1,'s')
z = []
p =
  -0.70711 + 0.70711i  -0.70711 - 0.70711i
g = 1
>> [n d]=zp2tf(z,p,g)
n = 1
d =

  1.00000  1.41421  1.00000


All low pass laplace filters have poles and no zeros.
Why does matlab's  results show a zero in the zp model?
I know this is an octave list and I should ask matlab why, but I am 
testing the new butter.m file for octive.
Doug Stewart



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