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## Re: Butter Filters

**From**: |
Doug Stewart |

**Subject**: |
Re: Butter Filters |

**Date**: |
Mon, 17 Feb 2003 18:32:21 -0500 |

**User-agent**: |
Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.2.1) Gecko/20021130 |

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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