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Topic: "Simple" filter design wrong answer
Replies: 1   Last Post: Sep 17, 1996 11:25 AM

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Dan Babitch

Posts: 3
Registered: 12/7/04
"Simple" filter design wrong answer
Posted: Sep 15, 1996 1:25 AM
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Here is a little problem that should work but does not. I'm using the Signal
Processing Toolbox 3 on Matlab 4.2c. I tell it to design a second order FIR
notch filter and display the frequency domain transfer function:

b=fir1(2,[.49 .51],'stop');
freqz(b,1,512);

The correct answer is b = [.5 0 .5], (this really is a notch at 1/2)but the
output of fir1 is completely wrong. If I use n=4 or more it always works
fine. If I try making the points farther apart, such as .25 and .75, it still
fails. If I try fir2 with many different appropriate choices of line
segments, it still always fails for n=2 and always works for n=4 or more.
If I run a firls design it also always fails for n=2 and always works for
n=4 or more.

If I ask for the notch to be some other frequency, I get the same problem.
There are no warnings or errors.

This is very strange. I feel I must be doing something terribly obvious
wrong.

Does anyone have any suggestions? I am a somewhat experienced user. I
never had this kind of results before, but I guess I never told it to do just
this problem before. I've never been this old before, either.

Thanks for your help.
Dan Babitch





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