Why are there differences in the filter coefficients generated via FDATool and from using the MATLAB command 'fdesign.lowpass'?
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I used the following code to produce a 70 tap filter:
fs = 203125;
Fpass = 9000;
Fstop = 12500;
Ap = 1;
Ast = 30;
Fp = Fpass/(fs/2);
Fst = Fstop/(fs/2);
d = fdesign.lowpass('Fp,Fst,Ap,Ast',Fp,Fst,Ap,Ast);
f = design(d,'equiripple');
However when I enter the same configurations into FDATool by setting fs = 203125, Fpass = 9000, Fstop = 12500, Astop = 30, Apass =1, Lowpass and equiripple, I get a 67 tap filter.
I would like to know the difference between these methods produce and why they produce different filters.
Accepted Answer
MathWorks Support Team
on 27 Jun 2009
FDESIGN uses the FIRGR function while FDATool uses the FIRPM function.
FIRGR and FIRPM both design equiripple filters but they use different implementations of the Parks-McClellan algorithm. Thus, they may return different answers.
In order to design filters graphically and ensure that the results are consistent with FDESIGN, use the FilterBuilder GUI instead of FDATool.
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