Why are there differences in the filter coefficients generated via FDATool and from using the MATLAB command 'fdesign.lowpass'?

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.

Answer 1

[MathWorks Support Team]

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.