# Berchin's FDLS arbitrary filter design algorithm

I'm trying to use Berchin's FDLS methods (for those that don't know what it is, it's a way to design arbitrary magnitude and phase of an IIR Eq)

I tried different things but neither gave me what i asked:

1. I tried to design an allpass filter with arbitrary phase response (nothing fancy just a linear phase and a short deviation for 5 points) and it didn't worked as planned. Actually what i want to do is to counter the phase change of an IIR filter

2. I tried a linear phase response for a $-12\textrm{ dB}$ notch and the same way it didn't worked (in black the desired response)

I used a samplerate of $316$, $M=158$ points and a filter order of around $50$.

Do I need to use more points to increase the filter order or is it just impossible to design my filters using that algorithm?

• @PeterK.: Although it is brief, you might consider making that an answer. Since the OP seems to have disappeared, there probably won't be any better resolution to the problem. – Jason R Sep 22 '13 at 23:59
• @JasonR: Done! And set to Community Wiki. – Peter K. Sep 23 '13 at 14:59

I haven't used the technique, but an IIR filter order of 50 sounds remarkably high, and possibly prone to numerical problems. Try starting out with much smaller values for M and filter order, and slowly ramping them up. The paper you quote uses $M=8$ and $N=9$.