Dear all,

I'm Andrea and this is my first post on DSPRelated.

I have a trouble that I can't solve.

I have a generic system whose output, in the frequency domain, has a module
proportional to the fourth power of angular frequency [ |.| proportional to w.^4
].

I have to build a FIR filter that compensates this phenomena. So the filter is a
sort of lowpass filter decaying with an inverse fourth power law.

I build up such a filter with the frequency sampling method (manual, not fir2),
but filtering the signal (sampled at 44100 Hz) I encounter some problems:

- filter's numer of taps must be a multiple of 1024 to have linear phase
(!!!)

- filtering the signal (with filter command) I obtain some unwanted zeros in the
frequency domain corresponding to the secondary lobes. But I can see these
secondary lobes only with a very high resolution FFT or FREQZ

- fir2 doesn't work (the ideal freq. response is very different from the
obtained one)

What I can do to solve?

Thank you very much,

Andrea