What are the steps for making a 'fft filter'?

Hi,

I am new to the fft filter and I want to know the steps of building a fft filter, e.g., what I need to know before I build this filter. In addition, my Matlab has Signal Processing Toolbox installed.

I am given a sequence with 131072 samples, the sampling rate is 500Hz. Also, I want to filter this sequence using low pass filter with cutoff frequency=20Hz.

My question is about the syntax of "fftfilt", how do I generate the coefficient vector b by providing the cutoff frequency(the freq range that i'm interested).

Thanks for your help, Kay

Answer 1

[Honglei Chen]

Hi Kay,

You need to first design the filter to get coefficient b and then pass it into the fftfilt. For example, assume you need a 10th order filter with cutoff of 20 Hz and sampling frequency of 500 Hz, you can do

h = fdesign.lowpass('N,Fc',10,20,500);
hd = design(h);
x = rand(131072,1);
y = fftfilt(hd.Numerator,x);

HTH

Answer 2

[Arturo Moncada-Torres]

Try checking this previous post. Although it is for a HPF, the main steps are practically the same ;-)