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

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Kay on 22 Sep 2011
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

Answers (2)

Honglei Chen
Honglei Chen on 23 Sep 2011
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);

Arturo Moncada-Torres
Arturo Moncada-Torres on 23 Sep 2011
Try checking this previous post. Although it is for a HPF, the main steps are practically the same ;-)

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