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

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Kay
Kay 2011-9-22
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

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回答（2 个）

Honglei Chen
Honglei Chen 2011-9-23
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
Arturo Moncada-Torres
Try checking this previous post. Although it is for a HPF, the main steps are practically the same ;-)

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