band pass filter a signal using FFT

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zozo
zozo 2012-4-16
评论： Joydeb Saha 2021-6-19
I have a random signal containing frequencies from 1Hz to 1000Hz (as viewed on a spectrogram).
I want to extract the signal containing freqs from 200Hz to 600Hz from it and zero out other frequencies (band pass filter). How can I do it in freq. domain efficiently? (since my signal is very long, doing it in time domain is time consuming using filter coefficient method)
Please help with an example.

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Wayne King
Wayne King 2012-4-16
With such a high sampling frequency and that many samples, why don't you downsample the signal using decimate()?
Since you are only interested in content between 100 and 300 Hz, you can downsample that signal a lot - by at least a factor of 25.
y = decimate(input,25);
That will give you data sampled at 2 kHz and make designing your filter much, much easier.
  2 个评论
zozo
zozo 2012-4-16
this works well..thank you sir. :)
Jan
Jan 2012-11-27
If this solves your problem, please accept this anser.

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更多回答（2 个）

Wayne King
Wayne King 2012-4-16
Have you considered fftfilt()?
You can design your FIR filter using fdesign.bandpass and then use fftfilt().
  2 个评论
zozo
zozo 2012-4-16
fs=1000;
t=0:1/fs:2-1/fs;
x=sin(2*pi*100*t)+cos(2*pi*200*t)+sin(2*pi*150*t)+cos(2*pi*60*t);
f = fs*linspace(0,1,length(x));
plot(f,abs(fft(x)))
i want to have only 100Hz,150Hz freqs in my output signal. How can I do this using fdesign.bandpass?
[I tried but Iam having problems specifying inputs to the function]
zozo
zozo 2012-4-16
Also, how do I convert the object 'd' into double to use it as an input to fftfilt() ?

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Wayne King
Wayne King 2012-4-16
d = fdesign.bandpass('Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2',75,100,150,175,50,0.5,50,1e3);
Hd = design(d);
y = filter(Hd,x);
If you want an IIR filter
Hd1 = design(d,'butter');
y1 = filter(Hd1,x);
plot(psd(spectrum.periodogram,y1,'Fs',1e3,'NFFT',length(y1)));
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zozo
zozo 2012-4-16
It is a signal recorded by an array of microphones. When viewed on a spectrogram, there was a line passing between the band of 100Hz and 300Hz. To further process it, I needed to scoop out signal from this band of frequencies(ignore other frequencies to reduce noise contribution) efficiently.
Joydeb Saha
Joydeb Saha 2021-6-19
How to remove the spikes from the data with a narrow bandpass filter? How to apply an inverse FFT algorithm to recreate the time-series data (filtered) from the frequency domain to the time domain? attaching the data.

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