how to filter the noise of a signal

Question

Luca Tognetti 2023-2-28

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https://ww2.mathworks.cn/matlabcentral/answers/1920640-how-to-filter-the-noise-of-a-signal

回答： Mathieu NOE 2023-2-28

Hi, I need to filter the signals when the value is close to 0 (where the noise is much present). as you can see from the picture there is a lot of noise. I attached the .mat file and the picture

I don't need a particular filter, i just need to clean it a bit so I ask you which is the best method.

thanks.

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Answer 1

Mathieu NOE 2023-2-28

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hello

I was surprised to see that your signal length is very long (730185 samples) and probably you are using a quite high sampling rate for what you really need

so basically I started downsampling your data with decimate (that includes also a low pass filter) and that is enough to already decrease significantly your noise level (which is high freq noise)

as a second step you can add some smoothing with smoothdata (equivalent to a low pass filter without phase distortion)

fyi, you can remove the decimation step if you really need to keep that high amount of data, but for plotting we rarely need more than a few thousands points

Zoom

load('matlab.mat');
% convert to double
F = double(F);
x = 1:numel(F);
% decimate first
r = 25;
Fd = decimate(F,r);
xd = 1:r:x(end);
% then low pass filter 
N = 25;
Fs1=smoothdata(Fd,'movmedian',N);
plot(x,F,xd,Fd,xd,Fs1)
legend('raw','decimated','decimated and smoothed');