Remove 50 Hz noise

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Paola 2018-6-25

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https://ww2.mathworks.cn/matlabcentral/answers/407241-remove-50-hz-noise

编辑： Walter Roberson 2020-2-21

I am recording 6 channels from a force transducer, but there is a 50 Hz noise I want to remove. I was considering to use a simple solution as iirnotch, but I don't have this function on my Matlab R2015a. How can I create a filter (filtfilt and fir1?) to eliminate this 50 Hz without a signal processing toolbox?

Thank you

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Star Strider 2018-6-28

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I thought you said that you do not have the Signal Processing Toolbox! I agree with Rik Wisselink that with it this is essentially trivial. However, you appear to be using its functions, so I will add this:

Fs = 20000;
Fn = Fs/2;
Wp = [47 53]/Fn;
Ws = [49 51]/Fn;
Rp =  1;
Rs = 50;
[n,Wn] = buttord(Wp,Ws,Rp,Rs);
[z,p,k] = butter(n,Wn,'stop');
[sos,g] = zp2sos(z,p,k);
figure
freqz(sos, 2^14, Fs)
set(subplot(2,1,1), 'XLim',[0 100])
set(subplot(2,1,2), 'XLim',[0 100])

The freqz call lets you see what the filter is doing in the frequency domain. There is also no need for the loop, because the filter functions operate column-wise. You simply need to transpose your matrix.

This should work:

ForceDataFilt = filtfilt(sos, g, ForceData');                       % Transpose & Filter
ForceDataFilt = ForceDataFilt';

I transposed ‘ForceDataFilt’ so it will match the rest of your code. Experiment with the filter passband and stopband frequencies to get the result you want. Remember that for a notch or bandstop filter, the stopband frequencies (here ‘Ws’) are within the passband limits, ‘Wp’. The passband and stopband attenuation values are ‘Rp’ and ‘Rs’ respectively.