Why does filtfilt provide me wrong results?

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pietro 2015-2-8

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https://ww2.mathworks.cn/matlabcentral/answers/176479-why-does-filtfilt-provide-me-wrong-results

评论： pietro 2015-3-19

Hi all,

I'm using filtfilt function to filter some signals since i would like to minimize the phase lag between the original signal and the filtered one.

Here my code:

LPsig= designfilt('lowpassiir','FilterOrder',10,'PassbandFrequency',1,'PassbandRipple',0.2, ...
         'SampleRate',Fs);
dfCoeff=double(LPsig);
FilteredSignalP=filtfilt(dfCoeff.Coefficients,1,Signal);

In this plot the filtered and non filtered signal are compared: http://s9.postimg.org/46qlzkz67/Immagine.jpg

There is the edge effect with a huge ripple. I found out that both effect are increased with the filter order, with a filter order of 10 both of them are negligeble. Could someone explain to me why? Anytime I have to filter data, I change almost randomly the filter parameter (filter order, filter type, passband ripple, etc) until I get a good result. How may I select the filter parameters?

Thanks

Regards

Pietro

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Answer 1

Erik S. 2015-2-8

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https://ww2.mathworks.cn/matlabcentral/answers/176479-why-does-filtfilt-provide-me-wrong-results#answer_167366

Why do you write dfCoeff = double(LPsig);

Everything is double precision already.

Call the filter function like

FilteredSignal = filtfilt(LPsig,Signal);

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Erik S. 2015-2-9

Can you send the data you want to filter? Do you know which answer to get from the filter?

pietro 2015-3-19

Hi Erik, I'm sorry for the delayed reply, I was convinced I had the email notification turned on. I will send you the data this evening, because I have limited internet connection at my workplace.

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