Why does filtfilt provide me wrong results?

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

Answer 1

[Erik S.]

Why do you write dfCoeff = double(LPsig);

Everything is double precision already.

Call the filter function like

FilteredSignal = filtfilt(LPsig,Signal);

[pietro]

Thanks for your comment. I didn't know this code simplification. Anyway this doesn't change the result.

[Erik S.]

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

[pietro]

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.