MATLAB: High-pass Butterworth filter and filtfilt

butterfiltfilt

Hello everyone, I'm trying to high-filter a signal beacause when I perform a fft of the original signal I find a contribution at very low frequency even if I take the fft of detrend(signal). I want to cut out that contribution. I wrote this simple script:
Fcp=3;  %cutoff frequency
[z,p,k]=butter(8,Fcp/(Fsp/2),'high');
sos=zp2sos(z,p,k);
%fvtool(sos,'Analysis','freq')
x101filtfilt=filtfilt(z,p,x101);
figure;
plot(t101,x101,t101,x101filtfilt);
grid on
title('PE101 (pressure signal)')
xlabel('t (s)')
ylabel('Pressure (kPa)')
legend({'Original PE101 Signal','Actual PE101 Signal (filtered and realigned signal)'});
The sampling frequency is 600 Hz and the signal x101 is about 180 s long. I receive the error: "Warning: Imaginary parts of complex X and/or Y arguments ignored". As a matter of fact the new signal x101filtfilt contains imaginary numbers. Where am I wrong? Thank you very much.

Best Answer

You have two errors that I can see.
1. You need both outputs from zp2sos:
[sos,g] = zp2sos(z,p,k);
2. Then use those in your filtfilt call:
x101filtfilt = filtfilt(sos,g,x101);
That should solve your imaginary number problem (unless ‘x101’ is complex).
I also analysed your filter with freqz:
freqz(sos, 2^16, Fsp)
It appears stable and that it will do what you want it to do.
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