filtfilt [y]=filtfilt(b,a,x) b: Real or complex valued vector or matrix a: Real or complex valued vector or matrix x: Real or complex valued vector or matrix This is an Octave function In theory, it forwards and reverse filters the signal and corrects phase distortion upto an extent by a one-pass filter but squares the magnitude response in the process. Practically though, the correction isn't perfect and magnitude response, particularly the stop band is distorted.