Question

Is there some process that can determine / remove an unknown DC offset from a non-periodic discrete time signal?

The signal in in question has a sample rate of 25Hz and has harmonics of interest between 0.25 and 3 Hz.

I have tried using highpass filters mixed results, first I used a 10th order guassian with Fc = 0Hz, this did a good job of removing the offset but it severly attenuated the AC aswell although it did leave the signal shape intact, next I used a 168th order equilripple with a stopband at 0Hz and passband at 0.25Hz, the phase shift was too severe and the signal shape too distorted, the distortion could probably be reduced if the pass-band was brought down to 0.1Hz but this would just further increase the phase shift which I need to keep to the very minimum.

Before and after applying x – LPF(x), as suggested by Paul R

Answer 1

I recommend using a notch filter at DC and using filtfilt to make it zero phase.

a = [1 , -0.98]; b = [1,-1];

y = filtfilt(b,a,x);

The closer the second value of a gets to -1 the narrower your notch will be.