I am trying to make a high pass filter for electroencephalographic (EEG) datasets, in order to get rid of very slow drifts. However frequencies around 0.3 Hz are very important for the research these data serve.
I use Matlab and I tried various things. It seems that a very low dB attenuation, of 3 dB is fine with FIR filters. For example I used that:
h=fdesign.highpass('Fst,Fp,Ast,Ap',0.005,0.01,3,1,250); % i needed to cut d=design(h,'butter'); fvtool(d)
However this filter makes a dc drift at the start of the data and I would not wish to loose these first datapoints.
I have read through your pages that a clever idea is to design a low pass filter that later I would subtract from the real data. I did it with an equiripple filter and it worked but left a baseline lift. I do not want to run now average removal as this is in a next step in the analysis protocol.