How can i implement a butterworth filter in multiple files.

Hi everyone,

For our programming exam we have to implement a butterworth filter on the markers of a 3d camera (biomechanics). The files consist of the x,y and z coördinates of the markers and are stored in a map data. In the map are subfolders for each subject and per subject we have the markers.

        % butterworth filter for the Videosignals [b,a]=BUTTER(N,Wn,ftype)
        % N = orde van de filter = 4
        % Wn = cutoff frequentie = 6hz/(0.5*VideoFrameRate)
        % ftype = type filter = lowpass

        [b,a]=butter(4,0.12,'low');

When i try to use the filter on all of the files I get the error that all my subscript indices must be either real positive or logical.

this is what I came up with:

 teller1=teller1+1; 
           videosta.(sprintf('v_sta%c',num2str(teller1)))=...
                VideoSignals(:,[stair_markers.RKNE, stair_markers.RANK, stair_markers.RASI,                         stair_markers.RPSI, stair_markers.RHEE, stair_markers.LKNE, stair_markers.LANK, stair_markers.LASI, stair_markers.LPSI, stair_markers.LHEE, stair_markers.STRN], [1,2,3]);
            videosta.(sprintf('v_sta_filt%c',num2str(teller1)))=...
                filtfilt(b,a,videosta.(sprintf('v_sta%c',num2str(teller1))));

where videosta is for markers in our stair trial, we also have a sit to stand trial.

I really have no clue of what to do with this error. And I hope someone can help me with the things described here.

Kind regards