MATLAB: Right filter syntax for the [z,p,k] syntax

filterfilter designSignal Processing Toolbox

Hi, I used the butterworth filter with the [b,a] syntax:
    filtord=4;  
    SampleRate=20000;
    d = fdesign.bandpass('N,F3dB1,F3dB2',4,100, 300,SampleRate);  
    hd = design(d,'butter');
    % zero phase shift by reverse filtering
    SignalFiltered=filtfilt(b,a, Signal);
Now I want to use higher order filter numbers where the help suggests to switch to the [z,p,k] syntax.
      Wn=[100,300]/(SampleRate/2);  % cutoff frequencies normalized by nyquist
        ftype='bandpass';
        filtord=10;
        [z, p, k] = butter(filtord,Wn,ftype);
        [sos,g]=zp2sos(z,p,k);
        hd=dfilt.df2sos(sos,g);
Simple question: would will be the appropriate filter command (SignalFiltered=filtfilt(??))? Or is there a missing piece of code? Thanks for your advice!
P.S.: Is the missing code the following:
[b,a]= sos2tf(hd.sosMatrix,hd.ScaleValues);
SignalFiltered=filtfilt(b,a, Signal);
?????

Best Answer

In general you don't want to switch back to b and a because you then have the numerical issue again.
If you have at least MATLAB R2011a, you can do
SignalFiltered = filtfilt(hd.sosMatrix,hd.ScaleValues,Signal);
Otherwise, you can do
hd.PersistentMemory = true;
y = filter(hd,Signal);
SignalFiltered = filter(hd,fliplr(y));
twice, one forward and one backward to mimic the behavior of filtfilt. You may need to set PersistentMemory to true to retain the state
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