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

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);

?????

Answer 1

[Honglei Chen]

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

[Peter]

Thanks for your assistance - unfortunately there ist still a phase shift (which I do not understand, because the filtfilt function is also based on the filter function). Nevertheless I found the filtfilthd.m file on the matlabe file exchange and it works pretty well.

      SignalFiltered = filtfilthd(hd,Signal);

[Honglei Chen]

filtfilt do something special to set the initial condition to reduce the transient, this is probably the reason you see some phase shift. Looks like filtfilthd duplicates that behavior.

Answer 2

[Peter]

Thanks for the advice. Could you give me a code snippet for the reverse filterung using filter (unfortunately I can't use the filtfilt function as mentioned above). Thanks a lot!

[Honglei Chen]

I updated the answer above