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

2013 6 27
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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);
?????

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Honglei Chen
Honglei Chen 2013-6-27
编辑：Honglei Chen 2013-6-27

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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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Peter
Peter 2013-6-28
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
Honglei Chen 2013-6-28
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.

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Peter
Peter 2013-6-27

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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!

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