Right filter syntax for the [z,p,k] syntax
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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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Respuesta aceptada
Honglei Chen
el 27 de Jun. de 2013
Editada: Honglei Chen
el 27 de Jun. de 2013
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
2 comentarios
Honglei Chen
el 28 de Jun. de 2013
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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