Right filter syntax for the [z,p,k] syntax
9 views (last 30 days)
Show older comments
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);
?????
0 Comments
Accepted Answer
Honglei Chen
on 27 Jun 2013
Edited: Honglei Chen
on 27 Jun 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 Comments
Honglei Chen
on 28 Jun 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.
More Answers (1)
See Also
Categories
Find more on Digital Filtering in Help Center and File Exchange
Products
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!