How i use the command Butter?
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I have two questions. The first question is: How do I choose the cutoff frequency of the Butterworth filter The second question is how to filter this data? Attached the link to download the data? https://docs.google.com/open?id=0BzSixbWRFmQLbmRRSC0tUEdMMlU
Thank you. I use this algorith
for m=1:endoffile
[a,b]=butter(4,6(210/2), 'low')
exit(1:4,m)=filter(d,f,arq(1:4,m))
However, the results are very different from the actual data.
3 Comments
Daniel Shub
on 14 Nov 2012
In general masking built-in functions with variables is a bad idea. Masking EXIT is only asking for problems.
Accepted Answer
Wayne King
on 20 Nov 2012
You cannot use filtfilt unless your input vector is more than 3 times as long as your coefficient vectors (the order of your filter).
Because filtfilt works on the columns of your data matrix, you need to transpose your data matrix assuming that you want your filter to operate on the dimension with 37 samples, not 4
More Answers (2)
Wayne King
on 13 Nov 2012
Edited: Wayne King
on 13 Nov 2012
Nobody can answer how to choose the cutoff frequency for you. That depends on your filter design. Suppose I wanted to filter data below 150 Hz and the data were sampled at 1 kHz. I would do the following. First I'll create some data and then filter it.
t = 0:0.001:1-0.001;
x = cos(2*pi*100*t)+randn(size(t));
[B,A] = butter(10,150/(1000/2));
y = filter(B,A,x);
If you want to view the magnitude response of your Butterworth filter, enter:
fvtool(B,A,'Fs',1000)
Obviously you have to use the correct sampling frequency.
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