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 件のコメント
Daniel Shub
2012 年 11 月 14 日
In general masking built-in functions with variables is a bad idea. Masking EXIT is only asking for problems.
採用された回答
Wayne King
2012 年 11 月 20 日
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
その他の回答 (2 件)
Wayne King
2012 年 11 月 13 日
編集済み: Wayne King
2012 年 11 月 13 日
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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