How i use the command Butter?

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.

[Jan]

What is "6(210/2)"?

[Daniel Shub]

In general masking built-in functions with variables is a bad idea. Masking EXIT is only asking for problems.

[Marcelo Costa]

I forgett 6/(210/2). 6 is a cutoff frequency and 210 is acquisition frequency. I try using this code now: [b,a] = butter(4,6/(210/2)); arq2=filtfilt(b,a,arq(:,m)); and the error is: Error using filtfilt>getCoeffsAndInitialConditions (line 149) Data must have length more than 3 times filter order.

Error in filtfilt (line 67) [b,a,zi,nfact,L] = getCoeffsAndInitialConditions(b,a,Npts);

Error in testefiltro (line 12)
arq2=filtfilt(b,a,arq(m,:));

Answer 1

[Wayne King]

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

[Marcelo Costa]

I made this and working.

Thanks.

Answer 2

[Wayne King]

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.

Answer 3

[Marcelo Costa]

Hello Again This is my command.

clear all
f2=12.5; frequency of cutoff
Fs=210; acquisition frequency
Files=dir('*.txt');
[z,n]=size(Files);
for x=1:z
   arq=load(Files(x).name);
   [v,p]=size(arq);
for m=1:p;
[b,a] = butter(4,f2/(Fs/2));
arq2=filtfilt(b,a,arq(:,m));
res=arq -arq2;
end
savefile=['filtrado','.',Files(x).name];
save(fullfile(savefile),'res','-ASCII');
end

my txt files is saved like this 4 rows X 37 coluns.

But always appers this msg Error using filtfilt>getCoeffsAndInitialConditions (line 149) Data must have length more than 3 times filter order.

Error in filtfilt (line 67) [b,a,zi,nfact,L] = getCoeffsAndInitialConditions(b,a,Npts);