How do I only get vectors out of the butter function?

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Grace Wagstaffe
Grace Wagstaffe 2019-5-23
编辑： Jan 2019-5-23
I am using the function butter as follows.
freq = 125;
rate = 300;
t = [0:0.1:4*pi]
s = sin(t);
[b,a] = butter(2,freq/(rate*2),'low');
dataIn = s;
dataOut = filter(b,a,dataIn);
When I look at b it is a 2x2 matrix, however to use the filter function it must be a vector.
I am using the R2017a.
I have also tried modifying the matlab example in browser and I get a vector
Where am I going wrong?

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回答（2 个）

Jan
Jan 2019-5-23
I cannot confirm this. With standard Matlab functions, butter replies vectors. So check, if you use the official function:
which butter -all
  2 个评论
Grace Wagstaffe
Grace Wagstaffe 2019-5-23
I am using the butter function from
C:\Program Files\MATLAB\R2017a\toolbox\shared\controllib\graphics\+ctrluis\+toolstrip\+dataprocessing
Jan
Jan 2019-5-23
编辑：Jan 2019-5-23
I'd expect this one:
C:\Program Files\MATLAB\R2017a\toolbox\signal\signal\butter.m
What does this reply:
which butter -all
?

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Star Strider
Star Strider 2019-5-23
Your filter design has other problems. Your actual sampling frequency is:
Fs = 1/mean(diff(t));
or 10, and the Nyquist frequency (that you use to normalise your desired frequency) is half of that.
Your butter call should be:
[b,a] = butter(2,freq/(Fs/2),'low');
Note also that ‘freq’ must be less than ‘Fs/2’, so a passband frequency of 300 will fail.
It is also better to use filtfilt than filter to do the actual filtering.

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