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

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?

[Alex Mcaulley]

Following butter documentation a and b are always row vectors (also running your code).

Answer 1

[Jan]

I cannot confirm this. With standard Matlab functions, butter replies vectors. So check, if you use the official function:

which butter -all

[Grace Wagstaffe]

I am using the butter function from

C:\Program Files\MATLAB\R2017a\toolbox\shared\controllib\graphics\+ctrluis\+toolstrip\+dataprocessing

[Jan]

I'd expect this one:

C:\Program Files\MATLAB\R2017a\toolbox\signal\signal\butter.m

What does this reply:

which butter -all

?

Answer 2

[Star Strider]

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.