MATLAB: “butter” filter: Undefined function

butterfilterundefined function

Hi everyone,
I'm having some problems in creating a lowpass butter filter. I am using the function below:
fc = forceplatesInfo.frequency; % forceplates
[b,a] = butter(4,15/(fc/2),'low');
Where:
– fc is the frequency of the force plates (in this case it is 2000Hz)
It is a simple code but I keep getting this error: "Undefined function 'butter' for input arguments of type 'double'".
I used the matlab tutorial and i still can't figure this out.
Any ideas? Thanks Emanuele

Best Answer

The error message means, that you do not have the Signal Processing Toolbox, which is required for the butter command. But you can calculate the parameters for a lowpass Butterworth filter by your own:
function [B, A] = ButterLow(n, W)
V  = tan(W * 1.5707963267948966);
Q  = exp((1.5707963267948966i / n) * ((2 + n - 1):2:(3 * n - 1)));  
Sp = V * Q;
% Bilinear transform:
P = (1 + Sp) ./ (1 - Sp);
Z = repmat(-1, size(P));
G = real(V ^ length(Q) / prod(1 - Sp));
% From Zeros, Poles and Gain to B (numerator) and A (denominator):
B = G * real(poly(Z'));
A = real(poly(P));
Related Question