"butter" filter: Undefined 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

Answer 1

[Jan]

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));

[Emanuele Giartosio]

Thanks! The Signal Processing Toolbox trial was expired 2 day ago and I didn't notice it.

Answer 2

[Honglei Chen]

Do you have Signal Processing Toolbox installed? You can find out by typing

>> ver

in the command window.

HTH

[Emanuele Giartosio]

Thanks! The Signal Processing Toolbox trial was expired 2 day ago and I didn't notice it.