"butter" filter: Undefined function

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1 个投票

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

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Answer 1

Jan 2017-5-5

编辑：Jan 2017-5-8

在 MATLAB Online 中打开

2 个投票

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));