Wednesday, May 7, 2014

Implementing filters in Matlab using ellipord and filtfilt


We can create a custom function in Matlab using the built in ellipord and filtfilt functions included in the signal processing toolbox.

Here is a lowpass filter with explanation in the comments. Once we create this lowpass function, we can call this function and simply pass the function our input signal, sampling frequency, and cut-off frequency.

function [y,N,B,A]= lowpass(x,fs,fc);
% Description: This function uses an elliptical filter
% to filter out high frequencies.
%
% [y,N,B,A] = lowpass(x,fs,fc)
%
% Variable Definition:
% x: Input signal
% fs: Input signal sampling frequency in Hz
% fc: cutoff frequency
%
% y: Filtered signal (output)
% N: Filter order
% B: Feedforward (FIR) coefficients
% A: Feedback (IIR) coefficients
Rp = 0.5;   % Ripple in dB (passband)
Rs = 20;    % Attenuation
mf = fs/2;  % Maximum frequency
Wp = fc/mf; % Cuttoff frequency
Ws = 1.2 * Wp; %Stopband frequency
% First, determine the minimum filter order to meet specifics
[N,Wn] = ellipord(Ws,Wp,Rp,Rs);
% Next, Design the filter (find the coefficients)
[B,A] = ellip(N,Rp,Rs,Wn,'low');
% Now, apply the filter (filter the signal)
y = filtfilt(B,A,x);
% Plot the filter
figure('Color',[1 1 1]);
freqz(B,A,2^14,fs);

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