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Zero-Phase Filtering

Design a lowpass Butterworth filter with a 1 kHz 3-dB frequency to implement zero-phase filtering on data sampled at a rate of 20 kHz.

type myZerophaseFilt.m
function output  = myZerophaseFilt(input) %#codegen

[B,A] = butter(20,0.314);
output = filtfilt(B,A,input);

end

Use codegen to create the MEX file for myZerophaseFilt.m.

codegen myZerophaseFilt -args {zeros(1,20001)} -o myZerophaseFilt_mex -report
Code generation successful: To view the report, open('codegen/mex/myZerophaseFilt/html/report.mldatx').

Generate a noisy sinusoid signal as input to the filter.

Fs = 20000;
t = 0:1/Fs:1;
comp500Hz = cos(2*pi*500*t);
signal = comp500Hz + sin(2*pi*4000*t) + 0.2*randn(size(t));

Filter input data using both MATLAB® and MEX functions.

FilteredData = myZerophaseFilt(signal);
MexFilteredData = myZerophaseFilt_mex(signal);

Plot the 500 Hz component and the filtered data.

tms = t*1000;
plot(tms,comp500Hz)
hold on
plot(tms,MexFilteredData)
plot(tms,FilteredData)
hold off

xlabel('Milliseconds')
ylabel('Amplitude')
axis([0 25 -1.8 1.8])
legend('500 Hz component','MEX','MATLAB')

Figure contains an axes. The axes contains 3 objects of type line. These objects represent 500 Hz component, MEX, MATLAB.