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Apply Lowpass Filter to Input Signal

Assuming a sample rate of 20 kHz, create a fourth-order Butterworth filter with a 3-dB frequency of 2.5 kHz. Filter coefficients for butter must be constants for code generation.

type ButterFilt
function output_data=ButterFilt(input_data) %#codegen
[b,a]=butter(4,0.25);
output_data=filter(b,a,input_data);
end

Use the Butterworth filter to lowpass-filter a noisy sine wave.

t = transpose(linspace(0,pi,10000));
x = sin(t) + 0.03*randn(numel(t),1);

Filter the noisy sine wave using the Butterworth filter. Plot the filtered signal.

fx = ButterFilt(x);
plot(fx)

Figure contains an axes. The axes contains an object of type line.

Run the codegen command to obtain the C source code ButterFilt.c and MEX file:

codegen ButterFilt -args {zeros(10000,1)} -o ButterFilt_mex -report
Code generation successful: To view the report, open('codegen/mex/ButterFilt/html/report.mldatx').

The C source code includes the five numerator and denominator coefficients of the fourth-order Butterworth filter as static constants. Apply the filter using the MEX-file. Plot the filtered signal.

output_data = ButterFilt_mex(x);
hold on
plot(output_data)
hold off

Figure contains an axes. The axes contains 2 objects of type line.