# Write a Matlab script that accepts the file “FilteredEvidence.wav” and performs the signal processing operations

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Dat on 3 Dec 2022
Commented: Walter Roberson on 4 Dec 2022
Write a Matlab script that accepts the file “FilteredEvidence.wav” and performs the signal processing operations
This will plot 60 Hz frequency
I upload the FilteredEvidenceFile.wav in Matlab but I got errors message Unrecognized function or variable 'l' when run it.
clearvars;
close all;
ToneFreqHz = 60;
NumSamp = size(FilteredEvidence,l);
%Create an array of cosine sample.
CosArray = zeros(NumSamp,l);
for Samp = 1 : NumSamp
CosArray(Samp) = cos(2*pi*60*Samp/Fs);
end
%Multiply each input sample by the cosine sample.
DemodOut = FilteredEvidence .* CosArray;
%Remove component at 120Hz with a moving average filter.
FirTaps = 1/3675*ones(3675,1);
CosArrayFiltered = filter(FirTaps, l, DemodOut);

Fifteen12 on 3 Dec 2022
You use a variable l in your code, but I don't see it defined (line 6). Make sure you set l equal to some value earlier in your script!
Fifteen12 on 4 Dec 2022
l in this case is the dimension of FilteredEvidence you're finding the size of. I'm not sure what dimension you're looking for, though Walter Roberson's suggestion ( l = 1) is as good as any. l is used later on as the denominator coeffecient of the rational transfer function employed in the filter equation (last line).
If that denominator is coeff is 1, then go ahead and set l equal to 1.
Walter Roberson on 4 Dec 2022
audioread() always returns an N x channels array where N is number of samples. channels is 1 for mono, 2 for stereo (both mono and stereo are common) and higher for possibilities such as Dolby. audioread() never returns a 3D array. So the possible choices for l in size(FilteredEvidence,l) are only 1 (number of samples) or 2 (number of channels) and anything larger would return 1

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