# Filtering Frequencies

• MATLAB
I want to write a Matlab code which removes all frequency components from a .wav sound file except those within ##±25 Hz## of ##523 Hz## as well as its harmonics (up to the Nyquist frequency). I want to do this without using any built in filters.

Here is my code so far:

Code:
[s, Fs] = wavread('chord.wav'); % Reading the .wav sound file
sNew = zeros(size(s)); % creating a matrix of zeros the size of s

for i = 1:length(sNew)
for N=1:1:(Fs/(2*(523+25))); % number of harmonics up to Nyquist freq
while abs(s-(523.*N))<25 % while within ±25 range of each of the harmonics
sNew(i)=1; % replace the 0 element in sNew with a 1
end
end
end

sFiltered=sNew.*s; % multiplying ones & zeros matrix with s to remove unwanted freqs

wavplay(sFiltered,Fs); % playing the resulting filtered sound

But this code doesn't work. When I run the code it takes forever and freezes, and there is no output. What is wrong here?

Any help with fixing this code is greatly appreciated.

Last edited:

FactChecker
Gold Member
On anything that freezes, look hard at any 'while' statement to make sure it is not getting stuck in there. You can put some displays inside the loops to see what is happening. That being said, Matlab is VERY slow at programmed loops like that, so it might just be taking a long time.

roam
Your code produces no output because you have written your while loop in such a way that it easily goes infinite. Look at the relationship between the loop conditional and the inside of the loop! There is nothing inside the loop that could change the result of evaluating the conditional required to enter the loop.

It is like coding, while 2 is less than 10, subtract 2 from 5. This would produce an infinite loop because subtracting 2 from 5 cannot change 2 being less than 10. Your loop is similar. The inside of the loop cannot change the value of the conditional.

Another problem in your code is that you refer to the entire vector s in the while loop conditional statement. I think you mean to refer to just element i of s, like s(i). This will not fix the first problem I mentioned, but it will get you closer.

roam and FactChecker
Your code produces no output because you have written your while loop in such a way that it easily goes infinite. Look at the relationship between the loop conditional and the inside of the loop! There is nothing inside the loop that could change the result of evaluating the conditional required to enter the loop.

It is like coding, while 2 is less than 10, subtract 2 from 5. This would produce an infinite loop because subtracting 2 from 5 cannot change 2 being less than 10. Your loop is similar. The inside of the loop cannot change the value of the conditional.

Another problem in your code is that you refer to the entire vector s in the while loop conditional statement. I think you mean to refer to just element i of s, like s(i). This will not fix the first problem I mentioned, but it will get you closer.

Thank you so much for the hint, it helped me a lot.