How do I zero out a frequency of an audio signal?

I am writing a matlab script that is supposed to do the following: I have a corrupted audio signal and my goal is to remove the tone that corrupts the singal. So far I have found the power spectral density of the audio signal and I have found the DFT of the audio signal by using the fft(x) command. The next step is for me to zero out the frequency associated with the tone causing the corruption. I am stuck on how to do this because I know the frequency associated with the tone, but I'm not sure how to zero it out.

I need to zero out X[K] and X[K-N] between kt-200 and kt +200

Thank you in advance

Answer 1

[Image Analyst]

Did you try something like

spectrum = fft(signal);
spectrum(kt-200 : kt+200) = 0;
denoisedSignal = ifft(spectrum);

if kt is an index.

Otherwise, scan your spectrum looking for a tall spike and zero it out.

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and tell us what K, N, and kt are, if you know them.

[Image Analyst]

Yes that's correct. The fft result basically puts the zero frequency element at the first element and the last element. Then the middle of the vector is the highest positive frequency and most negative frequency. So that's why to zero out 4 kHz you must zero out +4 kHz and -4 kHz. For your homework you need to figure out what element 4 kHz and -4 kHz occur at. Knowing fs and the number of elements in the vector should enable you to do that. Worst case you might be off by a factor of 2.