Question on use of fft and using positive frequency values only

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Hendrik2k1
Hendrik2k1 on 2 Jun 2022
Edited: Paul on 2 Jun 2022
I have two outputs of an carried out fft from filtered signal. I now that there is only a 770 Hz sin signal left in x_filtered, sample frequency fs is 48000.
fft_xf_shift = fftshift(fft(x_filtered,fs));
m_sh = abs(fft_xf_shift);
Seems to me that fft gives out a double sided frequency spectrum. With fftshift I zero-center the spectrum. Next I calculate the absolute values and then save in m_pos only the positive values because the negative frequency values are of no intereset in this case.
Next I plot the 20*log10 magnitude over frequency:
m_pos = m_sh(fs/2+1:fs);
f=0:1:fs/2-1;
figure;
plot(f,20*log10(m_pos/max(m_pos)))
The plot give the correct magnitude at 770 Hz. Even if I have the highest magnitude reading at 770 Hz in the final plot, when I plot m_pos, which is the right snipped of the fft_shifted value, the highest reading is at 771 Hz or at position 771 of the vector.
I certainly have a misconception here. Can you help me? Thank you.

Accepted Answer

Paul
Paul on 2 Jun 2022
Edited: Paul on 2 Jun 2022
Is this just an indexing question? If I understand the code snippet correctly, the frequency vector starts at 0, and incremented by 1, so f(771) = 770. So the peak would also be at m_pos(771), corresponding to 770 Hz.

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