How to generate the audio spectrum using fft in C++? [closed]

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There are quite a few similar/related questions on SO already which are well worth reading as the answers contain a lot of useful information and advice, but in essence you need to do this:

  • Convert the audio data to the format required by FFT (e.g. int -> float, with separate L/R channels);
  • Apply suitable window function (e.g. Hann aka Hanning window)
  • Apply FFT (NB: if using typical complex-to-complex FFT then set all imaginary parts in the input array to zero);
  • Calculate the magnitude of the first N/2 FFT output bins (sqrt(re*re + im*im));
  • Optionally convert magnitude to dB (log) scale (20 * log10(magnitude) or 10 * log10(re*re + im*im));
  • Plot N/2 (log) magnitude values.

Note that while FFTW is a very good and very fast FFT it may be a little overwhelming for a beginner – it’s also very expensive if you want to include it as part of a commercial product. I recommend starting with KissFFT instead.

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