iOS FFT Accerelate.framework draw spectrum during playback

Question

UPDATE 2016-03-15

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Answer 1

Firstly you're not applying a window function prior to the FFT - this will result in smearing of the spectrum due to spectral leakage.

Secondly, you're just using the real component of the FFT output bins to calculate dB magnitude - you need to use the complex magnitude:

magnitude_dB = 10 * log10(re * re + im * im);