In an implementation of phase vocoder using Python, I found the author get an approximation of the fundamental frequency of STFT using the following formula:
Freq0 = B/A * abs(B/A)
where B is the fft of the first windowed frame and A is the fft of the second whose starting point is Ra(analysis hop size) behind that of B. I try my best to understand this formula but still can't figure out why it approximates the fundamental frequency. Can anyone explain this? It'd be great if detailed mathematical derivation is given. Thanks in advance.