universal radix-4 FFT + iFFT fast fourier transform #created: 2017 #author marcin matysek (r)ewertyn.PL
marcinmatysek1@gmail.com #open-source
https://fast-fourier-transform-ifft-radix-4.blogspot.com/
//assumption: if N==signal period in table tab[] then resolution = 1 Hz but N=2^b;
//when we have signal period 22000 Hz and N=2^15=32768 then MAYBE
//the fundamental frequency is 0,671 Hz
//that means that in F(1) is 1*0,671 Hz =0,671 Hz
//in F(2) is 2*0,671 Hz =1,342 Hz
//in F(9) is 9*0,671 Hz =6,039 Hz
//in F(30) is 30*0,671 Hz =20,13 Hz that means in F(30) we will see what value has our signal in 20,13 Hz