Can anyone help me understand this FFT alignment formula?

Question

Hi,

 

I was experimenting with different FFT frame periods, and then needed to re-align things in the TimeDomain after the IFFT. I used an oscilloscope and some trial and error probing to figure out that there seems to be a perfect match to re-align if I use the following formula. Can anyone figure out why the numbers work, or is this just wishful thinking... :\

fs2 + (fs1 * 18.5) samp

seems to give me the correct signal delay re-alignment where fs2 is the higher of the two FFT sizes

Comments

OK, this 18.5  was just a coincidence with the certain signal flow and settings I had I think --

please ignore my formula.

What is the correct alignment in sampl, for example to re-align an FFT of 1024 with an FFT of 8192 ?

Answer 1

Hi Cristian, rather than comparing the latency differance of two different FFTs why not compair the latency of just ones output with respect to its own input and then the other. It may give more obvious results.  The last time I tested this, the latency from input pre overlap delay compencators to output post delay compencators was exactly 7.5 frames, i.e. an FFT of 1024 would be an overall delay of (1024*7.5) samp.

This may well have change as the newer FFTs have bigger frame sizes and no longer revers the partials every half frame, so it's quite posiable the latency has improved as well.