I’m a beginner with FFT concepts and so what I understand is that if I put in 1024 signals, I’ll get 513 bins back ranging from 0hz to 22050Hz (in the case of a 44100Hz sampling rate). Using KISS FFT in Cinder the getBinSize function returns the expected 513 values for an input of 1024 signals. What I don’t understand is why duplicate peaks show up. Running a test audio sample that goes through frequencies (in order) of 20Hz to 22000Hz I see two peaks the entire time. It looks something like:
_____|________|_____
As the audio plays, the peaks seem to move towards each other so the second peak really does seem to be a mirrored duplicate of the first. Every example I’ve been through seems to just go ahead and plot all 513 values and they don’t seem to have this mirroring issue. I’m not sure what I’m missing.
Ok, after reading up on this I found the solution. The reason for the mirroring is because I use an FFT on real numbers (real FFT). The normal FFT as everyone knows works on complex numbers. Hence the imaginary part is "set" to 0 in the real FFT, resulting in a mirroring around the middle (or technically speaking the mirroring is around 0 and N/2).
Here is a detailed discussion: http://www.edaboard.com/thread144315.html
(the page is no longer avaliable, but there is a copy on archive.org)
And read p 238 – 242 on this book (Chapter 12). It’s fantastic, so buy it. I think there is a free pdf version on the author’s website: http://www.dspguide.com/