FFT output spectrum generation

Hi,

I understand your point. But the when you actually coding, you can't just do a pack based on a fixed number, right? because the exponent value is different. So, are you suggesting to write a case statement to hard code the logic? If not, could you please give me some hints on this, because the only other way I figure out to do this is through a shifting register. And besides this, I still have one question. How to control the FFT output? By that, I mean way to control FFT to start streaming or stop streaming at our wish. Thank you very much!