pepelevamp:
the ability to manipulate in frequency space, not just observe in frequency space.
in audio work it is crucial. hence why i am trying to find a reliable way to do inverse FFT. im curious about using the CMSIS libraries. its how i got onto this thread
Using the FFT to manipulate the spectrum is not a good idea.
The spectrum of the FFT as a bank of band pass filters has the shape of sin(x)/x. (It's basically a bank of moving average filters, shifted in the frequency domain.) This means that it has very poor band-pass characteristics, it has pretty high bumps outside of the pass band, so it will interfere with other frequencies.
You can improve this by using a window function before applying the FFT.
https://www.dsprelated.com/freebooks/sasp/DFT_Filter_Bank.html
Depending on the application, you'll be better off using a bank of band-pass filters that were specifically designed for that purpose.
Pieter