Apple FFT Accelerate Framework Inverse FFT from Array of Real Numbers

Question

I am using the accelerate framework FFT functions to produce a spectrogram of a sound sample. This part works great. However, I want to (effectively) manipulate the spectrum directly (ie manipulate the real numbers), and then call the inverse again, how would I go about doing that? It looks like the INVERSE call expects an array of IMAGINARY numbers, but how can I produce that from my manipulated real numbers? I have tried making the realp array my reals, and the imagp part zero, but that doesn't seem to work.

The reason I ask this is because I wish to run an FFT on a voice audio sample, and then run the FFT again and then lifter the low part of the cepstrum (thus hopefully separating the vocal tract components from the pitch) and then run an inverse FFT again to produce a spectrogram showing the vocal tract (formant) information more clearly (ie, without the pitch information). However, I seem to be running into problems on the inverse FFT, into which I am passing in my real values (cepstrum) in the realp array and the imagp is zero. I think I am doing something wrong here and the results are unexpected.

Answer 1

You need to process the complex forward FFT results, rather than the real magnitudes, or else the shape of the IFFT result spectrum will be distorted. Don't consider them imaginary numbers, consider them to be part of a 2D vector containing the required angular phase information.

If your cepstrum lifter/filter alters only the real magnitudes, then you can try using the amount of change of the real magnitudes as scaling factors to alter your forward complex FFT result before doing a complex IFFT.