N step fft in D language

[matovitch]

I think you are not aswering his question (but maybe I am wrong).

If you want a Fourier transform with less frequencies than 
temporal samples you can perform a fft to get a result of same 
length like this :

9 2 7 6 1  8 (amplitude)
0 2 4 6 8 10 (frequency)

Then transform it like this :

11 13 9 (amplitude) (integrate the amplitudes)
  1  5 9 (frequency) (according to your frequencies)

This is a trivial example...I hope it's clear enougth...