Hi, Taek-

I don't know what is meant by frequency "8". At what Hertz is your frequency?

Re. the multiples of two or small factors of 3 and 5-- it is referring to the fact that "120" can be decomposed into only factors of 2, 3 and 5:
120 = 2 * 2 * 2 * 3 * 5

Zeropadding does not add information to the output spectra, but it does alter the frequencies that are reported; often it is used for interpolation, for exmaple (but again, it does not *add* information).

Re. your 3dcalc command-- I don't really understand it. So, firstly, you are processing on a surface? And you are picking out the volume with index 8? I don't think you are getting phase information out by just selecting a single volume (unless you have left out intermediate steps). For a real component "Real" and an imaginary component "Imag" from the complex number, the phase is given by: arctan(Imag/Real).

--pt