1. A method of detecting pitch in input voice signals, the method comprising: performing a Fourier transform on the input voice signals after performing a predetermined pre-processing on the input voice signals; performing an interpolation on the transformed voice signals; calculating a spectral difference from a difference between spectrums of the interpolated voice signals; calculating a spectral auto-correlation using the calculated spectral difference; determining a voicing region based on the calculated spectral auto-correlation; and extracting a pitch using a spectral auto-correlation corresponding to the voicing region.