1. A method of detecting a pitch in input voice signals implemented by a processor, the method comprising: performing, using the processor, a Fourier transform on the input voice signals after performing a pre-processing on the input voice signals; performing an interpolation on the transformed voice signals; calculating a normalized local center of gravity (NLCG) on a portion of a spectrum of the interpolated voice signals in a local region, instead of the entire spectrum; calculating a spectral auto-correlation using the calculated NLCG; 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, wherein the calculating of the NLCG includes calculating the NLCG on a portion of the spectrum in the local region, instead of the entire spectrum, so that a center of gravity on a spectrum in the local region among spectrum of the interpolated voice signals is included within a predetermined range, and wherein the calculating of the spectral auto-correlation comprises automatically performing a normalization when the NLCG is included within a predetermined range, wherein the NLCG is calculated by the equation cA ⁡ ( f i ) = 1 U ⁢ ∑ j = 1 j = U ⁢ iA ⁡ ( f i - U / 2 + j ) ∑ j = 1 j = U ⁢ A ⁡ ( f i - U / 2 + j ) - M where M represents a predetermined value, A represents the voice signal, U represents the local region, f represents the spectrum and i represents a time.