1. A method of processing a speech signal, comprising: converting, by an analog-to-digital converter, the speech signal to digital signals; converting, by a windowing unit, the digital speech signal into short-time frames; applying, by a spectrum analyzer unit, a Fast Fourier Transform to each of the short-time frames to obtain an original spectrum; deriving, by a spectrum deriving unit, a varied spectrum based on the original spectrum; applying, by a discrete cosine transform unit, discrete cosine transform to compute original cepstrum coefficients for the original spectrum and varied cepstrum coefficients for the varied spectrum; and generating a set of frontend feature vectors for each of the short-time frames.