1. A method of processing a speech signal, comprising the steps of: a. converting, by an analog-to-digital converter, the speech signal to digital signals; b. converting, by a windowing unit, the digital speech signal into short-time frames; c. applying, by a spectrum analyzer unit, a Fast Fourier Transform to each of the short-time frames to obtain an original spectrum; d. deriving, by a spectrum deriving unit, a varied spectrum based on the original spectrum; e. 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 f. superposing the original cepstrum coefficients and varied cepstrum coefficients to produce a set of frontend feature vectors for each of the short-time frames.