Low-band extension of CELP speech coder by harmonics recovery

Most of the telephone speech being transmitted in public networks is bandlimited to 300-3400 Hz. Narrowband speech is lacking in the information from low-band (0-300Hz) and high-band (3.4-8kHz) that are found in wideband speech (0-8kHz). As a result, narrowband speech is characterized by the reduced intelligibility and muffled quality, and degraded speaker identification. Therefore a bandwidth extension method, one of the techniques providing wideband speech quality without any additional information has been explored. By using the conventional technique to recover the missing high-band speech, we estimated the spectral envelope and the excitation for high-band using statistical recovery and spectral folding, respectively. In addition, we proposed an approach that exploits the harmonic synthesis method for the reconstruction of low-band speech over the CELP speech coding. Spectral distortion measurement and listening test are performed to evaluate the proposed method. Evaluation with synthesized speech shows the quality improvement.