This study investigates a band extension technique for the narrow-band speech encoded with G.711, the most common codec for digital speech communications such as VoIP. The proposed technique is based on the full wave rectification that generates high-band harmonics by nonlinear processing. In order to improve the conventional technique, this study focuses on the parameter control according to the characteristics of speech data. From the subjective evaluation, it is indicated that the proposed technique may potentially outperform the conventional technique.