The effect of highband harmonic structure in the artificial bandwidth expansion of telephone speech

The quality of narrowband telephone speech can be improved by artificial bandwidth expansion (ABE), which generates missing frequency components above the telephone bandwidth using only information from the narrowband speech signal. Straightforward bandwidth expansion methods do not reproduce the harmonic structure of voiced sounds properly, but a pitch-adaptive technique can be used to approximate the correct alignment of harmonic frequencies. In this study, pitchadaptive highband alignment was implemented into an existing ABE method, and the quality of the modified method was studied with formal listening tests in Finnish andMandarin Chinese. The effect of the highband harmonic structure was found unimportant for the perceived speech quality. Consequently, computationally expensive pitch adaptation was found to be unnecessary for the bandwidth expansion of telephone speech.