Subjective voice quality evaluation of artificial bandwidth extension: comparing different audio bandwidths and speech codecs

Artificial bandwidth extension (ABE) methods have been developed to improve the quality and intelligibility of telephone speech. In many previous studies, however, the evaluation of ABE has not fully reflected the use of ABE in mobile communication (e.g., evaluation with clean speech without coding). In this study, the subjective quality of ABE was evaluated with absolute category rating (ACR) tests involving both clean and noisy speech, two cutoff frequencies of highpass filtering, and input encoded at different bit rates. Three ABE methods were evaluated, two for narrowband-to-wideband extension and one for wideband-to-superwideband extension. Several speech codecs with different audio bandwidths were included in the tests. Narrowband-to-wideband ABE methods were found to significantly improve the speech quality when no background noise was present, and the mean quality scores were slightly but not significantly increased for noisy speech. Widebandto-superwideband ABE also showed significant improvement in certain conditions with no background noise. ABE did not cause significant decrease of the mean scores in any of the tests.