Combining equalization and estimation for bandwidth extension of narrowband speech

Current public telephone networks compromise voice quality by bandlimiting the speech signal. Telephone speech is characterized by a bandpass response from 300 to 3400 Hz. The voice quality is perceived as being much worse than for wideband speech (50-7000 Hz). We present a novel approach which combines equalization and estimation to create a wideband signal, with reconstructed components in the 3400 Hz to 7000 Hz range. Equalization is used in the 3400-4000 Hz range. Its performance is better than statistical estimation procedures, because the mutual dependencies between the narrowband and highband parameters are not sufficiently large. Subjective evaluation using an improvement category rating shows that the reconstructed wideband speech using both equalization and estimation substantially enhances the quality of telephone speech. We have also evaluated the performance on the narrowband output of several standard codecs. Overall, the use of equalization for part of the highband regeneration makes the system more robust to phonetic variability and speaker gender.