Combined estimation/coding of highband spectral envelopes for speech spectrum expansion

The paper addresses the problem of expanding the bandwidth of narrowband speech signals, focusing on the estimation of highband spectral envelopes. It is well known that there is not enough mutual information between the two bands. We show that this happens because narrowband spectral envelopes have a one-to-many relationship with highband spectral envelopes. A combined estimation/coding scheme for the missing spectral envelope is proposed, which employs this relationship to produce a high quality highband reconstruction, provided that there is an appropriate excitation. Subjective tests using the TIMIT database indicate that 134 bits/sec for the highband spectral envelope are adequate for a DCR (degradation category rating) score of 4.41. This is an improvement of 22.8% over a typical estimation of highband envelopes using the usual mapping functions, in terms of DCR score.