Wideband extension of telephone speech using a hidden Markov model

In this paper we propose an algorithm to recover wideband speech from lowpass-bandlimited speech. The narrowband input signal is classified into a limited number of speech sounds for which the information about the wideband spectral envelope is taken from a pre-trained codebook. For the codebook search algorithm a statistical approach based on a hidden Markov model is used, which takes different features of the bandlimited speech into account, and minimizes a mean squared error criterion. The new algorithm needs only one single wideband codebook and inherently guarantees the transparency of the system in the base-band. The enhanced speech exhibits a significantly larger bandwidth than the input speech without introducing objectionable artifacts.