Noise robust bandwidth extension of telephone speech for mobile and landline communications

Abstract

The present day telephone infrastructure limits the bandwidth of telephone speech to 300–3400 Hz, whereby conversational speech falls mostly within 50–8000 Hz. The ensuing band-limited narrow-band (NB) speech has a considerable loss in sound quality, in terms of naturalness and intelligibility. The purpose of bandwidth extension (BWE) is to utilize the receiver end NB speech and to add relevant frequency information to the missing low and high frequencies. This novel BWE algorithm utilizes the linear source filter model, which separates NB speech into two roughly independent components, the excitation and the spectral envelope. These components are then extended and convolved together to attain reconstructed wideband (RWB) speech. Excitation extension involves harmonic reconstruction for voiced frames and some modulations and filtering steps for unvoiced frames. Spectral envelope extension utilizes a Gaussian mixture model trained via the expectation maximization algorithm, which is involved in the mapping of NB noise robust features to the corresponding RWB features. Noise analysis is used to isolate the best feature set for real world applications. Results from a subjective listening test indicate this algorithm has an improvement in sound quality, while having the lowest known distortion metric among other speaker independent bandwidth extension algorithms.