Speech bandwidth extension using Gaussian mixture model-based estimation of the highband mel spectrum

The quality and intelligibility of narrowband telephone speech can be enhanced by artificial bandwidth extension. This study combines Gaussian mixture model-based (GMM) mel spectrum extension with a filter bank implementation for generating the missing spectral content in the highband at 4-8 kHz. The narrowband mel spectrum is calculated from input speech and the GMM is used to estimate the mel spectrum in the highband. An excitation signal for the highband is generated as a combination of upsampled linear prediction residual and modulated noise. The excitation is divided into sub-bands that are weighted and summed to realize the estimated mel spectrum. The bandwidth-extended output is obtained as the sum of the artificial highband signal and narrowband speech. Listening tests indicate that this method is preferred over narrowband speech and over a previously presented artificial bandwidth extension method which is implemented in some mobile phone models.