Block-based speech bandwidth extension system with seperated envelope energy ratio estimation

The major issue in extending bandwidth of narrowband speech signal (0-4kHz) is the estimation of high-band portion (4-8 kHz) of spectral envelope. It is found that, apart from the shape of high-band spectral envelope, the relative energy level of the missing high band to the observable low band is also crucial to the system performance. In this paper, the two-fold problem is solved by two different estimation rules. In memoryless bandwidth extension systems, the missing high-band information is estimated from narrowband speech using the current frame only. As the narrowband-to-wideband mapping is a one-to-many problem ([1]), memoryless system is likely to cause hissing and whistling artifacts. Our method treats envelope shape estimation on a block basis. Detected narrowband speech block is either one word or a sequence of words, which is modeled by CDHMM (continuous density hidden Markov model) and mapped to a wideband CDHMM pre-trained by original version of the speech block. High-band energy level, present as normalized energy ratio to observable low-band energy, is estimated on an MMSE rule. Both subjective and objective evaluations show that hissing and whistling artifacts are reduced and the spectrally extended wideband speech (0-8kHz) is pleasant to listen.