Block-based bandwidth extension of narrowband speech signal by using CDHMM

In this paper we present a block-based bandwidth extension system to enhance the quality of narrowband speech signal (0-4 kHz). In memoryless bandwidth extension systems, the missing high-band components are estimated from narrowband speech using the current frame only. As the narrowband-to-wideband mapping is a one-to-many problem, this memoryless system is likely to cause hissing and whistling artifacts in the reproduced speech. Our method estimates high-band components via narrowband-to-wideband state sequence mapping using continuous density hidden Markov model (CDHMM) on a block basis. The speech block is either one word or a sequence of words in narrowband utterance. CDHMM estimation method avoids the one-to-many property of low-band and high-band dependency. Both subjective and objective evaluations show that hissing and whistling artifacts are reduced and the spectrally extended wideband speech (0-8 kHz) is pleasant to listen.