Very low bit rate (VLBR) speech coding around 500 bits/sec

Abstract

New solutions to Very Low Bit Rate speech coding have been recently proposed based on speech recognition and speech synthesis technologies, [1,2,3,4,5,7,8]. In the continuation of the work described in [8], this paper presents a complete encoding scheme around 500 bits/sec. The proposed solution is based on automatic recognition of elementary acoustical units using HMM modelling. An unsupervised training phase is used to build the HMM models and the codebook of synthesis units. The decoded speech is then obtained by concatenating the corresponding synthesis units based on a HNM-like decomposition of speech. A new unit selection process is proposed integrating some prosody constraints. Through this approach, the size of the synthesis codebook is independent of the targeted bit rate. A complete description of the unit selection process and of the associated prosody modelling is given, together with the quantisation scheme of the overall set of encoded parameters.