Improvement of the restricted temporal decomposition method for line spectral frequency parameters

Line spectral frequency (LSF) parameters have not been used for the temporal decomposition (TD) method due to the stability problems in the linear preditive coding (LPC) model. To overcome this deficiency, Kim et al. introduced a method called restricted temporal decomposition (RTD)[4]. This method enforces a constraint on the event vectors to preserve their LSF ordering property, which is needed to ensure the stability of the corresponding LPC synthesis filter. Also, additional constraints are enforced on the event functions to reduce the computational cost of TD. The RTD method, however, has not completely guaranteed the LSF ordering property for the event vectors. This paper proposes an improved algorithm named modified RTD (MRTD) to solve this problem. Moreover, basing on the geometric interpretation of RTD method we find it necessary to have a new property imposed on the event functions namely the well-shapedness property, which is satisfied by adding one more constraint on them. It is also shown by experiments that spectral information of speech can be coded efficiently using MRTD based vector quantization.