A multi-stage perspective on CELP speech coding

The author addresses methods for high-performance innovation coding in a code-excited linear prediction (CELP) speech coder context. He argues that care should be taken in order to ensure that innovations do not extend outside the Voronoi regions of the LTP-stage. With this cell-conditioned approach, it is shown that innovation gain can be coded with 1 bit only (i.e. sign only). The previous method is extended to the case of delayed decision (tree search methods). It is demonstrated that, in this case, the innovation vectors should be placed near the boundaries of the LTP-Voronoi regions. This is in contrast to the strict one-at-a-time (OAT) procedure presented by P. Heddin and A. Bergstrom (1991), where innovations should be placed well inside the regions. Simulation studies clearly demonstrate that high-quality speech coding is indeed feasible using this 1 bit gain coding procedure