Multi-Track Codebook in Low-Rate Celp Coding

In this work, we propose a new technique to model the fixed codebook in low-rate code-excited linear prediction coders. The multi-track codebook presented in this paper reduces the coder bit-rate while maintaining a high speech quality. Unlike the algebraic codebook that is adopted in the 8 kb/s G.729 standard, the multi-track representation uses a pulse density of five pulses per 10 ms speech frame, and permits various pulse magnitudes. Four tracks of disjoint pulse positions sets are constructed and evaluated for each analysis frame. However, only the pulses of the optimal track are quantized and transmitted to the receiver. While the search of the best individual pulse positions is limited to one segment interval of 2 ms, the pulse magnitudes are optimized for the whole analysis frame. The excitation signal in the multi-track codebook is encoded with 2.2 kb/s. Informal test listening and objective test measures have revealed that at the same bit allocation, the multi-track codebook results in significantly higher objective and subjective coded speech quality than the algebraic codebook