Code-excitation is an attractive way of modeling the input to the LPC filter at low bit rates. The excitation consists of a codebook, populated from Gaussian random number. This leads to two disadvantages: 1) highly computational load and storage capacity; 2) crucial role of long term prediction. The paper proposes an effective Pulse Adaptive Code-Excited Linear Predictive (PACELP) speech coding algorithm. This scheme allows a much less computational power to determine the optimal excitation sequence when pulse adaptive search procedure is introduced and much fewer storage capacities to put the codebook and its related parameters. The codebook of PACELP is obtained from Regular-Pulse Excitation sequences using varying threshold clustering algorithm. Besides, this scheme omits the long term prediction because pitch information can be easily expressed when the initial phase is introduced to the codebook. The computer simulation results show that at 4kb/s, PACELP can produce a clear and natural-sounding synthetic speech and its realtime implementation on TMS320C25 is being under way.
Bibliographic reference. Liu, Z. Yong (1991): "An effective pulse adaptive code-excited linear predictive coder at 4kb/S", In EUROSPEECH-1991, 835-838.