Efficient excitation model for low bit rate speech coding

The authors describe a novel efficient excitation model, the adaptive density pulse (ADP), for a synthesis filter. This ADP is a pulse train whose density (spacing interval) is constant within a subframe but is varied subframe by subframe. The results of an investigation of the effects of the ADP parameters on the synthesized speech quality are presented. ADP excitation is introduced to the CELP (code excited linear prediction) coding method, which is known as one of the most promising ways for producing high-quality speech below 8 kbps. A CELP coder with an ADP (ADP-CELP) is described. The ADP excitation makes it possible for the CELP coder to have flexibility in bit-allocation to the excitation. The authors also explain that computational complexity in selecting the best excitation from a codebook, which has been the primary drawback of CELP, is reduced by utilizing the sparseness of ADP excitation. The authors evaluated 6.45 kbps and 4 kbps ADP-CELP coders by computer simulation. ADP excitation improved the performance of conventional CELP in terms of segmental signal-to-noise ratio