8Kbit/s LD-aCELP Speech Coding with Backward Pitch Detection

This paper presents an 8 Kbit/s speech coding algorithm whose delay is 2.5 ms on the base of G.728 algorithm to reduce coding rate. Adaptive codebook structure is introduced in the proposed algorithm, which is composed of the recent excitation. The algorithm uses double codebook structure which contains adaptive codebook and normalized fixed codebook. In codebook searching, the algorithm detects backward pitch, and then searches the adaptive codebook subtly round the pitch period T. After searching 64 adaptive code words, 8 gains, 256 fixed code words and 8 gains, the best excitation is obtained. Testing with the average segment SNR and perceptual evaluation of speech quality (PESQ), the improved algorithm is close to G.728 in speech coding quality. And it achieves desirable balance in coding rate, delay time and coding quality.