A high quality 4-kbit/s speech coding algorithm based on MDP-CELP

This paper presents a high quality 4-kbit/s speech coding algorithm based on the CELP algorithm. This coder operates on speech frames of 20 ms. Its main features are as follows: modification of a target vector for LSP-VQ which improves the subjective quality of the coded speech signal, a dispersed-pulse structure which improves pulse-based fixed-codebook performance at a low bit rate, a high performance fixed-codebook consists of a partial algebraic codebook and a random codebook, backward mode switching which improves fixed codebook performance for unvoiced and stationary noise regions, a stationary noise generator which significantly improves the subjective quality under background noise conditions. Subjective testing indicates that the quality of this coding algorithm is better than that of ITU-T G.726 (32-kbit/s ADPCM) under error-free and noise-free conditions and equivalent to that of ITU-T G.729 (8-kbit/s CS-ACELP) under some background noise conditions.