VARIABLE BIT-RATE METHODS FOR LOW-DELAY SPEECH CODERS

Several efficient Variable-Bit-Rate (VBR) methods suitable for low-delay Delayed-Decision Tree-Code (DDTC) and CELP speech coders are presented in this paper. These methods are based on modifying the existing codebook(s) by means of block or lattice codes. To achieve a reduced rate operation of the DDTC coder a novel technique based on a parity code is developed and up to 2 dB SNR improvement over conventional methods is demonstrated. For extended rate operation of the LD-CELP coder an approach based on a lattice code is used. The modified or additional lattice codebook has a geometrical (algebraic) structure, allowing the search procedure to be performed efficiently and with minimal additional memory. Simulation results of these VBR methods for a 16 kbit/s DDTC coder and for the 16 kbit/s LD-CELP coder proposed for the CCITT recommendation (G.728) are presented.