A novel paradigm based on pitch-adaptive windows is proposed for solving the problem of encoding the fixed codebook excitation in low bit-rate CELP coders. In this method, the nonzero excitation in the fixed codebook is substantially localized to a set of time intervals called windows. The positions of the windows are adaptive to the pitch peaks in the linear prediction residual signal. Thus, high coding efficiency is achieved by allocating most of the available FCB bits to the perceptually important segments of the excitation signal. The pitch-adaptive method is adopted in the design of a novel multimode variable-rate speech coder applicable to CDMA-based cellular telephony. Results demonstrate that the adaptive windows method yields excellent voice quality and intelligibility at average bit-rates in the range of 2.5-4.0 kbps.

Pitch adaptive windows for improved excitation coding in low-rate CELP coders

In this paper, we propose a successive bit-vector search approach to reduce the computational complexity of coded excited linear predictive coding (CELP) speech coders. By decomposing each codevector into several bit-vectors, the convolution process of the linear predictive coding model can be simplified. With a suggested matching criterion, the proposed method removes a portion of unlikely codevectors in a bit-vector by bit-vector fashion. Theoretical analyses and simulation results show that the proposed search method reduces a lot of the computational load for general codebook searches and maintains nearly the same speech quality as the traditional search approach. The proposed method, which reduces the complexity of the CELP speech coders in VLSI realization, can be flexibly embedded with a simple RISC processor to achieve a low-cost speech coder.

http://ir.lib.stust.edu.tw/bitstream/987654321/9854/1/successivebit-vectorsearchalgorithmforcelpvocoders.pdf

Hong Kook Kim

Papers

A Low-Bit-Rte Extensiion Algorithm to the 8kbit/s CS-ACELP Based on Adaptive Fixed Codebook Modeling