Wideband speech coding at 16 kbit/s for a videophone application

Abstract

This paper presents results on wideband 7 kHz speech coding at 16 kbit/s where the proposed CELP algorithm is implementable on a single floating point DSP. As a basic coder structure, the long-term predictor is implemented as an adaptive codebook, while a sparse Gaussian codebook with non-overlapping vectors is used for the stochastic excitation. In order to meet the complexity requirements, several methods for efficient codebook search are adopted. With these methods, it is shown that the computational effort for the basic coder structure can be reduced to 12.4 MIPS with a 7 bit stochastic codebook. A two-stage hierarchical search through the adaptive codebook is investigated. This search method reduces the computational effort further although at the cost of a small degradation in coder performance. The coder is evaluated in an absolute category rating (MOS) test using both a hi-fi handset and a loudspeaker, and compared to the CCITT standard coder G.722 at 48–64 kbit/s. The speech quality with the basic CELP structure is judged to be comparable to the G.722 coder at 48 kbit/s.