Low complexity speech coding at 1.2 to 2.4 kbps based on waveform interpolation

Abstract

Low-rate speech coding technology has recently made a significant progress with the introduction of new interpolative algorithms (Shoham, 1993; Kleijn and Haagen, 1995). The inherent complexity of these algorithms is, however too high too be commercially useful for low-cost applications. In this paper we propose new approaches to low-complexity speech coding at coding rates of 1.2 and 2.4 kbps. The proposed methods utilize all the advantages of interpolatve coding but greatly simplify the analysis and synthesis operations to a point where low-cost two-way digital speech communication can be easily implemented on DSP or host platforms. At 2.4 kbps, the complexity of the proposed coder is about 7.5 and 2.5 MFLOPS for the encoder and decoder, respectively. At 1.2 kbps, the complexity is about 6 and 2.3 MFLOPS for the encoder and decoder, respectively. The small computational load of these coders make them suitable for multi-tasking environment and low-cost terminals. Informal subjective evaluation shows that, at 2.4 kbps, good communication quality is obtained. Communication quality is less than toll quality but the perceived coding effects are not annoying and do not prevent long sustained two-way conversation with high degree of intelligibility. The quality does not significantly degrade at 1.2 kbps and it is considered sufficient for messaging applications.