Low rate speech coding research has recently gained new momentum due to the increased national and global interest in digital voice transmission for mobile and personal communication. The Telecommunication Industry Association (TIA) is actively pushing towards establishing a new "half-rate" digital mobile communication standard even before the current North-American "full rate" digital system (IS54) has been fully deployed. Similar activities are taking place in Europe and Japan. The demand, in general, is to advance the technology to a point of achieving or exceeding the performance of the current standard systems while cutting the transmission rate by half.
The voice coders of the current digital cellular standards are all based on code-excited linear prediction (CELP) or closely related algorithms. See M. R. Schroeder and B. S. Atal, "Code-Excited Linear Predictive (CELP): High Quality Speech at Very Low Bit Rates," Proc. IEEE ICASSP'85, Vol. 3, pp. 937-940, March 1985; P. Kroon and E. F. Deprettere, "A Class of Analysis-by-Synthesis Predictive Coders for High Quality Speech Coding at Rates Between 4.8 and 16 Kb/s," IEEE J. on Sel. Areas in Comm., SAC-6(2), pp. 353-363, February 1988. Current CELP coders deliver fairly high-quality coded speech at rates of about 8 Kbps and above. However, the performance deteriorates quickly as the rate goes down to around 4 Kbps and below.