An enhanced full rate speech coder for digital cellular applications

In the digital cellular environment, acoustic background noise and channel bit errors (both bursty and random) may severely degrade the performance of speech coders such as the current European GSM full rate standard. This paper presents a CELP-based, enhanced full rate coder, which allocates 11.9 kb/s for speech and 10.9 kb/s for error protection for an aggregate channel rate of 22.8 kb/s. This coder provides toll quality speech which is significantly better than 16 kb/s ITU standard G.728 in clean conditions; also, it maintains robust performance over adverse conditions. The noteworthy features of our coder include: 10 msec frame size, leading to lower coder delay; improved error control coding; improved detection and masking of bad frames; low-complexity predictive tree-searched multi-stage vector quantization of the line spectrum pairs (designed using a noisy channel vector quantization algorithm); and low-complexity tree-searched multi-stage vector quantization of the fixed excitation