Efficient pitch filter encoding for variable rate speech processing

Analysis-by-synthesis techniques are used in a wide variety of speech coding standards and applications for rates below 16 kbps. The presence of a long-term predictor, commonly known as the adaptive codebook, is critical to coder performance at the lower rates. Unfortunately, the encoding rate and computational requirements for high-quality encoding of pitch filter parameters can be excessive. Several popular approaches explore the trade-off between predictor order, allocated bit rate, and computational requirements for long-term predictor optimization. We investigate the relative performance of several long-term predictor structures and present a new approach to vector quantization of the pitch filter coefficients having a subjective quality equivalent to other schemes, but at a lower coding rate and requiring significantly less closed-loop computation. The performance is evaluated in a variable-rate CELP coder at an average rate of 2 kbps and in Federal Standard 1016 CELP.