Long-Term Quantization of Speech LSF Parameters

This paper addresses the problem of coding the LSF parameters of LPC speech coders on a "long-term" basis, i.e. beyond the usual #20 ms frame duration. The objective is to provide efficient LSF quantization for a speech coder with very large delay but very- to ultra-low bit-rate and good quality. To do this, a long-term model of the time-trajectory of the LSF vectors is applied on long segments of speech to capture the inter-frame correlation of the vectors over each whole segment. Using this model, it is shown that only a reduced set of LSF vectors need to be quantized to derive quantized LSF vectors at every original location. Experiments show that large gains in bit-rate over usual frame-by-frame quantization can be achieved (up to more than 50%) while preserving signal quality.