Low bit rate speech coding using spectral trajectory modeling

In the U.S. Federal Standard coder for 2400 bps, a data frame containing 54 bits of encoded signal is transmitted every 22.5 ms. In each frame, 25 bits encode the spectral features (10 Line Spectrum Frequencies—LSF). In this paper we describe a method for reducing the transmission rate while preserving most of the quality and intelligibility. This method is based on modeling the spectral trajectories with polynomial functions and on encoding these functions for segments of speech extending over multiple frames. Here 10 polynomials are computed by fitting them to the 10 LSF trajectories in the least‐squares sense. Then the polynomial coefficients are encoded for the whole segment instead of directly encoding the LSF vectors. The spectral parameters are thus reduced (compressed) to [(P+1)/N]×100%, where P represents the order of the polynomials and N the number of frames for each segment. Different compression rates can be achieved. For example, for P=5 and N=10 the spectral features are encoded using 40% l...