Low-rate quantization of spectral information in a 4 kb/s pitch-synchronous CELP coder

A new efficient algorithm for quantizing the spectral information for a pitch-synchronous CELP (PSCELP) speech coder is proposed. LPC analysis in the PSCELP is carried out once per pitch period. Direct quantization of the pitch synchronous LSF vectors would lead to a variable-rate codec, which is inconsistent with the objective of achieving a fixed-rate speech coder operating at 4 kb/s. Hence, a linear trajectory of LSF vectors is selected which can be encoded by one LSF vector each 20 ms. This conversion exploits the high correlation between successive pitch periods of the LSF parameters to achieve joint quantization. A coding rate of 1.2 kb/s is achieved for the LSF information with no noticeable degradation. The proposed algorithm employs linear interpolation at the decoder to recover the spectral parameters for the individual pitch periods used in the pitch-synchronous reconstruction of the speech signal. The comparison simulation results show that this algorithm produces comparable performance to that of LSF's linear interpolation quantization in a time-synchronous CELP coder.