LPC quantization requirements for the GPP-CELP coder

Abstract

Code-excited linear prediction coding with generalized pitch prediction (GPP-CELP) requires linear prediction filtering of the stochastic codebook output prior to addition of the adaptive codebook (ACE) component. The ACE component represents a sequence of past reconstructed samples passed through a low-pass filter to reflect the reduced pitch periodicity of the higher speech frequencies. The spectrum of the residual manifests broad peaks leading to significantly narrower distributions in the LPC parameter space. Additionally, the quantization error of the residual may be masked by the significantly greater energy of the ACE component. This work compares the quantization requirements for the information required to represent the time-varying LPC filter of the GPP-CELP coder with that of the classical CELP coder. With non-predictive coding of the LPC information a bit-rate reduction from 20 bits/20 ms to 16 bits/20 ms appears feasible without introducing noticeable degradation due to quantization.