Bit Allocation in Time and Frequency Domains for Predictive Coding of Speech

Adaptive predictive coding with dynamic bit allocation is presented for speech encoding at low to medium bit rates (6.4 kbits/s to 16 kbits/s). In this system, a split-band predictive coding scheme and a bit allocation scheme are employed in order to remove the redundancies due to a periodic concentration of the prediction residual energy, as well as the nonuniform nature of the speech spectrum. Quantization bits are dynamically allocated, both over the subbands (in the frequency domain) and over the subintervals (in the time domain), in accordance with the distribution of the residual energies in the time-frequency domain. Optimum bit allocation is derived based on the mean square error criterion on the speech waveform. The SNR gain is presented as the sum of the spectral SNR gain G_f, equivalent to the prediction gain, and the temporal SNR gain G_T. Although G_T is much smaller than G_f, temporal bit allocation greatly improves the actual SNR performance of the APC system to more than the value expected from its SNR gain in the bit rate range of less than 2 bits/sample. A study on the segmental SNR performance for various coder designs shows that the coder design using three subbands, four subintervals, and a fourth-order predictor in each subband is most appropriate for speech encoding in the bit rate range of 6.4 kbits/s to 16 kbits/s. This system is evaluated in terms of the segmental SNR and subjective speech quality. The results show that the system results in a substantial improvement compared with the conventional full-band APC system in regard to SNR performance and predictor loop stability. It is also shown that this system can provide speech quality subjectively equivalent to 7 bit log-PCM at 16 kbits/s, and to 6 bit log-PCM at 9.6 kbits/s.