High-quality speech at low bit rates: Multi-pulse and stochastically excited linear predictive coders

We will present in this paper some recent developments in low bit rate speech coding research. Two new speech coding algorithms - multi-pulse LPC and stochastic coding (code-excited linear prediction) - have been proposed recently to achieve high quality speech at bit rates below 10 kbits/sec. Both of these algorithms use a linear filter to synthesize speech at the receiver but they differ in the methods used to generate the excitation for the linear filter. The multi-pulse model assumes that the excitation can be represented with sufficient accuracy as a sequence of pulses (typically 4 to 8 pulses every 5 msec). In stochastic coders, the excitation is selected from a random codebook of white Gaussian sequences. The optimum excitation in both these coders is chosen to minimize a subjective error criterion based on properties of human auditory perception. Although these coding algorithms are complex requiring over 10 million multiply-add operations per second, new fast digital signal processor chips offer the possibility of their real-time implementation.