A multiple rate low rate voice codec

Algorithms for a multiple rate (2400, 800, and 400 bps) CODEC are described. The U.S. Government standard LPC-10 algorithm is used for the 2400 bps rate. Lower rates are obtained by applying the source coding techniques of vector quantization and trellis coding to the LPC analysis output. In this paper, the algorithms used are described, along with data rate control and formatting techniques for use with a fixed rate modem. Simulation test results for different vector quantizer codebook sizes and structures are presented, along with DRT test results showing the voice quality achieved at each rate. Fortran simulations of the algorithms have been performed. Real time implementation of the CODEC is in progress, with completion expected by the end of 1985.