A compact, flexible LPC vocoder based on a commercial signal processing microcomputer

A very small, flexible, high-quality, full-duplex 2.4-kbit/s linear predictive vocoder has been implemented with commercially available integrated circuits. This fully digital realization is based on a distributed signal processing architecture employing three Nippon Electric Company (NEC) μPD7720 signal processing interface (SPI) single-chip microcomputers. One SPI implements the LPC analyzer, a second implements the Gold pitch and voicing decision algorithm, while the third μPD7720 implements the excitation generator and synthesizer. An Intel 8085-based 8-bit microcomputer is used for data transfer, control and multiplexing functions, and communications with the host terminal. The LPC chip set achieves high flexibility by accepting run time initialization options from the Intel 8085. These parameters include choice of linear predictive model (
\leq 15
), analysis and synthesis frame size, and speech sampling frequency, as well as choice of speech input and output coding formats (linear or μ-255 law) and choice of analog or digital pre- and deemphasis. A total of 16 integrated circuits is used in the LPC vocoder with a power dissipation of 5.5 W and occupying 18 in
²
of circuit area.