A compact digital channel vocoder using commercial devices

A very small, high-quality, full-duplex 2.4 kbps channel vocoder based on the JSRU Belgard design has been implemented with commercially available integrated circuits. This fully digital realization is based on a distributed signal processing architecture employing six (6) Nippon Electric Company (N.E.C.) µPD7720 Signal Processing Interface (SPI) single chip microcomputers. Three of the µPD7720s implement a Belgard-type analysis filter bank by dividing the 19 channels into groups of 6, 6, and 7 channels. Similarly, a Belgard synthesizer including excitation generation is realized by two SPI's which process and combine the 10- and 9-channel subbands comprising the output speech. A one-chip implementation of the Gold pitch and voicing decision algorithm is achieved with the sixth µPD7720. An intel 8085-based 8-bit microcomputer is used for data transfer, control and multiplexing functions, and communications with the host terminal. Finally, the design features a compact analog subsystem based on a telecommunications industry LSI CODEC-with-filters chip. A total of 26 integrated circuits is required with a power dissipation of 11.5 watts and occupying about 70% of a 7" × 7" wirewrap board (excluding power supply). A very compact, full-duplex 2.4 kbps LPC vocoder has also been designed based on the same architecture as the channel vocoder but requires only three µPD7720s. A total of 16 integrated circuits are used in the LPC vocoder with a power dissipation of 8.6 watts, occupying one-half of a 7" × 7" wirewrap board.