Branch metric computer for a Viterbi decoder of a punctured and pragmatic trellis code convolutional decoder suitable for use in a multi- channel receiver of satellite, terrestrial and cable transmitted FEC compressed- digital television data

摘要

A branch metric computer for the Viterbi decoder of a convolutional decoder primarily intended for use in a multi-channel compressed-digital television receiver capable of receiving FEC data transmitted over satellite, terrestrial and transmission channels. The branch metric computer employs a RAM, having an effective depth of 2.sup. x storage locations and an effective storage-location width of 16 m, which is preloaded with two selectively programmed I and two selectively programmed Q lookup tables, wherein each table entry is made up of four m- bit values,. The RAM is designed to operate with either QPSK punctured- coded I and Q signal inputs or any one of a plurality of different PAM or QAM higher alphabet or an 8-PSK pragmatic trellis-coded 2.sup.x -bit I and 2.sup.x -bit Q signal inputs to the convolutional decoder that are forwarded as signal inputs to the RAM for controlling the particular I and Q lookup-table entries read out from the RAM. The branch metric computer further comprises (1) a group of eight Manhattan-distance metric computing summers organized into first and second sets of four summers for adding certain combinations of the four m-bit I and Q lookup-table entries read out from the RAM and (2) a network including four comparators with each of them forwarding to the Viterbi decoder the lesser sum output from corresponding ones of the summers of the first and second sets.