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

In a convolutional decoder of a series of convolutionally-encoded symbol packets applied as in-phase (I) and quadrature-phase (Q) digital input signals thereto, wherein said convolutional decoder includes a branch metric computer and a Viterbi decoder;

• the improvement wherein said branch metric computer comprises:
• a memory having an effective depth of 2.sup.x storage locations, where x is a first given integer larger than one, each storage location having an effective width sufficient to store an entry of at least one I-defining lookup table and an entry of at least one Q-defining lookup table, wherein each of said entries of an I-defining lookup table and a Q-defining lookup table consists of a group of first, second, third and fourth m-bit metric values, where m is a second given integer larger than one, said memory including a control input for preloading said 2.sup.x storage locations with at least one I-defining lookup table and at least one Q-defining lookup table, and 2.sup.x -bit I and 2.sup.x -bit Q signal inputs for reading out said one I-defining lookup table entry stored in the storage location defined by the value of a 2.sup.x -bit signal currently applied to said 2.sup.x -bit I signal input and said one Q-defining lookup table entry stored in the storage location defined by the value of a 2.sup.x -bit signal currently applied to said 2.sup.x -bit Q signal input;
• a group of summers including a first summer for adding the first m-bit metric value of the readout one I-defining lookup table entry to the first m-bit metric value of the readout one Q-defining lookup table to derive an (m+1)-bit sum output therefrom, a second summer for adding the second m-bit metric value of the readout one I-defining lookup table entry to the second m-bit metric value of the readout one Q-defining lookup table to derive an (m+1)-bit sum output therefrom, a third summer for adding the third m-bit metric value of the readout one I-defining lookup table entry to the third m-bit metric value of the readout one Q-defining lookup table to derive an (m+1)-bit sum output therefrom, and a fourth summer for adding the fourth m-bit metric value of the readout one I-defining lookup table entry to the fourth m-bit metric value of the readout one Q-defining lookup table to derive an (m+1)-bit sum output therefrom;
• and
• means for forwarding the respective sum outputs from four summers of said group to said Viterbi decoder.