Abstract: The present invention relates in general to a channel CODEC to increase the transmitting effect on a communication channel of a communication system, and more specifically to a branch metric module of a viterbi decoder. The module includes an operator for operating and outputting the differential magnitude of two signals, one signal being a code word generated to perform the viterbi decode and the other being a signal transmitted through the channel. An adder sums and outputs the data which is outputted to it from the operator. A receiving code word converter converts the magnitude of any of bits by the linear sampling quantization process, and non-linearly converts it in accordance with a preestimated or predetermined format. A number of operators calculate the magnitude between two signals at the absolute value to convert the magnitude of an inputted generative code word and the magnitude of the non-linear converted receiving code word.