New designs for soft-decision threshold decoders are developed to significantly reduce the complexity and increase the coding gain as compared to previously proposed architectures. The approximate a posteriori probability (AAPP) decoder considered here is a digital approximation of the a posteriori probability (APP) decoder introduced by Massey (1976). These designs are useful for high-rate self-orthogonal convolutional codes with a long constraint length. The study case considered in this paper is for a rate 23/24, constraint length 4056 code used in a typical microwave digital radio. The new decoder architecture provides a four-fold decrease in complexity and an additional 0.3 dB coding gain as compared to previous designs.>