Analog decoding and beyond

In 1998, Hagenauer and Loeliger et al. independently proposed to decode error correcting codes by analog electronic networks. In contrast to previous work on analog Viterbi decoders, the work both by Hagenauer and by Loeliger et al. was inspired by turbo-style decoding of codes described by graphs. Large gains, in terms of speed or power consumption, over digital implementations were envisaged. Since 1998, much effort has been spent towards turning these ideas into working chips. While only decoders of "toy" codes have so far been successfully manufactured, extensive simulations of such circuits have not revealed any fundamental problems. Some progress has also been made in analyzing the effects of transistor mismatch. While much remains to be learned, the author feels confident that analog decoders will eventually find their way into applications. The present paper, rather than reporting on circuit details, offers some thoughts on "the bigger picture"- the underlying principles, motivations, and possible directions of future research.