Abstract: The purpose of coding in the V.90 digital modem is to shape the transmitted signal spectrum and constellation instead of error correction. The computational load of spectrum shaping is much heavier than that of constellation shaping. The sign bits selection is a problem of searching the optimum path in the trellis, thus Viterbi algorithm can be used. This is equivalent to placing a conventional decoder in the (V.90) digital modem encoder. Logically speaking, the (V.90) digital modem decoder is like a traditional encoder. From the above basic idea, we have represented the modification rules constrained by 2-state trellis as a convolutional code, derived all the formula needed for decoding, and finally proposed a decoding algorithm for the V.90 digital modem.

Key words: public switched telephone network(PSTN), V.90, decoder, modem