A trellis shaping method is described that may be used for suppressing DC components and/or Nyquist frequency components from the outputs of a PCM (56K) modem. The technique is based on convolutional codes. The code is generated through the use of a Viterbi decoder. Data bits are mapped for transmission into a set of n magnitudes and (n−k) sign bits s. The sign bits s are passed through (HT)−1 to get preliminary sing bits t=s (HT)−1 of size n. (HT)−1 is a matrix of size (n−k) by n which represents the left inverse of the syndrome-former matrix HT of convolutional code c=b G, defined so that G HT=0. The convolutional code is then added to sign bits t through an XOR operation to give final sign bits s (HT)−1+b G. After transmission, the final sign bits are passed through HT to give an output of (s (HT)−1+b G) (HT))=s, for recovery of the data bits.