Apparatus as in claim 4 wherein, a first set (A) is chosen when a last binary symbol of a previous codeword (Z) is a 0 and a second set (B) is chosen when Z is a 1 and valid codewords for sets A and B are by the steps of:removing binary words that contain more than one of j consecutive 1's and more than k consecutive 0's from each of two lists of 2n possible codewords for sets A and B, respectivel