The parity bits of the digital words are selected to satisfy the matrix equation HT = 0, where H is a rectangular 3 ??? 7 matrix and T is a 1 ??? 7 column matrix made up of a digital word from the data block, for example the first digital word of the data block 80 in FIG. 3 would be D1, D2, D3, D4, P1, P2, P3.