In view of the above situation, we now consider a method of coding, as shown in FIGS. 8 to 10 wherein the m1/n1, m2/n2 coding method is divided into three major coding groups, i.e. the first coding group consisting only of Group A codewords corresponding to the first datawords m1=00 to 73, the second coding group consisting of Group B and Group C codewords corresponding to the first datawords