8. The method according to claim 7; in which the step of grouping said M-bit data words by weight includes the step of grouping said M-bit data words into a first group consisting of M-bit data words having an equal number of "1" and "0" bits, a second group consisting of M-bit data words having the number of "1" bits greater than the number of "0" bits, and a third group consisting of M-bit data words having the number of "0" bits greater than the number of "1" bits.
10. The method according to claim 8; in which the step of assigning alternately supplies M-bit data words from said two coding maps for M-bit data words having unequal numbers of "1" and "0" bits.