The method in accordance with claim 1 wherein the frame length ij equals xQ where x is an integer and Q is the least common multiple of the denominators of the subchannel rates normalized by the fixed channel rate to an irreducible ratio, i??? Q/L +1 where D is the difference between Q and the number of bits per Q bits required for all of said subchannels, ##EQU15## and j is an integer equal to (x