A multiplexer in accordance with claim 5 wherein the frame length ij equals xQ where x is an integer and Q is at least common multiple of the denominators of the subchannel rates normalized by the fixed channel rate to an irreducible ratio, i??? Q/D +1 where D is the difference between Q and the number of bit per Q bits required for all of said subchannels, ##EQU17## and j is an integer equal to (