in the environment of hdtv systems therefore without limiting the applicability of the invention to hdtv systems the invention will be described in such an environment such an hdtv system is described in co pending u s patent applications ser no 60 295 616 entitled packet identification mechanism at the transmitter and the receiver for an enhanced atsc 8 vsb system 60 301 559 entitled proposal for a potential revision of the atsc dtv standard using enhanced coding schemes 60 324 482 entitled key elements for the newly proposed atsc standard 60 280 782 entitled proposal for an improved atsc dtv standard the entire disclosures and content of which are incorporated herein by their reference a preferred implementation of the current invention will support a flexible payload data rate in preferably a 6 mhz channel and with a flexible cnr in one aspect of this invention as shown in fig3 combined robust and standard bit streams x u2032 1 and x u2032 2 are input to enhanced coding block 305 which comprises a first trellis encoder device described in detail with respect to fig4 fig4 illustrates the enhanced coding block 305 which comprises a trellis encoder providing an upper coding scheme to achieve 2 vsb mapping or enhanced 8 vsb encoding with a 16 state rate 1 3 trellis encoder to achieve a rate 1 4 or 1 2 robust stream encoding it should be understood that this encoder may additionally be converted to a 32 state or higher state to provide even greater error correction capability as shown in fig4 the x u2032 1 bit stream is input to the 1 2 input of a first stage multiplexer 410 the contents of two twelve symbol delay registers d 405 comprising historical values of the bit stream are combined with.