A transmitting system and a method of transmitting digital broadcast signal are disclosed. The method of transmitting digital broadcasting signal in a transmitter includes forming a data group including mobile data, known data sequences, trellis initialization data bytes, wherein the known data sequences are sequentially concatenated with the trellis initialization bytes, interleaving data in the data group, wherein the data group includes a plurality of data blocks and scalable data blocks, wherein the known data sequences inserted in a predetermined area of the data group, trellis encoding on the interleaved data and initializing memories for the trellis encoding using initial bits of the trellis initialization data bytes in the interleaved data, wherein the initial bits of the trellis initialization data bytes are calculated using exclusive or operation with predetermined memory values of the memories for the trellis encoding, transmitting a broadcast signal including the trellis encoded data during a slot.