A digital broadcast system and processing method that can transmit/receive digital broadcast signal are disclosed. More specifically, when a known data sequence is inputted to a trellis encoder, by having the transmitting system initialize a memory within the trellis encoder and trellis-encode the inputted data, thereby transmitted the processed data, and by having the receiving system estimate known data information, which is to be used for frequency synchronization and channel equalization, the receiving performance of the receiving system may be enhanced in a situation undergoing severe and frequent channel changes.