A method of processing broadcast data in a transmitting system includes randomizing enhanced data; Reed-Solomon (RS) encoding and Cyclic Redundancy Check encoding the randomized enhanced data to build an RS frame; encoding the enhanced data in the built RS frame at a coding rate of at least ½ or ¼; first interleaving the encoded enhanced data; deinterleaving the first interleaved enhanced data; first multiplexing enhanced data packets including the deinterleaved enhanced data with main data packets including main data; randomizing the main data in the multiplexed enhanced and main data packets; second interleaving the enhanced data in the multiplexed enhanced and main data packets and the randomized main data to output a data group having the interleaved enhanced data and the interleaved main data; trellis encoding data in the data group in a trellis encoding unit; and second multiplexing the trellis-encoded data with field synchronization data and segment synchronization data.