A transmitting system, a receiving system, and a method of processing broadcast signals are disclosed. Herein, the transmitting system includes an RS frame encoder, a block processor, a group formatter, and a trellis encoding module. The RS frame encoder performs error correction encoding on an RS frame payload including mobile service data so as to form an RS frame, divides the RS frame into a plurality of portions, and outputs the divided RS frame portions. The block processor performs one of ½-rate encoding and ¼-rate encoding on each bit of the mobile service data included in each portion. The group formatter maps a portion including symbols of the ¼-rate encoded mobile service data and symbols of the ½-rate encoded mobile service data to a corresponding region of a data group. And, the trellis encoding module performs trellis encoding on the symbols of the ¼-rate encoded mobile service data and the symbols of the ½-rate encoded mobile service data of the data group.