Digital broadcasting system and method for transmitting and receiving digital broadcast signal

An apparatus for processing a digital broadcast signal, the apparatus comprising:

• a processor configured to process the digital broadcast signal in at least two transmission modes, wherein the processor comprises:
• a signaling encoder configured to encode signaling data including a transmission parameter for a second transmission mode;
• a group formatter configured to form a first data group including mobile service data and place holder data for main service data, an MPEG-2 header and non-systematic RS parity data for a first transmission mode and form a second data group including mobile service data and the transmission parameter for the second transmission mode,
• wherein the second data group includes a region in which a scalable number of mobile service data packets are inserted,
• wherein the transmission parameter includes a group extension mode field specifying the number of mobile service data packets included in the region;
• a packet formatter configured to remove the place holder data for main service data and non-systematic RS parity data in the first data group, replace the place holder data for the MPEG-2 header with an MPEG-2 header having a packet ID of mobile service data packets in order to form and output 118 mobile service data packets including data in the first data group, and form and output M numbers of mobile service data packets including data in the second data group, wherein M is an integer equal to or greater than 118 and equal to or less than 156;
• a multiplexer configured to multiplex the output 118 mobile service data packets and 38 main service data packets for the first transmission mode, and multiplex the output M numbers of mobile service data packets and N numbers of main service data packets for the second transmission mode when the N is not zero, wherein a sum of M and N is 156;
• and
• a transmission unit configured to transmit the processed digital broadcast signal.