A data processing apparatus operable to map data symbols received from a number of sub-carrier signals of Orthogonal Frequency Division Multiplexed OFDM symbols into an output data stream, the number of sub-carrier signals being determined in accordance with one of a plurality of operating modes, for which a predetermined number of sub-carrier signals is assigned and the data symbols including first sets of data symbols and second sets of input data symbols, the data processing apparatus comprising a controller, an address generator and an interleaver memory, the controller being operable, when operating in accordance with a first interleaving process to read out from the interleaver memory a first set of the data symbols into the output data stream using addresses generated by the address generator, to write into the interleaver memory a second set of the data symbols received from the sub-carrier signals of an even OFDM symbol using the sequential order of the first group of input data symbols, and the controller is operable in accordance with an second interleaving process, to read out from the interleaver memory a first set of the data symbols into the output data stream using read addresses determined in accordance with a sequential order of the first set of input data symbols, and to write into the interleaver memory a second set of the data symbols received from the sub-carrier signals of an odd OFDM symbol at write addresses determined by the address generator, such that while data symbols from the first set are being read from locations in the interleaver memory, input data symbols from the second set can be written to the locations just read from, wherein the number of the sub-carriers which are available from a previous OFDM symbol is different from the number of the sub-carriers which are available from a current OFDM symbol, and the controller is operable to determine before reading out the first data symbols from the interleaver memory, whether the read address is valid for the previous OFDM symbol, by determining the number of sub-carrier signals in the previous OFDM symbol in accordance with the operating mode used for the previous OFDM symbol, comparing the read address with the number of sub-carriers which are available for the previous OFDM symbol, and if the read address is greater than the number of sub-carriers which are available, determining that the read address is not valid and not reading a data symbol from the interleaver memory at the read address, and re-generating the read address, and to determine before writing the second data symbols into the interleaver memory, whether the write address is valid for the current OFDM symbol, by determining the number of sub-carrier signals in the current OFDM symbol in accordance with the operating mode used for the current OFDM symbol comparing the write address with the number of sub-carriers which are available for the current OFDM symbol, and if the write address is greater than the number of sub-carriers which are available, determining that the write address is not valid and not writing a data symbol to the interleaver memory at the write address, and re-generating the write address.