Research And Implementation Of The Digital Baseband Of DAB Transmitter

DAB (Digital Audio Broadcasting) is a new generation broadcast system after amplitude modulation broadcast and frequency modulation broadcast. DAB has overcomed the respective shortcomings of amplitude modulation broadcast and frequency modulation broadcast, that is, too wide baseband, too big transmitting power, poor sound quality, and weak ability to resist the multipath transmission. DAB can provide better sound quality and more baosdcasting program than modulation broadcast under the same baseband and channel environment. DAB has outstanding advantages as following:1. high quality sound signals which can reach CD's quality.2. stronger ability to resist multipath interference which can ensure perfect receiving effect in the moving state with high speed.3. small transmision power, wide overlay area, and high efficiency with spread spectrum which can reduce the baseband width.4. ability to transmit additional data.This paper mainly studies the realization of the digital baseband of DAB transmitter. Being composed of channel encoding and COFDM modulation, the digital baseband consitutes the core of DAB transmitter. The channel encoding is made up of sections of energy difusing, convolutional encoding that can be punctured, redundancy codes puncturing, time interleaving, which respectively perform the functions of adding interference to the source codes through the PRBS, generating convolutional encoding, puncturing redundancy codes and carring out the bits interleaving between the different frames. The COFDM modulation part is made up of blocks partition, COFDM symbol forming, DQPSK symbol mapping, frequency interleaving, difference modulation and COFDM modulation and so on. They can perform the functions of constructing DAB data blocks and distributing them into COFDM symbols; mapping the suit of bits in the COFDM symbols into the DQPSK symbols; finding out the carrier corresponding the DQPSK symbols according to the frequency interleaving table; modulating the the DQPSK symbols differedntly onto the carriers; carring out the IFFT procession and COFDM modulation; modulating the COFDM signals (in-phase and quadrature) into intermediate frequency signals through the digital upper transducer.In this paper, only one FPGA chip is used to perform the two functions of channel encoding and COFDM modulating. By doing this, the reliability and the operation speed of the baseband are improved while the cost of the baseband is reduced.