Design And Implementation Of Digital-analog Simulcast FM Broadcasting Transmitter Based On FPGA

The digital-analog simulcast FM audio broadcasting is considered to be the besttechnology during the transition from the analog FM audio broadcasting to digitalbroadcasting. According to a certain criteria, the digital broadcasting signal issuperimposed on the analog FM audio broadcasting band (88~108MHz), which can belaunched together with the analog FM audio broadcasting signal. Both the analog FMprograms and digital broadcasting programs can be provided without serious interfere atthe same time. The solution of digital-analog simulcast can take full advantage of theexisting analog FM audio spectrum resources to achieve the transition from analog FMaudio broadcasting to digital broadcasting smoothly.The basic concept of digital-analog simulcast was analyzed in the article firstly. Andthe main standards of digital-analog simulcast FM broadcasting have been researched,which consists of DRM+standard of Europe and the FM HD Radio standard of the UnitedStates. The processing of analog FM stereo coding and modulation has been simulatedwith matlab. And the spectrum of OFDM signal has been provided by simulation. With theproblem of spurious in FM stereo structure based on DDS, an analysis model of DDSspurious resource is proposed. The source, location and effection of the different DDSspurious have been researched by the analysis model. In addition, an OFDM modulatorbased on the improved IFFT algorithm is designed through the research of the interleavingmodule and IFFT module in channel coding of digital broadcasting, which can savememory resources.A digital-analog simulcast FM audio broadcasting transmitter is designed based onFPGA secondly, which consists of a front-end audio acquisition module, a minimumsystem of FPGA module and a digital-analog simulcast DUC module. The circuits ofanalog audio amplifier, acquisition and digital audio receiver have been designed toprovide audio signal. And the FPGA circuit based on the Altera company’s Cyclone IIIseries as the core, which is designed to meet the requirements of the FM stereo coding, theOFDM baseband signal processing and other timing logic. Moreover, the control platformof the transmitter is built based on the Nios II embedded soft-core processor. Furthermore,the interface circuits and hardware drivers have been designed for the transimitter. Thecircuit of digital-analog simulcast DUC is designed based on the core structure, whichcontains two quadrature digital up converters. Baseband FM signal and the OFDM signal are modulated onto the RF carrier separately. And then, the RF signal of analog programand the digital program are combined to digital-analog simulcast signal. Finally, thedigital-analog simulcast signal is sent to the antenna after an analog low-pass filter whosecut-off frequency is110MHz.Finally, the system performance of the transmitter has been tested, especially thefunction of front-end analog acquisition. Then the bandwidth of FM stereo signal, thefrequency offset of FM and the phase niose of RF carrier have been tested. At last, thecenter frequency, the bandwidth and the shoulder attenuation of OFDM RF signal havebeen tested. The test results show that the transmitter processes a good performance in theactual testing and with structural integrity. By the different software configuration, thetransmitter can complete the experiments of digital-analog simulcast according to thestandard of DRM+or FM HD Radio. Therefore, a common hardware platform is providedfor the experiments of digital-analog simulcast FM audio broadcasting in China, which hasa very broad application prospects.