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The digital broadcast DAB technology can provide CD-quality programs by adopting appropriate sound source codec and error correction technology, and improves the anti-interference ability in the transmission process by adopting a broadband multi-carrier transmission method. Even high-speed mobile reception does not affect the quality of listening. DAB technology enables multiple sets of programs to be reused in one frequency band, improving spectrum utilization. At the same time, DAB can also transmit a large number of data services, such as broadcast messages, still pictures, computer programs and so on. Therefore, DAB is also known as Digital Multimedia Broadcasting (DMB).With the development of digital broadcasting DAB, there are more than 1000 DAB/DMB stations in the world, including Beijing, Shanghai, Guangdong, etc., but the DAB/DMB decoding core chip is still the bottleneck, and the development of receiving terminals is seriously lagging behind. ID200 is a DAB/DMB baseband decoding core chip developed by Xi'an Xixin Microelectronics Co., Ltd. in recent years. Its integration degree, power consumption, performance and price index are all at the international level. Based on this chip and core technology, it can not only simplify DAB. The design of the digital audio broadcasting receiver can also reduce the cost of the DAB digital audio broadcasting receiver, thereby increasing its market competitiveness.
DAB baseband chip ID200 introduction
The ID200 is a small, low-power DAB decoder (DAB baseband decoder and MPEG L2 audio decoder) for mobile and portable devices. The chip can perform full-speed decoding of 1.5Mbps for DAB transmission frames, including digital mixing, synchronization/tracking and demodulation. The baseband decoder can simultaneously decode two audio, data or video programs within the transmission frame. The decoded audio program can be played by an on-chip MPEG L2 audio decoder, and the data/video program can be further processed and displayed by the off-chip CPU. 1Mb SRAM is integrated on the chip.
The ID200 DAB baseband decoder architecture is shown in Figure 1. The function of DAB baseband decoding can be summarized as follows: The system first converts the analog DAB IF signal into a digital intermediate frequency signal through an ADC, and then uses the digital method to separate and filter the IQ component to obtain a complex form of the OFDM time domain signal. The samples of each subcarrier are obtained by FFT and the frequency deinterleaving is completed. Then, the result of the FFT calculation is used for DQPSK demodulation and time deinterleaving is completed, and finally the channel decoding is performed by the Viterbi decoder, thereby recovering the DAB program source data. The program source (MPEG audio stream) is then played by an audio decoder to obtain high quality sound.
Figure 1: Schematic diagram of the DAB baseband decoder structure.
DF50C hardware structure
The DAB receiver DF50C requires the ID200 chip + RF chip, as well as the ADC and audio DAC (drive headphones). The FM receiver uses a single-chip solution. DAB reception and FM reception share the same antenna. The DAB+FM module requires an MCU chip to control the RF chip, ID200, audio DAC, and FM chip. Receiver manufacturers can also implement unique interface design and function expansion through this MCU chip. Baseband decoding does not require additional SRAM.
The hardware structure of the DF50C is shown in Figure 2. The functions of each part of the receiver are:
â—† The RF chip uses the MAX2170. The RF module performs the tuning function and moves the RF DAB signal from Band III (174 to 230 MHz) or L Band (1452 to 1492 MHz) to the intermediate frequency (2.048 MHz or 38.912 MHz). According to the DAB protocol, the RF module must have the ability to receive signals from these two bands;
â—† The A/D module converts the baseband analog signal of the RF output into a digital signal, thereby facilitating the realization of the subsequent DAB digital decoding;
â—† The baseband decoding module is the core part of the DAB receiver. DAB channel decoding and OFDM demodulation, DQPSK demodulation and the like are completed. The data processed by the baseband decoding includes sound broadcasting and data services. The sound broadcasting service can continue to perform sound decoding, and the data service is sent to the corresponding decoding unit for specific service decoding.
â—† MCU uses Microchip's P18f45j10, MCU control module to manage the configuration of each module in the entire system, as well as control the operation and code stream of each module. At the same time, it also provides human-computer interaction functions, such as responding to keyboard information and controlling LCD display information. It provides bus interfaces such as SPI and I2C to facilitate communication with external modules.
The baseband decoding and source decoding portion of the DAB receiver is shown in the dashed box in Figure 2.
Figure 2: Block diagram of the DAB receiver.
Main technical indicators of DF50C
DAB receiving performance:
â—† Digital Broadcasting Standard DAB ETSI 300 401
â—† China DAB National Standard GY/T214.2006
â—† High-speed mobile reception in L-band and 3-band
â—† Sensitivity: -97dBm/3 band, -97dBm/L band
â—† Automatic identification and reception of DAB modes 1, 2, 4
â—† MPEG L2 audio source decoding, support 48KHz, 24KHz sampling frequency, automatic recognition channel mode, support single channel, dual channel, stereo;
â—† LCD display, display DAB program name, dynamic text, program type, DAB signal strength and BER
â—† Time display, including year, month, day, hour, minute
â—† Provides extended JTAG & USB interface
â—† Low-power DAB time-sharing mode
â—† Module power consumption: 140mW@128Kpbs DAB reception
â—† DAB+FM receiver prototype typical power consumption: 240mW
â—† Power: 3.0V / 1.8V
â—† DAB+FM module size: 35 x 65 x 3 mm2
â—† DAB+FM module PCB design: only 2 layers of metal
FM receiving performance:
â—† Automatic search mode and manual search mode
â—† FM radio frequency display
â—† RSSI signal strength display
â—† Support RDS data and text decoding
â—† FM IC power consumption: 54mW @ FM reception
DF50C software design
FIC decoding implementation
The FIC is a fast information channel in a DAB transmission frame. The FIC is not time-interleaved. Therefore, it is called a fast information channel. Since it provides an index of each subchannel and service, it is also the part that the receiver must first process. At the same time, the FIC is not static, so the decoding of the FIC should be performed periodically to ensure that the receiver responds to changes in the FIC in time. Through the FIC decoding flowchart 3, the FIC decoding can clearly obtain the detailed information of each service component, and the MMI program in the MCU can control the baseband according to the index information and select the program that it likes.
Figure 3: FIC decoding flow chart.
MMI software design
DF50C MMI software design can be divided into two parts, as shown in Figure 4, part of which is sub-channel composed of FIC decoding module and business information decoding module and dynamic text decoding module, and the other part is human-machine interface module, including human-machine interface menu Control module, baseband register setting module, audio control module, FLASH storage module, keyboard input module, LCD display control module, RF control module, etc. The decoding module solves various program related information and then manages and calls the human interface module according to the needs of the user. The reason why the decoding module is separated from the human interface module is because the decoding module has less connection with the hardware, and most of the other modules include the hardware interface driver, and also consider the flexibility of the human interface menu design structure, and the decoding module Independently, it is beneficial to the transplantation of the program on different hardware, and it can still be reused in the case of changing the structure of the human interface.
Figure 4: MMI software design.
DF50C system test
Figure 5 is a DAB receiver DF50C system based on ID200. The DF50C can correctly receive DAB signals at each frequency in Band III (174 ~ 230 MHz) and L Band (1452 ~ 1492 MHz), and the sensitivity can reach -97db. Stable reception of L-band and III-band in high-speed movement is achieved. When decoding a DAB audio stream with a bit rate of 128 kbps, the power consumption of the core modules (including RF chips, AD chips, baseband chips, and MCUs) is 130 mW.
Figure 5: DAB Receiver DF50C Board.
The DF50C system integrates FM chip, clock chip, flash chip and infrared chip, and completes DAB, FM manual and automatic frequency search, timer switch, program collection and infrared remote control. The technical specifications of the DF50C meet the design requirements, achieving a low-cost, low-power, multi-functional DAB receiver design. The introduction of this product fills the gap between China's DAB core chip and machine technology, and promotes the development of DAB in China.
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