FM radio (FM) has always occupied a very important position in people's entertainment life. Most manufacturers engaged in consumer electronics design have embedded FM parts in products such as MP3, smart phones, PDAs and so on. The RDS data broadcasting system (Radio Data System) uses subcarriers in FM stereo to provide users with various data information services, such as radio program lists, emergency announcements, and road traffic information, while transmitting audio programs. Although there are few radio stations with RDS in China except Taiwan and Hong Kong, this technology has developed quite maturely abroad, and the demand is also increasing rapidly. The modules introduced in this article are based on this application. , Provide FM reception and RDS information service for car audio system, the following mainly introduces the design of FM module and audio processing and RDS data processing.
1 S14730 / 31 peripheral circuit design
S14730 / 31 is the earliest highly integrated CMOSAF / FM radio receiver chip in the industry. Because S1473X's built-in DSP's advanced digital low and intermediate frequency architecture requires only a few peripheral components and no manual adjustments, it can achieve all AM / FM receiving functions from antenna input to audio output. The module is not only small in size and light in weight, but also supports multiple serial control methods, so it is especially suitable for car audio and other small portable devices. S14730 / 31 is a multi-functional solution, its main functions include: advanced automatic search algorithm; mute function; automatic calibration of digital frequency modulation; FM stereo processing; for analog and digital voice output.
Because S1473X integrates all the functions of antenna input to audio output on a single chip, the radio circuit design process with S1473X as the core becomes simple. The peripheral circuit of S1473X is shown as in Fig. 1.
Precautions:
(1) All ground wires must be directly connected to the ground on the PCB board.
(2) Pins 1 and 20, not connected, are left floating.
(3) Pin 2 is connected to the antenna interface of FM, and pin 4 is connected to the antenna interface of AM.
(4) S14730 / 31 should be placed near the antenna as much as possible, and the path of the two pins of AMI and FMI should be as short as possible.
2 Introduction of RDS data broadcasting system
RDS (Radio Data System) data broadcasting system is a high-tech that has appeared in the field of wireless broadcasting in recent years. It uses existing FM equipment to load some additional data of non-sound data into the additional channel of the FM stereo channel after modulation. In this way, on the basis of these additional data, the RDS data broadcasting system has the processing data, The ability of words, symbols and information can be used for traffic information broadcasting, broadcast paging, background music, has the remarkable characteristics of large-area networking and high reliability. Based on the RDS scheme, it will show more and more technical advantages .
2.1 RDS broadcast signal
The realization of the FM automatic search technology is based on the RDS data source, namely RDS broadcast data, which requires the support of the local broadcast system. The FM band of the RDS receiver is in the range of 87.5 to 108.0 MHz, and the adjacent stations are separated by at least 100 kHz, and the subcarrier data is loaded on 57 kHz. The data content includes radio station type, program type, traffic announcement, standard time, weather forecast, etc. At the same time, it provides an open interface to provide data text application channels for special users. The essence of RDS broadcasting technology is a working method that uses subcarriers of FM stereo broadcasting to carry data information. The RDS data is modulated by FSK to become an RDS data signal with a carrier wave of 57 kHz. The RDS data signal is mixed with the sound signal played by the radio station itself to become a composite signal, which is then transmitted by FM modulation.
2.2 Introduction to RDS Common Functions
PI (Program IdenTIficaTIon): It is a 16-bit binary number, which indicates the transmission of a radio station. If the radio finds that two transmitters use the same code, it can be considered that the programs transmitted by the two are the same. The radio can then decide which transmitter station to use based on the stronger signal and better quality.
PS (Program Servicename): is an eight-character string used to identify the station being accepted. The PI code can only be read by the machine, while the PS is designed to let the audience see and know which radio station is currently listening.
PTY (Program Type Code): used to confirm the current program belongs to one of the 32 types of programs defined in the RDS standard, such as news, current affairs, weather, children's programs, etc.
TP (Traffic Program IdenTIficaTIon): radio station identification with traffic program information, there is only one TP. It indicates that you are listening to the radio station, in fact, you may want to publish traffic and travel notices while listening to the program. Car drivers can use it as a way to find traffic information and travel news in unfamiliar areas.
RT (Radio Text): Used to display related information of the program being played, such as the name of the currently playing song, singer, etc., similar to the additional information of CD / MP3.
RT + (Radio Text Plus): It is an expanded function of RT and a new function added in the new standard revised by RDS in 2009. Mainly used to display some details such as song singer tracking, scrolling news headlines, some information of the current radio station, such as the radio station's website, hotline, etc. RT + downward is fully compatible with RT.
2.3 Data format of RDS
RDS signal transmission follows an internal protocol, which specifies the specific format of RDS data and its corresponding meaning. From the perspective of signal transmission, every 8 bytes of data is called a data packet, which is the most basic unit of signal transmission. A type of data packet is called a group. There are 32 types of group types: group 0A, group 0B, group1A, group 1B, ..., group 15A, and group 15B.
1 group = 4 block = 104 b
1 block = 16 b data + 10 b check digit
From the perspective of the meaning of the data content, the RDS data is organized into sets of sections. Each group of data corresponds to the concept of a data packet at the sending angle, and is also composed of 8 bytes of data. However, each set of data in a set of RDS data is different, and each content has a different specific meaning. Further down, each group of RDS data is divided into 4 sections of data, each section occupies two bytes, and different sections have different specific meanings. A certain number of groups of RDS data are combined to form a complete set of RDS data. With the passage of time, the specific content of the adjacent sets of RDS data may change, but the nature of the content is fixed and uniform. Figure 2 is the definition of each block and specific bits of RDS data.
Among them: A3 ～ A0 represents the number of the data group, the size is from 0 to 15; B0 represents the version type of the data group, which is divided into A and B; PI code represents the unique identification of the radio station, each group of data groups contains PI code; TP indicates whether the current radio station contains traffic information; PT4 ~ PT0 have 5 bits in total, used to display the current program type; checkword + offset A, B, C, D is 10 b check digit plus offset, used for data synchronization And error checking.
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