FM radio has always played a very important role in people's entertainment life. The manufacturers engaged in consumer electronics design embed the FM part in products such as MP3, smart phones and PDAs. The RDS data broadcasting system (Radio Data System) utilizes subcarriers in FM stereo to provide users with various data information services, such as broadcast program lists, emergency announcements, and road traffic information, while transmitting audio programs.
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Although there are few radio stations with RDS functions in addition to Taiwan and Hong Kong, the technology has matured abroad and the demand is constantly increasing. The modules introduced in this paper are based on this application. To provide FM receiving and RDS information services for car audio systems, the following mainly introduces the design of FM module and audio processing and RDS data processing.
1 S14730/31 peripheral circuit design
The S14730/31 is the industry's first highly integrated CMOS AF/FM radio receiver chip. Because the S1473X's built-in DSP's advanced digital low-IF architecture requires only a few peripheral components and requires no manual tuning, it can perform all AM/FM reception functions from antenna input to audio output.
The module is not only small in size, light in weight, but also supports a variety of serial control methods, so it is especially suitable for car audio and other small portable devices. The S14730/31 is a versatile solution with key features including: advanced automatic search algorithm; mute function; auto-calibration digital FM; FM stereo processing; for analog and digital speech output.
Since the S1473X integrates all the functions of the antenna input to the audio output on a single chip, the radio circuit design process based on the S1473X is simple. The peripheral circuit of the S1473X is shown in Figure 1.
Precautions:
(1) Pins 1 and 20 are not connected and are suspended.
(2) Pin 2 is connected to the antenna interface of the FM, and pin 4 is connected to the antenna interface of the AM.
(3) All ground wires must be directly connected to the ground on the PCB.
(4) Place the S14730/31 as close as possible to the antenna so that the paths between the AMI and FMI pins are as short as possible.
2 RDS data broadcasting system introduction
The 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 modulate additional data of non-sound data onto the additional channel of the FM stereo channel. Thus, based on these increased data, the RDS data broadcasting system has processing data. The ability of words, symbols and information can be used for traffic information broadcasting, broadcasting paging, background music, with large-area networking and high reliability. Based on the RDS scheme, more and more technical advantages will be revealed. .
2.1 RDS broadcast signal
The implementation of the FM automatic search technology should be based on the RDS data source, that is, the RDS broadcast data, that is, the support of the local broadcast system is required. The FM band of the RDS receiver is in the range of 87.5 to 108.0 MHz, and the adjacent stations are spaced at least 100 kHz apart, and the subcarrier data is loaded at 57 kHz.
The data content includes station type, program type, traffic announcement, standard time, weather forecast, etc., and provides an open interface to provide data text application channels for special users. The essence of RDS broadcast technology is a way of working with data information using subcarriers of FM stereo (FM) broadcast.
The RDS data is modulated by FSK to become an RDS data signal with a carrier frequency of 57 kHz, and the data signal of the RDS is mixed with the sound signal played by the radio itself to become a composite signal, which is then transmitted by FM modulation.
2.2 Introduction to RDS Common Functions
PI (Program Identification): is a 16-bit binary number indicating the transmission of a 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 to use based on the stronger signal and better quality.
PTY (Program Type Code): Used to confirm which program of the 32 program types defined in the RDS standard, such as news, current affairs, weather, children's programs, etc.
TP (Traffic Program Identification): Station identification with traffic program information, only one TP. It indicates that the radio station is being listened to, and actually it is listening to the program that may have to publish traffic and travel announcements. Car drivers can use it as an alternative to finding traffic information and travel news in unfamiliar areas.
PS (Program Servicename): is an eight-character string that identifies the station being accepted. The PI code can only be read by the machine, while the PS is designed to be visible to the listener and know which station is currently listening.
RT (Radio Text): Used to display information about the program being played, such as the name of the currently playing song, the singer, etc., similar to the CD/MP3 additional information.
RT+ (Radio Text Plus): It is an expansion function of RT and a new feature added to the new standard revised by RDS in 2009. Mainly used to display some details such as song singers, scrolling news headlines, some information about the current radio station, such as the radio's website, hotline, etc. RT+ is fully compatible with RT.
2.3 RDS data format
The RDS signal transmission follows an internal protocol that specifies the specific format of the RDS data and its corresponding meaning. From the signal transmission point of view, every 8 bytes of data is called a data packet, which is the most basic unit of signal transmission. One type of data packet is called a group, and the group type has 32 types of group 0A, group 0B, group1A, group 1B, ..., group 15A, 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 by a set of sub-components. Each set of data corresponds to the packet concept of the transmission angle and is also composed of 8 bytes of data. However, each set of RDS data has different data, and each content has different specific meanings. Further down, each set of RDS data is divided into four sections of data, each section occupies two bytes, and different sections also have different specific meanings.
A certain number of sets of RDS data are combined to form a complete set of RDS data. As time goes by, the follow-up of the broadcast program, the specific content of the adjacent set of RDS data may change, but the nature of the content is fixed and uniform. Figure 2 is a definition of the various blocks and specific bits of the RDS data.
Among them: A3 ~ A0 indicates the number of the data group, the size is from 0 to 15; B0 indicates the version type of the data group, divided into two types A and B; PI code indicates the unique identifier of the station, each group of data groups contains PI code; TP indicates whether the current station contains traffic information; PT4~PT0 have 5 bits for displaying the current program type; checkword+offset A, B, C, D are 10 b check digits plus offset for data synchronization And error checking.
3 software implementation of RDS function
This article discusses the RDS receiving function through the car-used chip S1473X. Because the S1473X has a built-in digital signal processor and has the decoding function of RDS data, it does not need to be equipped with a separate decoding chip, and it also eliminates the control data synchronization and error check in software programming, only by sending instructions and reading instructions. The return value can be easily done. The following is familiar with the instruction format of the S14 73X.
3.1 S1473X instruction format
The S1473X exchanges information with the system controller through commands and responses. To perform an action, the system controller sends a byte instruction and several bytes of related parameters. S1 473x performs the corresponding action and returns the corresponding status byte and several response bytes according to the type of the command. The complete control process of the S1473X consists of the following four parts:
Parameter ARG: sent by the main controller, is a parameter describing the command, which is used to further specify the specific behavior of the command. For example, a command tuned to a specified frequency, whose parameters contain the specified frequency;
Status STATUS: Returned by S1473X, each command has a status of 1 B to indicate the execution of the command. Such as the interrupt status, whether it is ready, etc.;
Command CMD: sent by the main controller to control the behavior of the S1473X, such as powering down, powering down or tuning to a specified frequency;
Response RESP: Returned by S1473X, when the controller sends a command to read data, the read content is returned by S1473X. For example, FM_RDS_STATUS reads the data status of the RDS.
3.2 Implementation of RDS function
The RDS information of the current channel is obtained by sending the command 0X24, that is, the function FM-RDS_STATUS, and a null value is read from the RDS stack. The information of the RDS includes the synchronization status flag bit, the stack status flag bit, the information of the entire group (blocks A, B, C, D) and the error information transmitted by each block.
In addition, the attribute command 0X1500, RDS_INT_SOURCE is responsible for the interrupt configuration related to RDS. The attribute command 0X1501, RDS_INT_FIFO_COUNT is responsible for setting the minimum value of the RDS group data stored in the RDS stack before the RDSCREV flag is set. Because some RDS functions display, such as displaying the radio program name, song name and other information, several sets of RDS group data are required to be transmitted, so the RDS stack must have a certain storage capacity to display the information completely. The process of implementing the entire RDS function is shown in Figure 3.
4 Conclusion
In recent years, China's terminals such as mobile phones, MP3s, PDAs, etc., which provide information service functions, are developing faster and faster, and their functions are becoming more integrated. Intelligent terminals that integrate multiple information service functions are the development direction of these portable devices, and many consumers consume They hope to add FM function to their handheld products, and the data broadcasting service such as RDS uses the existing wide coverage of the broadcast network, the network construction investment is very small, and many FM chips have integrated the RDS decoding function, so the RDS function The implementation is simple and easy. I believe that FM+RDS will have a wider range of applications.
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