“The transmission of broadcast data in FM stereo broadcasting is a major development in the multiplexing application of FM sound broadcasting. In recent years, data broadcasting has developed very rapidly internationally, and corresponding technical specifications have been formulated. Such as the Broadcasting Data System (RDS) specification promulgated by the European Broadcasting Union and the Radio Broadcasting Data System (RBDS) specification in the United States. In my country, data broadcasting is still in the experimental stage.
Authors: Li Hua, Zheng Ming, Fan Xiong, Li Qing
The transmission of broadcast data in FM stereo broadcasting is a major development in the multiplexing application of FM sound broadcasting. In recent years, data broadcasting has developed very rapidly internationally, and corresponding technical specifications have been formulated. Such as the Broadcasting Data System (RDS) specification promulgated by the European Broadcasting Union and the Radio Broadcasting Data System (RBDS) specification in the United States. In my country, data broadcasting is still in the experimental stage. The system adopts RDS technology, combined with self-developed encoder and receiving device, and utilizes the existing FM broadcast surplus frequency band to transmit data information. The successful application of this technology is the reuse of broadcast frequency resources to provide an advanced information dissemination medium for the coming information society.
1 Principle of data broadcasting
1.1 Technical principle
The so-called RDS technology uses FM multiplexing technology to add a sub-carrier channel in the surplus frequency band of FM broadcast to transmit data information.
According to the tests conducted by CCIR with various sub-carriers and modulation methods? Under multipath transmission conditions, the interference is minimal when the center frequency is three times the pilot signal frequency (19kHz) and is phase-locked with it. Therefore, the data information to be transmitted can be subjected to double sideband amplitude modulation of the suppressed subcarrier on the 57kHz subcarrier, and then combined with the stereo composite signal to form a baseband modulation signal, and then the VHF main carrier is frequency modulated. The spectrum of baseband modulation signal of FM stereo broadcasting with RDS channel is shown in Figure 1.
In Figure 1, the RDS data signal occupies a baseband frequency of 57±2.4kHz, which does not interfere with the stereo broadcast, nor does it degrade its quality, and at the same time, it will not be interfered by the broadcast program.
1.2 Information sending format
According to the RDS standard, the transmission of the broadcast data signal adopts a data structure in which data blocks are repeated continuously. The largest data unit is a group, and each code group consists of 4 blocks, which are set as A, B, C, and D respectively; each block has 26 bits, including 16 information bits and 10 check bits. Check bits are used for error identification, correction and data synchronization. The transmission rate of the data stream is 1187.5bit/s. The RDS data format is shown in Figure 2.
1.3 Source encoding and decoding
The code used in this system is an optimal shortened cyclic code for correcting burst errors, and its generator polynomial?
The matrix G is generated accordingly. The transmitter multiplies a 16 × 26 G matrix by a 16 information bit mx, and the result is added modulo two with the offset word (10 bits) unique to each block to generate a 26-bit data series.
The RDS decoder at the receiving end obtains the RDS clock signal and the RDS data signal. The received error-free bit series is multiplied by the check matrix H to obtain the syndrome (check word) corresponding to the offset word, thereby establishing the block synchronization and group synchronization of the data stream. In fact, adding an offset word to each block is equivalent to adding an error to each block, that is, the offset word is equivalent to a series of errors. If there is no other bit error on the channel, the offset word can be found by calculating the syndrome in the received information, so as to determine which one of A, B, C, and D the received data block is, that is, synchronization is achieved. .
2 Information transmission system and encoder
2.1 Block diagram of transmitting system
The computer is used to edit various data information to be sent, such as advertisements, road condition information, etc., and is sent to the FM-RE602 encoder by the control management program through the RS232 serial port. FM-RE602 is an RDS signal encoder. It receives the data information sent by the host and generates a data link that conforms to the RDS specification. After waveform transformation, the data stream is subjected to double-sideband amplitude modulation to suppress the sub-carrier with the 57kHz sub-carrier signal. Finally sent to the VHF/FM transmitter.
As can be seen from Figure 3, the use of FM stereo broadcasting to send data information only needs to add a computer and FM-602 encoder on the original basis to achieve the purpose.
2.2 FM-RE602 encoder
The RDS encoder is an important part of the transmitting system. At present, there is no such product in China, while the foreign RDS encoder circuit is complex and expensive. To this end, the FM-RE602 encoder that conforms to the RDS specification is developed.
The data information received by the encoder from the host is a binary data series. Since the spectrum of the rectangular pulse is very wide, in order to save the frequency band in the data transmission process, it is necessary to transform the 1 and 0 in the binary data series into the fixed waveform shown in Fig. 4 respectively.
The waveform is sent to a multiplier circuit as a modulated signal, multiplied by the 57kHz subcarrier, and its output is a two-phase differential phase-shift keying signal (DPSK), whose spectral distribution is shown in Figure 5.
Different from foreign encoders adopting the hardware filtering method to obtain the output waveform shown in Figure 4, the FM-RE602 encoder uses the waveform synthesis method, that is, the ideal waveform is sampled, stored in a 1K-byte memory, and passed through D The /A converter outputs the corresponding waveform. The hardware block diagram of the FM-RE602 encoder is shown in Figure 6. The single-chip MCU uses 80C31, and its serial communication port is driven by the MAX232 chip, and then connected to the host through a 9-pin plug, three-wire Tx, Rx, and GND.
In Figure 6, the interrupt frequency is 1187.5Hz. After receiving the external interrupt 0 of the 80C31, the CPU counts to 26 and sends a piece of data. Its control program takes out the corresponding waveform data from the EPROM according to the state of 0 and 1 in the data block, and forms the desired waveform after passing through the DAC0832.
The multiplier selects MC1496, one way is 57kHz carrier signal, the other way is D/A output signal, multiplied to get the signal that suppresses the carrier. Because of the square wave modulation, this signal contains many harmonic frequency components. In order to obtain a pure signal, band-pass filtering is required. The filter selects a MAX275 fourth-order band-pass filter with fO=57kHz, Bf=4.8kHz, and Q=f0/Bf=11.875. The signal has been tested and fully complies with the RDS specification.
3 Data information receiving device
The data receiving device is a variety of information terminals distributed in the coverage of the radio station signal, such as LED large-screen Chinese character Display billboards. Each receiving device must be equipped with a specific receiving card to Display various information sent by the transmitting end. Since each receiving device has its own address and group address, the receiving devices distributed in different regions display different information or public information.
The hardware block diagram of the receiving card is shown in Figure 7. The receiving card works at a specific frequency in the FM frequency band, so the front-end part is similar to a common FM radio, including high-frequency amplification, frequency mixing, and mid-amplification. After the mixed signal is amplified, it is sent to the RDS decoding chip through a 57kHz band-pass filter. The RDS decoding chip selects SAA6579T, which directly samples the input signal and outputs the clock and data. The output clock signal is connected to the external interrupt input port of the MCU as a synchronization signal, and the interrupt service routine reads the data through the P1.0 port.
Although the data signal of the binary series sent by the transmitter can be recovered through the hardware circuit, it is still necessary to find the synchronization word, correct the error, remove the redundant bit through software methods, and send the valid data information to the information display device through the serial port. , and its software flow is shown in Figure 8.
Using the FM stereo baseband signal to transmit data information, it only needs to add an FM-RE602 encoder to the transmitter on the basis of the existing FM stations in various places to transmit information to the information receiving devices within the coverage of radio waves. It has the characteristics of wide coverage, low investment and quick effect. Tests in Wuhan, Harbin and other places show that? The system runs stably, the data transmission is reliable, and there is no interference to the main channel signal, which fully meets the design requirements. There is no doubt that the successful application of this technology provides a new means of information transmission, which is bound to have a profound impact on the national economy and social life.