In the FM stereo radio broadcast data transmission is a FM sound broadcasting major development in multi-tasking applications. In recent years, data broadcasting developing very rapidly in the international arena and develop the corresponding technical specifications. Such as the EBU issued by the Radio Data System (RDS) specification and the United States Radio Broadcast Data System (RBDS) standard. In China, the data broadcast is also exploring the experimental stage. The system uses RDS technology, developed in conjunction with their encoder and receiver, using the surplus over the existing FM radio band to transmit data information, with coverage of large area. Less investment. Salient feature of quick. The successful application of this technology is a re-use of radio frequency resources for the information society is approaching to provide an advanced information communication media.
Principles of a data broadcasting
1.1 Technical Principle
FM RDS technology is the use of the so-called multi-tasking technology, the surplus in the FM radio band added a sub-carrier channel to transmit data.
According to international non-Consultative Committee (CCIR) organized in various sub-carrier and modulation by the tests showed the multi-path transmission conditions, the center frequency of pilot signal frequency (19kHz) to three times, and with lock-in when the The most disruptive to the school, therefore, you can send the required data to the 57kHz sub-carrier to suppress the sub-carrier amplitude modulation of bilateral band, and then combined with the stereo signal together constitute the base-band modulated signal, and then the main carrier of the VHF FM. Channel FM with RDS stereo radio baseband modulation signal frequency spectrum as shown in Figure 1.
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In Figure 1, RDS data signal occupies 57 ± 2.4kHz baseband frequency, it does not interfere with stereo radio, it will not reduce its quality, the same time, it will not be radio interference.
1.2 Message sent
Accordance with the RDS standard, broadcast data signal is sent using a continuous data block duplication of data structures. The largest data unit as a group, each code group consists of four, namely set ABCD; each have 26 bits, 16 bits of information, 10 parity bits. Parity bit for error identification. Amendments and data synchronization. Data stream transmission rate of 1187.5bit / s. RDS Data format shown in Figure 2.
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1.3 Source Coding and Decoding
The system encoding is used in an optimal error-correcting shortened cyclic code burst, which generates polynomial
g x = x10 + x8 + x7 + x5 + x4 + x3 +1
Corresponding generator matrix G. The transmitter through a 16-bit of information mx with 16 × 26 G-matrix multiplication, the result then the bias associated with each block-specific word (10 bit) mode 2 increases, it produces a 26-bit data series.
RDS receiver decoders receive RDS clock signal and the RDS data signals. Received error-free bit-Series and check matrix H multiply, we get the word corresponding to the bias associated with equation (checksum word), in order to establish data flow block synchronization and group synchronization. In fact, it added the word bias inside each one, it is equivalent to bit-error added to each one, the word that is the equivalent of bias error series. If no other channel error, on receiving the information inside can be found by calculating the bias associated with type characters, to determine the received data block is ABCD in which a, that is, to achieve the synchronization.
2 launch system and the encoder information
2.1 Launching System Block Diagram
The computer is used to edit the various data to be sent information, such as advertising. Traffic information, etc., by the control management program through the RS232 serial port to send to the FM-RE602 encoder. FM-RE602 is the RDS signal encoder, it receives data sent by the host and generate consistent with RDS standard data link, the data stream through the wave transformation, with 57kHz sub-carrier signal with suppression of bilateral sub-carrier amplitude modulation, finally sent to the VHF / FM transmitter up.
As can be seen from Figure 3, the use of FM stereo radio to send data only on the basis of an increase in the original computer and an FM-602 in the encoder can achieve their goals.
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2.2 FM-RE602 encoder
RDS encoder is an important launch system components, there is no such domestic products, while foreign RDS encoder circuit complexity. Expensive. To this end, developed a line with RDS standard FM-RE602 encoder.
Encoder data received from the host information is binary data series. Since rectangular pulse spectrum is very wide, in the data transmission process in order to save bandwidth, you need to binary data in the series 1 and 0 respectively, converted into a fixed waveforms as shown in Figure 4.
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The wave as a modulation signal sent to a multiplier circuit, with the 57kHz subcarrier multiplied, the output is the difference between the two sub-signal phase shift keying (DPSK), its spectrum distribution in Figure 5.
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With foreign countries to take the hardware encoder filtering method to obtain the output waveform shown in Figure 4 The difference is, FM-RE602 encoder using the waveform synthesis method, that is, after sampling the ideal waveform, stored in the 1K-byte memory, and through D / A converter output waveforms corresponding to. FM-RE602 encoder hardware block diagram shown in Figure 6. MCU MCU selection 80C31, its serial communication port via MAX232 chip driver, adopted 9-pin plug 3 wire Tx, Rx, GND received a master mode.
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Figure 6, the interruption frequency of 1187.5Hz, received 80C31 external interrupts 0, CPU count to 26, it sends out a data. Its control program according to the data block 0 and 1 in the state removed from the EPROM in the corresponding waveform data, after DAC0832, the formation of the desired waveform.
Multiplier chosen MC1496, all the way for the 57kHz carrier signal and the other way for the D / A output signal, multiplied suppress the carrier signal. Because it is square wave modulation, so that this signal contains many harmonic frequency components. To obtain pure signals, and subject to band-pass filter, the filter selection MAX275 4-order band-pass filter, the fO = 57kHz, Bf = 4.8kHz, Q = f0/Bf = 11.875. The signal has been tested in full compliance with RDS specifications.
3 data receiver
Data receiving device shall be located in the radio signal coverage within the various information terminals, such as large screen LED display billboard characters. Each receiving device must be accompanied by a specific receiver card to display from the transmitter to send a variety of information. Since each receiver has its own address and group address, so that in different regions of the receiving device to display different information or public information.
Receiver card hardware block diagram shown in Figure 7. FM receiver card work in a specific frequency band, so front-end part of the similarity with ordinary FM radios, including the high-frequency amplification. Mixing. In place and so on. Mixing signal amplification, after a 57kHz band-pass filter, sent to the RDS decoder chip. RDS decoder chip select SAA6579T, it directly samples the input signal and output clock and data. The output clock signal as the sync signal to the MCU external interrupt input from the interrupt service routine to read data through the P1.0 port.
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Although the hardware circuitry to restore the transmitter sent a binary series of data signals, but need to find a way through software synchronization word. Error correction. Remove redundant bits, the effective data information via the serial port to send to the information display device, its software processes as shown in Figure 8.
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The use of FM stereo baseband signal transmission of data information, simply in various places on the basis of the existing FM station, in the transmitter to increase a FM-RE602 encoder can be in the radio waves within the coverage area information receiving devices for information transmission, has a wide coverage. Less investment and. Quick features. The in Wuhan. Harbin and other places of the tests showed system is running stable and reliable data transmission on the main channel signal does not interfere with, fully meet the design requirements. There is no doubt that the successful application of this technology provides a new means of information transmission, it is bound to the national economy. A profound impact on social life.
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