A FM Dual Audio Communication Solution Suitable for Single Chip Microcomputer System

introduction

In many single-chip microcomputer application systems, electrical signals and non-electrical signals are often used to transmit control signals and data information to realize remote control or telemetry functions, such as optical signals, ultrasonic signals, electrical pulse signals, etc. In this FM wired/wireless remote control broadcasting system, dual-audio electrical signals are used to send and receive signals to realize remote control and management of FM wired/wireless broadcasting. This dual-audio communication has the characteristics of simple control, convenient implementation, reliable transmission, and low cost. It is also a more commonly used communication method. The key to realizing dual-audio communication in a single-chip microcomputer system lies in the design and application of dual-audio receiving circuits and the design of detection programs.

1 Basic principles of remote control broadcasting system

The basic principle is shown in Figure 1.

The communication of the remote control broadcasting system is a dual-audio communication method of telephone dialing and receiving, which is a reliable and mature communication technology. It replaces the traditional telephone dialing pulse communication method and becomes the main communication method of modern telephone communication dialing. It has low requirements for transmission circuits, long transmission distances, and has a variety of general dual-audio components for selection. The transmission signal and encoding can be obtained through an ordinary telephone. After being modulated by the FM agile frequency stereo frequency modulation modulator, two stereo channels are used. The left channel inputs the broadcast audio signal, and the right channel input is used to control the dual-audio signal to avoid the control signal interfering with the broadcast audio signal. The FM modulated signal can be transmitted to the receiver at the transceiver end through the cable TV network or through electromagnetic waves emitted into space. After the receiver antenna receives the RF signal, it detects and obtains the audio signals of the two stereo channels. The left-sound broadcast audio signal from the stereo output is amplified and output by the power amplifier; and the dual-audio control signal of the right channel is decoded by the dual-audio decoding circuit to obtain the binary code and send it to the OTP microcontroller port. The power supply of the receiver amplifier is turned on and off according to the machine number code set in the single chip microcomputer in the receiver, so as to achieve the purpose of remotely controlling various broadcasting points of the FM receiver.

2 Hardware circuit design of remote control broadcast system transmitter and receiver

2.1 Remote control broadcast system transmitter

The transmitter can be designed by yourself, or it can be modified from a good quality telephone with a dial-up digital display and telephone number storage function. As shown in Figure 2, it is a remote control broadcast system transmitter. Connect the +48V power supply according to the figure, adjust the resistance value of R1 so that the voltage at point A is 6-12V when the telephone is connected, and add an FM agile stereo frequency modulator and broadcast sound source.

The operation of the remote control broadcasting system transmitter can be done by pressing buttons like making a phone call, or by storing it in a memory, or by redialing, etc. The password of the remote control broadcasting point can be output and modulated by an FM agile stereo FM modulator.

2.2 Remote control broadcast system receiver

The working principle of the receiver is that when the broadcast audio and remote control signal are transmitted from the FM modulator to the FM receiver through space (wireless) or RF cable (wired), the broadcast audio and control dual audio code signals are output from the left and right channels respectively. The dual audio code signal outputs the binary code code to the P1 input port of the AT89C2051 microcontroller through the CD22204 dual audio decoder, detects the password and executes the control action of turning on or off the power amplifier. CD22204 is a product of the American Radio Corporation (RCA), and its internal working principle is shown in Figure 3. It has the ability to detect and output standard dual audio coding signals. Its working principle is that when the analog signal is added to the input end, the 50Hz interference is first suppressed, and then the pre-amplification is performed. The high-frequency filter circuit has a good filtering effect on the high-frequency spark. After zero-crossing detection, bandpass filtering, amplitude detection circuit, and time circuit processing, each key 0~9, *, #, etc. is pressed, and the DV has a positive pulse output. The decoded outputs on the output terminals D1, D2, D4, and D8 are output only when the EN enable terminal inputs a high level. The binary code output by CD22204 is positive logic, as shown in the receiving and CPU schematic diagram in Figure 4.

The output data signals of D1, D2, D4, and D8 are inverted by the inverter composed of BG1, BG2, BG3, and BG4 and then input to the P1.7, P1.6, P1.5, and P1.4 ports of AT89C 2051. The LCD display is designed to facilitate observation of communication and detection conditions. The ones digit displays the total number of code digits 5, the thousands digit displays the number of dynamically detected digits 5 to 1, the tens digit displays the dynamic local password 0 to 9, and the hundreds digit displays the dynamic code 0 to 9. When the input five-digit code is equal to the five-digit password of the local machine and a switch signal (0—on, #—off) is received, the power amplifier switch will be activated. If the password is incorrect or the on or off button is not pressed, there will be no action. [page]

When using the system, the code should be pre-stored in the memory or the redial key should be used to output the code at one time, so that the code can be input quickly and accurately. When the MCU detects that the input code is equal to the local password and on (*) or off (#), it will output a high level from the P3.5 port to make the BG6 conduction relay close and turn on the power amplifier, or output a low level to make the BG6 load and turn off the power amplifier, so as to achieve point-to-point or point-to-multipoint control.

3 Software Design of Remote Control Broadcasting System Receiver

3.1 RAM Allocation

10H The ones digit display register shows the password digit 5.

11H Tens display register, showing dynamic password number 0~9.

12H Hundreds digit display register, displays dynamic input code number 0~9.

13H Thousands display register, showing the number of dynamic password digits 5 to 1.

17H 1ms delay initial value register.

30H Unit digit password number, this machine is 1.

31H Ten-digit password, this machine is 2.

32H Hundreds digit password, this machine is 3.

33H Thousands password number, this machine is 4.

34H Ten thousand digit password, this machine is 5.

3.2 Program software design

The software consists of a main program, a password check subroutine, a liquid crystal display subroutine, a T0 timing interrupt, and a 10ms delay subroutine. The T0 interrupt is set to provide a 250Hz square wave drive signal for the liquid crystal, so that the liquid crystal strokes will not burn out. The 10ms delay is set for stable display. The liquid crystal display software is designed to send the font to the liquid crystal screen bit by bit from the units. In the password check subroutine, the negative logic binary code is read from P1.4 to P1.7, and the positive logic needs to be restored by inverting. A large loop and a small loop are set in the main program, the purpose is to re-install the set value after the loop is completed, so as to maintain abnormal input and delay a certain time to resume normal operation. The password check subroutine can also be designed in an interrupt mode.

4 Conclusion

Since the prototype designed and manufactured was put into use, it has stable performance, reliable operation, and normal operation, and basically no control errors have been found. This control scheme can also be extended to the closed-circuit television management system, but the receiving device must be specially set up. A method of controlling multiple users with one receiver can be adopted to reduce costs. It can be used for the control scheme of the camera pan/tilt to reduce the cost of the monitoring system. Further improve and enhance the design of the system and find problems in a large number of uses, solve problems and improve the level and quality of the system. This method is a one-way communication method and still has some shortcomings.