Led billboard is a public oriented information display terminal, which has a very wide range of civil and commercial value. However, most of the information displayed on LED billboards is fixed in the system in advance, which loses the real-time property of information and is particularly inconvenient for users to change the display contents. A few led billboards are directly connected with a microcomputer and the information is transmitted by a microcomputer. This system solves the problem of information refreshing. However, due to the wired connection, the random placement of the system is restricted, especially for the For the advertising company which faces multi region and multi client, it brings a lot of inconvenience to the real-time update and maintenance of the system. This paper puts forward a new scheme of remote control LED billboard by hand-held transmitter, which realizes the flexible change of billboard content and display mode, which is very convenient to use. In this scheme, PC is used as the upper computer, and MCU is used as the core of transmitter and receiver, which has a high performance price ratio.
1. System scheme
1.1 wireless remote control mode
Wireless remote control is to realize the non-contact remote control of the controlled target, which is widely used in the fields of industrial control, aerospace and home appliances. Compared with wired and infrared devices, wireless remote control and wireless transmission system improves the mobility degree of freedom. Thus, the application of wireless remote control device and wireless transmission system in the industrial field is increasing. The advantages of cable connection are that the installation cost is low (no wiring, underground engineering, no cable tray), and it improves flexibility and reduces maintenance cost.
In order to achieve the purpose of remote control, it is necessary to transmit the information and control commands to the LED screen of the display terminal through wireless transmission. The commonly used wireless transmission modes are: sound wave (ultrasonic wave), light wave (infrared ray) and electric wave. Considering the following advantages of radio wave transmission:
1) The transmission distance is much longer than the other two;
2) There are many kinds of available components, good performance and low price;
3) The technology is more mature.
Therefore, the system adopts radio wave transmission. That is, information is transmitted by radio waves between the handheld transmitter and the receiver.
The performance of high-frequency wireless transmitting and receiving module directly affects the remote control distance and communication quality. After investigation and demonstration, the system adopts integrated transmitting and receiving module. Its main advantages are good frequency consistency and no debugging.
1.2 data coding mode
Data coding is a kind of technology to express the database information that needs to be processed with close-up numbers. According to the qualitative characteristics of a certain data structure and target, the data is converted into code or coded characters, which represents the composition of data in data transmission, and serves as a set of rules and conventions for transmission, acceptance and processing. Because the data information to be processed by computer is very complex, the meaning of some databases is difficult to remember. In order to be easy to use and memorize, it is often necessary to encode the processed objects, and a code is used to represent a piece of information or a string of data. Data coding is very important in the management of computer, which can easily carry out information classification, check, total, retrieval and other operations. Therefore, data coding becomes the key of computer processing. That is, different information records should adopt different codes, and a code point can represent an information record. People can use coding to identify each record, distinguish processing methods, classify and check, so as to overcome the shortcomings of uneven projects, save storage space and improve processing speed.
Source coding and decoding is very important in wireless communication, which can improve the reliability and effectiveness of signal transmission. Billboards are placed outdoors, and there are many sources of interference. Therefore, in the design of this system, the reliability of encoding and decoding and anti-interference are the key to the success or failure of the design.
This system starts from the wireless communication theory, studies the coding and decoding technology and its influence on the system. After repeated comparison, the special chip suitable for the system is selected. The module also uses code division multiple access (CDMA) technology to encode and decode data. It can provide up to 531441 (312) address codes, which can completely eliminate any code address conflict and interference of unauthorized encoded data.
1.3 image lattice extraction and moving algorithm
Image lattice extraction is to use the algorithm to calculate the offset address in the image source database, so as to get the corresponding lattice data and generate new display data. The correctness of the extraction algorithm is directly related to the correctness of the display image.
Image moving is to calculate the offset address of the next frame data in the display lattice database by using the algorithm. The correctness of mobile algorithm is directly related to the synchronization of dynamic image or text and background and the scene effect, especially for color display screen.
1.4 system block diagram
The whole system is composed of three relatively independent subsystems, i.e. upper computer, handheld transmitter, receiver and display system. The structure diagram is shown in Fig. 1.
Figure 1 system overall structure block diagram
The task of the upper computer is to complete the input and editing of the display information, and then through the lattice extraction and moving algorithm, the display information is converted into the lattice data corresponding to the LED display screen, and the lattice data is transferred to the handheld transmitter through the serial port of the PC.
The function of the handheld transmitter is to transmit the lattice data from the upper computer and the commands received on the panel keyboard to the outdoor user group in the code division multiple access communication mode after coding and modulation, and can set and debug the display format of the display screen on site.
After amplification, shaping, demodulation and decoding, the receiver outputs the received high-frequency signal to the single-chip microcomputer in parallel. The single-chip microcomputer identifies and stores the received data, changes the display mode and content in real-time, and drives the LED display screen.
2. Hardware configuration
The hardware of the system mainly includes three parts: upper computer, handheld transmitter, receiver and display module.
2.1 upper computer
The upper computer refers to the computer that people can directly send out the control command. Generally, PC displays various signal changes (hydraulic pressure, water level, temperature, etc.) on the screen. The lower computer is a computer that directly controls the equipment to obtain the equipment status, which is generally PLC / SCM and so on. The command sent by the upper computer is first sent to the lower computer, and then the lower computer interprets the command into the corresponding timing signal to directly control the corresponding equipment. The standard application software is rich, the interface has strong universality, the program based on PC has strong compatibility and portability, and has high cost performance.
Because the upper computer system needs to complete the input, editing and animation effect design of display information (image and text), in addition to the central PC, it must also be equipped with corresponding input peripherals, such as scanner, camera, keyboard, etc. Internet interface is also essential, which can realize the remote download of information and the network management of advertising system.
The composition of the upper computer system is shown in Figure 2.
Figure 2 configuration block diagram of upper computer system
2.2 handheld transmitter
The transmitter is composed of single chip microcomputer, keyboard, encoder, transmitter, serial communication interface and UPS power supply system. Its structure diagram is shown in Figure 3.
Figure 3 transmitter structure block diagram
The function of single chip microcomputer is to store the image lattice from PC or EPROM into the transmitting buffer RAM temporarily, and then send it to the encoder in serial according to the command of keyboard.
The 89S52 of ATMEL company is selected as the MCU, which has the advantages of fast operation speed, stable performance and low price.
The keyboard is used to set the image moving speed, display mode and communication protocol of transmission.
Encoder is one of the key components of the system, which determines the reliability of communication. The main features of the chip are CMOS technology, low power consumption, very high noise immunity (multi frame synchronization), up to 12 bit 3-state address pins (up to 312 address codes can be provided), up to 6-bit data pins, wide range of operating voltage, single resistance oscillator, output form can be set as latch or transient.
The function of transmitter is to modulate the coded digital signal to high frequency carrier, and then transmit it after power amplification. It determines the wireless remote control distance of handheld transmitter. The integrated module of modulation, driving and transmitting is selected in this system. The module uses acoustic meter resonator to stabilize frequency, SMT resin packaging, good frequency consistency, no debugging, especially suitable for multi transmitter and multi receive wireless remote control and data transmission system. However, the frequency stability and consistency of general LC oscillators are poor. Even if high-quality trimming capacitors are used, it is difficult to ensure that the adjusted frequency point will not shift.
UPS power supply is used to ensure that the data in the ram of the transmitting buffer will not be lost when the handheld transmitter works outdoors. Because the transmission buffer needs a large amount of image lattice data, if the use of ultra large capacity nonvolatile memory, such as E2PROM, flash and fram, is not only high cost, but also difficult to purchase, so the system selects ordinary RAM as buffer register.
2.3 receiver and display module
The receiver is a circuit system composed of antenna, filter, amplifier and a / D converter. The navigation and positioning signal sent by GPS satellite is an information resource that can be shared by numerous users. For the vast number of land, ocean and space users, as long as users have the receiving equipment that can receive, track, transform and measure GPS signals, namely GPS signal receiver, they can use GPS signals for navigation and positioning measurement at any time.
The receiver and display module are composed of receiver, decoder, single chip microcomputer, display driver, LED screen and power supply system. The structure diagram is shown in Fig. 4.
Figure 4 receiver block diagram
The receiver demodulates the digital coded signal after amplifying and shaping the received high frequency signal. We choose the integrated receiving and demodulation module corresponding to the transmitting module. Single frequency receiver. Single frequency receiver can only receive L1 carrier signal, and measure carrier phase observation value for positioning. Because the ionospheric delay can not be effectively eliminated, the single frequency receiver is only suitable for the precise positioning of short baseline (15km). 2. Dual frequency receiver. The dual frequency receiver can receive L1 and L2 carrier signals at the same time. The influence of ionosphere on the delay of electromagnetic wave signal can be eliminated by using the difference of ionosphere delay caused by dual frequency. Therefore, dual frequency receiver can be used for precise positioning of thousands of kilometers. The series of modules adopt superheterodyne and secondary frequency conversion technology, and all RF receiving, mixing, filtering, data demodulation, amplification and shaping circuits are integrated in the module. The function is highly integrated, which avoids the instability of RF frequency debugging and super regeneration receiving circuit, and has the characteristics of high reliability, stable frequency and debugging free receiving frequency.
The decoder decodes the digital coded signal and outputs it to the MCU in parallel. We choose the special chip of code division multiple access (CDMA) serial decoding corresponding to the encoding.
Single chip microcomputer is responsible for the identification, storage and display mode conversion of received data. 89S52 of ATMEL company is selected as MCU, which has good versatility and high cost performance.
The display module is used to display advertisement information. The module includes LED display screen and display driver board, with display buffer and dynamic scanning mode to drive LED display, which has the characteristics of low power consumption and high brightness.
There are no special requirements for the power supply of the receiving display system, but considerable margin should be reserved and attention should be paid to ventilation and heat dissipation, because many fires caused by LED billboards are caused by the power supply system.
3. Software structure
The software of this system is mainly composed of three modules, i.e. the host computer control program, the transmitter master control program and the receiver master control program.
3.1 main control program of upper computer
The main control program of upper computer includes display information generating program, display dot matrix conversion program and serial communication program, which is the most complicated part of the three main control programs.
Display information generation program completes the input, editing and animation effect design of display image and text. Firstly, the main control program of DOS platform is combined with the underlying communication software, and the multi parameter input of source file, Internet interface and other input devices is completed through serial and parallel communication ports.
Display lattice conversion program is to convert the generated display information into the corresponding lattice data of LED display screen through lattice extraction and moving algorithm. For color display screen, RGB three primary color data of image information should be sampled separately, stored in blocks and transmitted in sequence; for continuous animation images, offset should be calculated by frame, and the key is to synchronize background image and animated text. The software of this part is written in turbo c3.0.
Serial communication program is relatively simple, attached to the lattice conversion program.
Limited by space and technical reasons, this paper only takes the sending program of Chinese character extraction in PC as an example to describe its principle and structure.
Set the display screen of the terminal to display 16 × 16 Chinese dot matrix. Therefore, in order to display advertising information on the terminal, the lattice data of Chinese characters contained in the information must be transmitted to the terminal. In turbo c3.0, when a Chinese character is assigned to a variable, the location code of the Chinese character is actually assigned to this variable. Each Chinese character is a 16 × 16 dot matrix, where the stroke passes is “1”, and the rest is “0”. In this way, from top to bottom, from left to right, a Chinese character consists of 256 dots, that is, 32 bytes. Chinese characters are arranged in the Chinese character library according to the order of location code. The area code is the row, the bit code is the column, and there are 94 digits in one area. In this way, the offset address of a Chinese character in the Chinese character library is (area code × 94 + bit code) × 32. The 32 byte Chinese character matrix is sent to the transmitter through the serial port of PC. The flow chart is shown in Figure 5.
Figure 5 PC software flow chart
3.2 main control program of transmitter
The main control software of transmitter is to receive the image lattice sent by upper PC through serial port, read keyboard command, and send operation command or display data to encoder in serial.
This part of the software is written in Keil C51 environment, it can directly translate C language into assembly language, generate binary code and write it into MCU, so the writing efficiency is higher. When triggered by the serial port, it indicates that the PC has image lattice data, and the single-chip microcomputer immediately receives the lattice data and stores it in RAM; when INT0 is triggered, it indicates that the keyboard has action, it immediately receives the key value to determine the type of command, and sends image lattice data or operation command to the receiver. Its function flow is shown in Figure 6.
Figure 6 transmitter main control program flow
3.3 main control program of receiver
The function of this part of software is to complete the recognition, storage, transformation and display drive of the received data, and can realize the local self-test and display the solidified image. When the decoding chip of the receiving device has output, the single-chip microcomputer interrupts the current display, receives the sign word, and then judges the content of the sign word, and determines whether it will receive the image lattice or the operation control command. If the lattice data is stored in a display buffer database, it will be displayed in the current way. If it is a command, change the display mode immediately.
Among them, the key is the real-time realization of image moving up, down, left and right. The specific implementation method is as follows:
1) Move the image up and down until one image is finished; move the lattice of the next image again. The process of moving up is similar.
2) The left and right movement of the image will shift each line of data of an image to the left (using the shift instruction of C language), and the shifted dot matrix will be stored in buf and displayed. The process of moving right is similar.
The function flow of receiver main control program is shown in Figure 7.
Figure 7 receiver main control program flow
4. Performance test
The system function and parameter test results are as follows:
1) Image input function scanner, camera, digital camera, image source file;
2) Text input keyboard, word / text source file;
3) Animation design function text or graphics relative to the background move, zoom;
4) Transmitter set function receiver serial number, image movement command, self-test command and transmission command;
5) The receiver self-test function displays blue lawn background and “welcome to” mobile characters;
6) Standby time of transmitter ≥ 120h;
7) Transmitter remote control distance ≥ 250m;
8) The buffer space of transmitter is 8 Mbyte;
9) LED display 320 × 640dip;
10) The power consumption of receiving display system is less than 4kw
The performance test and appraisal conclusions of the whole machine are as follows:
1) Through the remote control of LED billboard with handheld transmitter, the scheme is novel, practical and has high market promotion value;
2) Scientific hardware configuration, stable performance and high cost performance;
3) The software structure is reasonable, powerful and easy to use.