1 Introduction
With the expansion of the monitoring site and the increase in the number of probes, in the image display system of the monitoring terminal, it is often necessary to receive external data and display related information in real time along with the image on the monitoring screen so that the user knows the image information in a manner Information from where and how it relates, time, regional logo, real-time data is displayed on the screen. However, for the monitoring personnel, the attack on the monitoring terminal is a problem worth worrying about. Therefore, improving the technical solution and enhancing the security is an urgent requirement for the current situation. What the monitor wants to see is a true image that is verified, regardless of time, environment, probe position, angle and focal length. In the remote monitoring situation, the monitors of important departments are often not completely assured of the real-time and authenticity of the received images, especially if the high-tech objects are monitored in real time, then, after the monitor temporarily leaves the monitoring station for some time, Since it is not known whether the monitored party has attacked the monitoring system, there is often a requirement to confirm that the probe is working properly and the authenticity of the image is seen.
The author introduces the concept of confrontation to design a video surveillance system with security guarantee. The system uses a method of superimposing dynamic pseudo-random symbols on the screen to prevent the system from being disassembled while the system is running. Once the line is dismantled, the confirmation mark given by the main monitoring terminal will no longer be displayed, thereby detecting the behavior of the high-tech personnel of the monitored party to disassemble the monitoring head system at will, effectively preventing the pseudo-destruction of the signal transmission line from being falsified. With the introduction of images, the system can be applied in special occasions with high requirements and monitoring backgrounds without environmental features, such as special occasions such as blue sky, white wall and wilderness in the background and some important departments.
2 Principle and design ideas of image superposition
In order to achieve the purpose of image superposition, it is necessary to understand the principle of image transmission. In the current circuit technology, a method of sequentially transmitting pixels in turn (in turn) is adopted. At the transmitting end, the position of each pixel is sent to the receiving end one by one, and on the display screen of the receiving end, each pixel is also displayed one by one. Therefore, if you want to get the expected image at the receiving end, the sender and receiver must be synchronized. The so-called superposition is to ensure that the level of the icon signal is superimposed at the corresponding position of the video signal under the condition that the line and field signals separated by the video signal extraction circuit meet the synchronization requirements of the image signal to be superimposed.
In order to prevent the monitored object from modifying the line and introducing false images to confuse the monitor, this design introduces a confrontational idea: First, the designed monitor head additional circuit uses pseudo-random sequence coding to display various types of graphics or characters, and remote monitoring The center also uses the pseudo-random sequence coding corresponding to it to detect various types of graphics or characters. Since the codes used by the probes are different, the monitoring head can be prevented from being illegally exchanged. The second is to use the hardware video signal detection circuit to detect the separation phenomenon that may occur between the transmission line and the monitoring head device during the operation, and take corresponding measures.
3 system design
The image overlay application system introduced in this paper uses three main circuits: 89C52 type single chip microcomputer, MB90092 type special character superposition circuit and LA7806 type synchronous separator. Among them, MB90092 superimposes characters and graphics on the analog video input signal and produces mixed signals, 89C52 handles the icon and alarm to be displayed; LA7806 realizes the separation of the line signal and the field signal.
3.1 Hardware circuit design and module
The additional hardware structure of the probe portion of this video image overlay system is shown in Figure 1. The main working principle is that the 89C52 sends control commands to the MB90092 according to the control requirements, such as display control and VRAM write address control, and the video signal sent by the probe is separated from the original video signal by the LA7806 and then input into the MB90092; MB90092 is based on The instruction of the single chip reads the characters in the font MBM29F040 and mixes the image signal, the superimposed characters, the blanking pulse and the composite sync pulse according to a certain ratio, and sends out the video output signal. Since the line is disconnected, the video input/output signal is interrupted. Therefore, the MCU also determines whether the video input signal is present by detecting the LA7806, whether it has field sync, and whether the video output signal is judged by the output video signal detecting circuit. . For abnormal conditions, the 89C52 will receive an interrupt request and then issue a stop display character command to the MB90092 to alert the central monitoring room.
3.1.1 Video Clamp Circuit
The video clamp circuit is a video input signal preprocessing module. The composite video signal received by the MB90092 is 2Vp-p and requires a 1V DC offset. Therefore, it is necessary to clamp and amplify the 1Vp-p video signal level output by the camera. The amplifier circuit uses a MAX457 video amplifier with a magnification of 2. The circuit selects the same diode and triode with the same voltage drop. The potentiometer R2 is used to adjust the clamp voltage of the negative phase of D1 to 1V, and the DC signal of VV is superimposed with 1V. See Reference [2]. ].
3.1.2 Video signal synchronization separation circuit
Use SANYO's LA7806 synchronous processor (see Technical Reference [4] for detailed technical information). It separates the composite sync, composite blanking and other signals from the video signal and makes these signals work synchronously. Some designers use National Semiconductor's LM1881 line separator, which is highly integrated and can extract the sync signal from the video signal with a small number of peripheral circuits, but at a low signal-to-noise ratio (S/N). Under the condition, the composite sync signal separated by the LM1881 has a left-right sway phenomenon, which causes the whole system to be affected.
3.1.3 Character Overlay Module
FUJITSU's MB90092 is an OSD programmable LSI made in CMOS technology. Its functional block diagram is shown in Figure 2. It has a video signal generator, display memory (VRAM) and a font memory interface, requiring only a few external components. Character and graphic display capabilities are available. MB90092 can be used for screen character display of computer controlled video recorders, VCD and other video equipment. The circuit has a video signal input/output function, which can be used as a general-purpose OSD to superimpose the display of video signals and characters. An external 2M byte ROM can be provided, and two superimposing methods can be provided on the screen, which are called main screen superimposition and sub-screen superimposition. Both can appear on the monitor independently or overlapping. The main screen display supports a character display matrix of 24×32, a display capacity of 12 lines, 24 characters per line or Chinese characters, and each character can be a different color. The font size is controlled by standard (24×32 dot matrix), double width, double height, double width×double height, 4 times width×4 times height. The display position control sets the starting position in the horizontal and vertical directions, and sets the line spacing. MB90092's internal video signal generator supports NTSC or PAL, selects progressive or interlaced scanning, and can provide Y/C, composite video, R, G, B and other modes of output. The secondary screen can assist the main screen to display additional information, such as the drop-down menu function of windows. At the same time, the MB90092 can generate the synchronization signal by itself, without adding the video signal and the synchronization signal, and can directly output the superimposed character signal to the monitor, and display the text with different background colors and different Chinese characters on the monitor. The MB90092 has only 3 communication lines ( CS, SCLK, SIN) communicate with 89C52. Since the 89C52 itself does not have a serial peripheral interface, in the software design, the 89C52 ordinary I/O port analog serial communication is used, and the analog SPI mode is used for communication. Including serial clock, data input / output and chip select signal, but MB90092 does not input to the 89C52 communication line, this analog SPI system bus can be realized by only 3 lines.
The interface timing of MB90092 and external MCU is shown in Figure 3. First, the CS chip select is valid. On the rising edge of each clock, the bit of DATA data 1 enters the serial shift register inside MB90092, 8 bits is a byte, then CS and SCLK are respectively high, as one byte. At the end, the next byte is sent. In the serial transmission, the clock counts the received bit. By forcing CS to be high, the serial transmission can be reset, by forcing CS from high to low. The reset is cleared so that the subsequent 8-bit data is one byte of data. If CS goes high in the middle of 8-bit data, this data is invalid.
3.1.4 Chinese character font module
The Chinese character font module uses an external MBM29F040 type flash memory, in which the characters to be superimposed for storage are each composed of 24×32 lattices, that is, each character is represented by 96 bytes of data. According to the address interface relationship between the circuit and the external character memory and the order in which the circuit reads the dot matrix data, the desired characters are burned to the external memory in advance. Such an external character memory brings great convenience to the design. The system can change the character lattice at any time as needed.
3.1.5 buffer amplification module
The output buffer is amplified using the MAX470 amplifier, which contains four identical video amplifiers. Each input and each output are separated by a ground or power line. Each video output card requires two MAX470s. The main features of this circuit are bandwidth of 90MHz, gain of 2V/V, differential gain of 0.01%, differential phase of 0.03°, slew rate of 300/μs, switching time of 20ns, input capacitance of 5pF, and output can be set to High impedance to reduce power consumption.
3.1.6 Transmission line interruption detection
The purpose of line detection is twofold: one is whether there is a video input signal; the other is whether the video output signal is cut off. Among them, it is determined whether there is a video input signal by detecting whether there is field synchronization. After the video signal is separated by LA7806, the field sync signal is connected to P3.3 of 89C52 as an external interrupt source. Under normal circumstances, the video superimposed mixed signal (Vout) output by MB90092 has corresponding current and voltage. If the transmission line is cut off, the line current is also cut off. In order to ensure reliable operation, after a certain delay (using Schmitt trigger), if the detected analog line also detects the current stop, it can be confirmed that the line is cut off, so as to raise an alarm to the external interrupt of the microcontroller. Interrupt the request.
This design is to set a unit in the microcontroller. Since the ROM unit in the MCU is a Flash Memory, it will not be automatically restored. The MCU must detect the unit before sending a character to MB90092 in the running. If the unit is found to be set, it will permanently refuse to continue to send the superimposed characters. Whether the line returns to normal, the monitor can find the line change at any time thereafter.
3.2 Image overlay system software
This project needs to superimpose the image information on the original video signal to meet the monitoring needs and detect the fraudulent behavior of the video surveillance head. Therefore, the MCU should control the character image of the superimposed output, and also give the selected character and its color, position and background according to the arrangement of the pseudo-random sequence. The color, position, background, and the like of the character are given while giving the character instruction to be superimposed. By the pseudo-random variation of the pointer for each instruction entry address, the sequence of each instruction selection address forms a seemingly disordered pseudo-random sequence, which can more effectively prevent the fraud caused by the monitored party from identifying the identifier.
The software flow first initializes the program, and then retrieves the subtitle type and display position data from MBM29F040 to fill MB90092, and combines the input original video and its synchronous separated signal to output. The output causes an interrupt caused by an abnormal condition detected in the signal, causing a change in the external interrupt of the microcontroller, clearing the display character overlay flag, and the program stops transmitting characters.
4 remote detection alarm system
In order to enable the monitoring personnel of the central control room to discover the abnormal phenomenon of the equipment, the system should alarm the corresponding situation. The case where the line is cut off has been introduced in the section. The remote detection alarm system described in this section mainly detects the case where the probe is illegally replaced. This kind of situation can be detected by manual detection or computer. The manual detection can only see the big difference between the character and the character display. The display of the specific position and the similar shape of the icon that is difficult to distinguish with the naked eye must also rely on the remote computer to detect the alarm system. The system designed by the author is in the central monitoring room to match the conventional central monitoring system. By intercepting and analyzing each scene monitoring head image, an abnormality is detected and an alarm is issued.
Remote detection alarm is divided into 4 steps:
(1) intercepting the image of the probe using the functions provided by the original central monitoring system;
(2) processing the image to segment the superimposed logo;
(3) Identify the logo and get the relevant parameters;
(4) Matching with the corresponding pseudo-random sequence.
The process is shown in Figure 4.
The central monitoring system is equipped with monitoring software, which includes network communication programs, soft decoding programs and recording programs. The network communication program is connected to the probe of the monitored terminal to provide real-time monitoring and receive real-time video overlay images. The video program supports real-time monitoring while recording the data stream of the separated video signal monitored in real time to the hard disk to provide management and retrieval of the video data. The detection alarm system utilizes some of the functions provided by the central monitoring system.
The alarm system program can perform corresponding screen interception, segmentation and pseudo-random sequence matching according to the address displayed on the icon in the image superimposition process of the monitored terminal, and the selected characters and their characteristics, and the obtained separation result is superimposed on the monitoring scene. Processing is a complete one-to-one inverse process.
5 Conclusion
The monitoring system designed by the author loads the Chinese character font module, which has stronger editing function, higher anti-aggression and more obvious effect. The hardware design combines the software program controlled by the single-chip microcomputer to achieve the effect of anti-switching video equipment, which can ensure the real-time and accuracy of the monitoring image.
references:
[1]. MB90092 datasheet http://
[2]. 89C52 datasheet http://
[3]. LA7806 datasheet http://
[4]. MAX457 datasheet http://
[5]. LM1881 datasheet http://
[6]. ROM datasheet http://
[7]. MAX470 datasheet http://
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