Anti-jamming measures of CCTV

introduction

Closed-circuit television monitoring systems (CCTV) are increasingly used in construction projects. Due to the complex electrical environment in the building, various sources of interference are prone to form. If appropriate precautions are not taken during the construction process, various interference It will enter the closed-circuit television monitoring system through the transmission cable, resulting in the degradation of video image quality, system control failure, unstable operation and so on. Therefore, to study the nature of the CCTV monitoring interference source and understand the impact on the CCTV monitoring system, so as to take measures to solve the interference problem is very useful for improving the quality of the CCTV monitoring system and ensuring the stable operation of the system.

The source and influence of interference

There are two main types of transmission signals in CCTV monitoring systems: one is analog video signals, the transmission path is from the camera to the matrix, and then from the matrix to the display or video recorder; the first is the digital signal including the matrix and the camera Control information transmission between machines, digital signals of the computer part in the matrix. The possibility of general equipment becoming a source of interference is very small, so interference mainly enters the system through the signal transmission path. The signal transmission path of the CCTV monitoring screw system is that the interference that can be coupled into the system through the video cable and the twisted pair that transmits the control signal are: various high-frequency noise such as large inductance load start and stop, and the power frequency introduced by the unequal ground Interference, unbalanced transmission lines reduce the ability to suppress noise, and turn common frequency interference into differential mode interference. Impedance mismatch on the transmission line causes signal reflection to reduce signal transmission quality. Electrostatic discharge enters the device along the transmission line and causes damage or damage to the interface chip. The specific performance is as follows:

The effects caused by the impedance mismatch appear as ghost images on the video image. The signal transmission line will oscillate at the front and back edges of the pulse sequence. The existence of oscillation makes the threshold difference between high and low levels smaller. When the amplitude of oscillation is larger or other interference is introduced, the pulse level value cannot be correctly distinguished, resulting in longer communication time or communication interruption. Poor grounding and shielding will cause the transmission line's ability to suppress external electromagnetic interference, which is reflected in the video image is snowflake noise, moire interference and horizontal scrolling; spike interference on the signal transmission line causes communication errors. Unbalanced transmission lines can also cause spike interference on signal transmission lines. In addition to causing damage to the device, electrostatic discharge will also affect the data in the memory, causing some inexplicable errors in the device.

Anti-interference method

From the analysis of interference sources, it is understood that there is no special interference source. There are many theoretical discussions on eliminating or reducing the above interference. How to solve the interference problem for CCTV monitoring projects is rarely mentioned in the literature. The following are common among CCTV monitoring types. Discuss the interference and solutions.

Anti-interference measures in digital signal transmission

In weak current system engineering, the transmission of digital signals usually refers to long-line transmission. Common methods include: transmitting digital signals on power lines or video lines through modulation and demodulation methods; transmitting through industry-standard communication networks, such as RS422, RS845, RS485 ; Automatic transmission developed by ourselves. Comparing the three, the common ones are RS422 and RS485, so we focus on the anti-interference method of RS485 digital communication.

RS485 bus is a differential balanced electrical interface, which has strong anti-electromagnetic interference ability, but in actual engineering RS485 bus has not achieved the desired effect. The problem often appears in the following aspects: the first network topology is unreasonable, and it is not wired according to the bus network topology, becoming a star topology on the matter; the transmission line is incorrectly connected to the receiving and sending end equipment, weakening the anti-interference of the balance line Capacity; the third public twisted pair, no further anti-interference measures, such as shielded twisted pair. Although there are different ways to cause interference, there are only two manifestations of interference: one is that reflection increases the degree of signal distortion; one is that external interference is destroyed due to equilibrium conditions, and common mode interference becomes a string. The mode signal enters the transmission line.

About signal reflection. According to electromagnetic theory, the only way to reduce signal reflection on long lines is impedance matching. If the communication style topology is bus-type, impedance matching is easier to achieve, but if it is a star network topology, according to engineering experience, it can be matched according to Figure 1. The resistance RO on the sending end string and the characteristic impedance of the transmission line are connected at the receiving end as shown in the figure, where R1> R2, R = (R1 * R2) / (R1 + R2) = R0. When transmitting R0 is generally more than 5 times the internal resistance of the output of the drive gate, a higher transmission level can be obtained. The matching impedance of the reception is formed by a 5 volt power supply, and the absorption power consumption is reduced while the impedance is matched, which reduces both In addition, the increase in the matching resistance will not absorb too much signal power, and the signal level threshold difference becomes smaller.

Twisted pair as RS485 has a strong ability to suppress a pair of electromagnetic induction noise, but the ability to suppress noise caused by static induction is poor, so RS485 transmission line should use shielded twisted pair. The shielding layer of the twisted pair should be properly grounded. The “ground” mentioned here should be the “ground” that drives the bus logic gate, not the “chassis ground” or “protective ground”, but it is often not given on many actual devices. The ground connection terminal, so in this case, you need to draw a line to connect the shield to the ground of the drive logic gate integrated circuit.

Video signal interference

The interference of the video signal is shown on the image as ground flower dots and 50HZ horizontal grain scrolling. For the snowflake dot interference is caused by the attenuation of the signal on the transmission line and the coupling of high-frequency interference. This interference is relatively easy to eliminate. Add a video amplifier at a reasonable position between the control matrix to increase the signal to noise ratio, or change the path of the video cable to avoid the source of high-frequency interference. The problem of high-frequency interference can be basically solved. It is more difficult to solve the situation of 50HZ horizontal grain rolling and further adding high frequency interference, such as the output image of the camera in the elevator car. In order to suppress the above interference, first analyze the causes of the above problems.

There are generally three types of power supply required by the camera: DC 12V, AC 24V or 220V. In most engineering applications, the power supply of the elevator car is not taken from, but the power supply is additionally arranged to supply power to the camera. The output image is sent out from the stop or below the shaft through a flexible video cable. The video cable and power supply cable are bundled with the power line of the car. When the elevator is running, the electromagnetic field generated by the operation of the traction motor propagates along the lighting power line. Obviously it will affect the power supply cable and video cable of the camera. When the shielding layer of the video cable is not tight enough, high-frequency interference will be transmitted back to the monitor via the video cable. According to the theory of electromagnetism, the 50HZ rolling of horizontal stripes can completely eliminate the 50HZ power frequency interference. It can be inferred that this part of the interference is not coupled through the video cable, but from the power line and the unreasonable video line connection.

For the high-frequency interference in the image, because its frequency band is still within 8MHZ, the use of a shielding network with a void rate of about 50% can basically prevent high-frequency interference, but to achieve a 50% void rate, the number of shielding nets needs each There are more than 60 wavelength lengths, and such a high density will reduce the flexibility of the cable. A better method is to use a video cable with double shielding.

The video cable shielding layer is grounded. If the video signal “ground” and the display ’s “ground” have different potentials relative to the “grid ground”, that is, as shown in FIG. 3, the voltage difference between the two grounding points is different from the grid “ground”, then The power supply circuit is formed between the camera and the display through the power supply, so that 50HZ power frequency interference enters the display. From the electrical connection in the figure, it can be seen that there are two ways to eliminate 50HZ power frequency interference. The "ground" potential is exactly the same as the "grid ground" potential difference, or the path forming the ground circulation is cut off. Due to the complex engineering environment, it is difficult to make the "earth" at all locations completely equipotential. The resistance of the ground loop can be reduced as much as possible by increasing the cable diameter of the camera's power supply cable. Or use the method of cutting off the ground circulation circuit, one end of the camera or display is not grounded, and the power supply ground is usually not connected to the display end, although the interference cannot be completely eliminated but the interference of 50HZ can be greatly reduced.

From the above analysis, it can be seen that if high-frequency noise is coupled to the power cord, even if the shielding of the shielded cable of the video cable is good, the noise will be sent to the display, so the power supply cord of the camera should also be shielded. The above measures need to be fully considered during engineering design and construction. If the interference is found during system debugging, the method of modulation and demodulation can be used to filter out the noise, and a modulator is provided on the camera to convert the video signal. Moved to the frequency band of tens of megahertz, set a low-pass filter at the display end to filter out all signals lower than 8NHZ, and then demodulate to restore the video image.

Monitoring system power supply

There are only two power supply methods for the monitoring system: one is the centralized power supply method, that is, the power is drawn from one place, and the other is the distributed power supply. The camera takes the power supply near the installation location. From the perspective of anti-interference effect, the power supply is centralized The power supply method is better, which can basically eliminate the unequal reference potential everywhere.

Conclusion

The interference in the monitoring system is mainly reflected in the interference of digital communication lines and video images. The key to solving the interference is that anti-interference measures must be fully considered when the project starts construction, so that the interference problem can be solved fundamentally without waiting for the project Later, measures will be taken to repair the dead.

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