Closed-circuit television monitoring systems are used more and more in various fields. Under different environments, different installation conditions and different construction personnel, due to differences in line and electrical environment, or negligence in construction, it is easy to cause various types of interference. . These interferences will enter the closed-circuit television monitoring system through the transmission cable, resulting in the degradation of video image quality, system control failure, and unstable operation, directly affecting the quality of the entire system. Therefore, to understand the impact of video interference on closed-circuit television monitoring system, take corresponding measures to solve the interference problem for different situations, it is very beneficial to improve the quality of closed-circuit monitoring system engineering and ensure the stable operation of the system.

The main form of video interference is 1. A black bar or white bar appears on the monitor screen and scrolls up or down. This is the so-called 50Hz power frequency interference. Most of this interference is due to the potential difference between the grounding and the control center's improper grounding of the two devices, resulting in a loop entering the system; it may also be due to a drop in the power supply performance of the device itself.

2, the image has snow noise. This kind of interference is mainly caused by signal attenuation on the transmission line and coupling of high-frequency interference.

3, video images have ghosting, or the image is whitish, character jitter, or on the monitor screen to produce a number of equally spaced vertical bars interfere. This is due to impedance mismatch due to the fact that the characteristic impedance between the video transmission line or the equipment is not 75Ω.

4, twill interference, beating interference, power interference. The appearance of this kind of interference does not drown the normal image when it is light, but when the image distortion is serious, it cannot be watched. The cause of this kind of failure phenomenon is more and more complicated. For example, the quality of the video transmission line is not good, especially the shielding performance is poor, or because the power supply system power supply is caused by clutter, and there is a strong system nearby. Source of interference.

5, large area moire interference, also known as single-frequency interference. This phenomenon is mainly due to the short circuit caused by the video cable's core wire and shielded network, the failure caused by the open circuit, or due to the poor contact of the BNC connector.

The video interference encountered in the scene is nothing more than the above five conditions. Therefore, we encountered such phenomena in the scene. First of all, we must calmly analyze the types of interference that belong to which categories, identify the possible causes of interference, and finally come Exclude it. The following is a specific example to analyze the causes of video interference and how to eliminate it.

Figure 1 shows a schematic diagram of a typical video surveillance system. Such systems tend to produce images with twill interference or scrolling dark bars on the monitor. From the form of interference performance, we can determine that the main cause of interference is from line interference and power interference.

Vab=(Vo×75)÷[75×2+Rc+Rd]]+(Vi×75)÷[75×2+Rc+Rd]]

Among them, the first item is the effective video signal obtained by the load Voh=(Vo×75)÷[75×2+Rc+Rd]]. The second item is the effective interference signal obtained by the load Vih=(Vi×75)÷[75×2+Rc+Rd]];

When the cable is very short, the resistance of the inner and outer conductors can be ignored, Rc+Rd=0. At this time, the effective video signal Voh=(Vo×75)÷75×2+0)=Vo×75÷75×2=Vo/2=1 Vp-p. Since the disturbance induced electromotive force Vi is proportional to (Rc+Rd), Vi=0 and Vih=0 at this time.

It is worth noting that the interference signal Vi is determined by the longitudinal distribution parameters (impedance or resistance) of the cable, and is not a concentrated point source. The important point is that it is connected in series in the video signal transmission circuit, and the load is at the same time that the camera video signal is obtained. It must also acquire the interference signal.

The nature of the interference belongs to "additive interference" and it always exists regardless of the presence or absence of video signals. From the above formula, we can see that the most important method to eliminate interference is to sense the small electromotive force as much as possible. This should also be taken into account when handling.

Several major sources of interference A, the ground of the ground-interference front-end equipment and the ground of the control room equipment are different from the potential of the power-grid ground, that is, the potential difference between the two grounding points is different from the “ground” of the power grid. A power loop is formed between the camera and the matrix, and the video cable shield is grounded, so that the 50Hz power frequency interference enters the matrix, causing interference. For this kind of interference, because it is difficult to make the “ground” potential of each location exactly the same as the potential difference of “grid-ground”, a more effective method is to cut off the path of the formation loop and use a method of cutting off the ground loop, at the end of the camera. Do not ground and do insulation measures with the mounting bracket so that interference caused by grounding can be substantially eliminated. It is worth mentioning that, because the coaxial cable is too long, it will inevitably have joints in the middle. If the joints are not handled properly, the shield network will also cause such interference when it touches the metal trunking.

B. Power supply interference Caused by the power supply system's power supply is not "clean". The power source referred to here is not "clean" and refers to an interference signal superimposed on a normal power source. This type of interference signal on the power supply is mostly caused by equipment that uses thyristors in the power grid, especially high-current, high-voltage thyristor equipment. The pollution to the power grid is very serious, which leads to the power supply in the same power grid. Not "clean". For example, in this power grid, there are high-power thyristor frequency modulation speed control devices, thyristor rectifier devices, thyristor AC-DC converter devices, and so on, all of which will cause pollution to the power supply. For example, after an on-site system is installed, there is a twill interference phenomenon in the image, and no reason can be found. Later, it was discovered that since this system was installed in the summer, the room was already equipped with air conditioning during installation, and the air conditioner was in automatic operation. Once accidentally shutting down the air conditioning power supply, a miracle appeared and the image on the monitor was normal. Then plug in the air conditioning power supply, and the interference appeared again. The problem of being troubled for a long time was finally discovered. Later, UPS was installed to separate the system power supply from the air-conditioning power supply, eliminating interference. The solution to this situation is relatively simple, as long as the entire system is powered by a clean power supply or an online UPS.

C. The space radiation interference introduced by the transmission line This interference phenomenon is generated mostly because there is a strong, high-frequency space radiation source near the transmission system, the system front-end or the central control room. One of the solutions to this situation is to understand the surrounding environment when establishing the system and try to avoid or stay away from the radiation source; another way is to strengthen the shielding of the front-end and center equipment when the radiation source cannot be avoided. The transmission line is piped with steel and grounded.

D. Impedance mismatch The fault phenomenon caused by the mismatch of the characteristic impedance of the transmission line. This is due to impedance mismatch due to the fact that the characteristic impedance of the video transmission line is not 75Ω or the characteristic impedance of the device itself is not 75Ω. For such interference, the impedance of each device in the system should be matched as much as possible.

As more and more matrix cascading systems are now in place, cascaded matrices are generally used where control points need to be managed separately and managed in the main control room. In the cascade system, if the distance is close, the video line may be directly connected between the matrices. In this way, the interference is very easy to occur. Especially since the two matrices are not together, the ground potentials of the two grounds are inconsistent. The most prominent interference of this system is ground interference. After this kind of disturbance appears, it is very difficult to handle. For the suppression of such interference, there are methods that have been solved in the project.

For the interference generated by this type of system, there are the following suppression methods:

1. Where possible, cascading video signals and control signals use optical transceivers as much as possible to transmit video, so optical isolation can better solve this interference problem. However, because the price of optical transceivers is relatively high, and not every place has the conditions for laying optical cables, this method is not applicable everywhere.

2. Install video anti-jamming isolator between cascaded video signals. Video anti-jamming isolator is a modern high-tech patented product. When audio-visual equipment is connected to produce various cross-talk interference, the product is connected in series on the video line, which can effectively isolate various interference signals and restore the original image and improve the quality of signal transmission. . The video anti-jamming isolator is a low-distortion, high-performance video isolator consisting of a wideband, high-performance band-pass filter and a high-performance video operational amplifier. Its use enables the remote and near-end video cables to be electrically powered. Completely in isolation, this fundamentally completely avoids the interference caused by the ground potential difference, and at the same time can effectively suppress the system damage caused by lightning shock. Due to the isolation of the electrical signal, the video anti-jamming isolator can also effectively avoid the interference of the interference signal to the video signal due to the environment high frequency, pulse and the like.

Anti-jamming isolator features:

●Simple installation, just connect the receiving end of the video cable in series;

● This product does not require an external power supply;

● Adopts wideband, high-performance bandpass filters;

● Optically isolated video signals;

● Effectively suppress the black-print rolling interference caused by the difference in ground potential;

● Effectively suppress high frequency moire caused by the phase of the power supply;

●It can also suppress high-frequency pulse interference caused by insufficient cable insulation;

● For long-distance transmission of video signals, with phase adjustment, signal amplification;

● Effectively isolate the impact of lightning on the system equipment.

3. Use a modem to increase the signal frequency and transmit it.

E. The relationship between the interference wave propagation distance and frequency According to the relationship curve between the interference wave propagation distance and frequency, it can be seen that the lower the frequency is, the longer the propagation distance is. This also means that the lower the frequency, the greater the chance of interference and the greater the intensity. Therefore, using the signal to increase the frequency after transmission, in the center and then the signal modulation and filtering can also greatly reduce the interference.

Because each monitoring system has its own different characteristics, it will produce such interference, so correctly analyze the causes of interference, and take effective measures to suppress interference, to ensure the quality of the project and equipment to run well, the monitoring system looks Especially important.

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