CCTV system interference problem and method of grounding during construction

1. In video transmission, the most common fault phenomenon is that a black bar or white bar appears on the screen of the monitor, and it slowly scrolls up or down. Therefore, when analyzing this type of fault phenomenon, it is necessary to distinguish between two different causes of the fault.

To distinguish between the problem of power supply and the problem of ground loop, a simple method is to connect only the output signal of a camera with no power problem to the control host. If the above interference phenomenon does not appear on the monitor , It means there is no problem with the control host. Next, you can use a portable monitor to connect to the video output of the front camera, and check each camera one by one. If there is, deal with it. If not, the interference is caused by ground loops and other reasons.

2. Wood grain interference appears on the monitor. The occurrence of such interference will not overwhelm the normal image when it is slight, and the image cannot be viewed when it is severe (even destroying the synchronization). The reasons for this failure phenomenon are many and complicated. There are roughly the following reasons:

⑴ The quality of the video transmission line is not good, especially the shielding performance is poor (the shielding network is not a good quality copper wire network, or the shielding network is too thin to play a shielding role). At the same time, the line resistance of this type of video cable is too large, so that the signal is greatly attenuated is also the cause of the increased failure. In addition, the characteristic impedance of this type of video cable is not 75Ω and the parameters are beyond the specifications is also one of the reasons for the failure. Because the above-mentioned interference phenomenon is not necessarily a failure caused by a bad video line, the cause of this failure must be accurate and cautious in judgment. Only after excluding other possibilities, can we consider from the perspective of poor video line. If it is really a cable quality problem, the best way is of course to replace all such cables and replace them with cables that meet the requirements. This is the best way to completely solve the problem.

⑵ Caused by the power supply system is not "clean". The power supply here is not "clean", which means that the interference signal is superimposed on the normal power supply (50-week sine wave). The interference signals on this kind of power supply mostly come from the equipment using SCR in this power grid. In particular, high-current, high-voltage thyristor equipment has a serious pollution on the power grid, which results in the power supply in the same power grid not being "clean". For example, there are high-power thyristor frequency and speed control devices, thyristor rectifiers, and thyristor AC-DC converters in the power grid, which will pollute the power supply. The solution to this situation is relatively simple, as long as the entire system uses purified power or online UPS power supply can basically be solved.

(3) There are strong interference sources near the system. This can be judged by investigation and understanding. If this is the reason, the solution is to strengthen the shielding of the camera and ground the video cable.

3. Failure caused by short circuit or open circuit of the core wire of the video cable and the shielding network. The manifestation of this kind of failure is that a deep and chaotic large-area network interference is generated on the monitor, so that the image is completely destroyed, and the image and the synchronization signal cannot be formed. This situation often occurs on BNC connectors or other types of video connectors. That is to say, when this kind of fault phenomenon occurs, it is not always that the signals of all channels of the entire system are faulty, but only appears on the channels with bad connectors. As long as the joints are carefully checked one by one, it can be solved.

4. Failure phenomenon caused by mismatch of characteristic impedance of transmission line. The manifestation of this phenomenon is that there are several vertical bars with equal spacing on the screen of the monitor. The frequency of the interference signal is basically an integer multiple of the line frequency. This is caused by the impedance mismatch caused by the characteristic impedance of the video transmission line not being 75Ω. It can also be said that this interference phenomenon is caused by the combination of the characteristic impedance and distribution parameters of the video cable that do not meet the requirements. The method of solution is generally solved by the method of "start-end series resistance" or "terminal-parallel resistance". In addition, it is worth noting that when the video transmission distance is very short (generally within 150 meters), the use of the above-mentioned impedance mismatch and distribution parameters of the video cable is too large will not appear the above-mentioned interference phenomenon. The fundamental solution to the above problem is to ensure quality when purchasing video cables. If necessary, the cable shall be sampled and tested.

5. Space radiation interference introduced by the transmission line. Most of this interference phenomenon occurs because there are strong, high-frequency spatial radiation sources near the transmission system, system front end, or near the central control room. One solution to this situation is to understand the surrounding environment when the system is established, and try to avoid or stay away from the radiation source; the other method is to strengthen the shielding of the front end and central equipment when the radiation source cannot be avoided. The pipeline of the transmission line is made of steel pipe and well grounded.

In fact, all grounds are the same, and grounding is only a matter of means. It depends on what your grounding is used for, so don't mess up the grounding connection problem without conditions, otherwise you will make many serious mistakes. . First of all, from the perspective of general circuit design, the grounding has no potential and the grounding is stable, but it is actually-impossible. In the actual production of the circuit, errors often occur due to dirty grounding. In general, The low-frequency circuit often adopts the single-point grounding method and the high-frequency circuit often adopts the multi-point grounding method, but it must be noted that the high-frequency grounding is mostly a large-area grounding. Why?

First of all, the theory of low-frequency circuit grounding is originally different from the theory of high-frequency grounding. Is it true that the audio circuit has a constant rule, that is, single-point grounding? If you do n’t see it, the low-frequency hum The so-called point is the area where the cross-sectional area is close to zero.The audio circuit, especially the rear stage, often hums due to the lack of single-point grounding.I have heard the loop current, the resistance of the wire, and you can guarantee the metal you If zero impedance is impossible, then you must accept the fact that there is a potential difference in grounding, and there is current when there is a potential difference, which is the source of hum, and it is actually a current area with a very small flow range, but there is one here. The phenomenon is that your large-area grounding is on the chassis. If you use a single-point grounding output ground, the current will less affect the input grounding, and the current hum can be eliminated from the chassis because of the grounding current. But this is just a means.

There are many grounding methods, and the single point is just one of them. The low-frequency circuit uses wires to connect each unit circuit to the chassis. There will be no large loop, no large loop current, no large loop Current input and output are safe, but what about high-frequency circuits?

The grounding theory of high-frequency circuits is deeply affected by the skin effect. What is the skin effect? ​​The skin effect refers to the situation that the current distribution of the wire will be denser on the metal surface in a high-frequency operating environment, which means that your wire will Unlike a wire, it will become a full reactance. The higher the frequency, the more severe the reactance of the wire, so you can use the wire to ground? Don't be stupid, when the antenna is almost the same, do you still want to use wires? If the circuit has many units, you divide it into many circuit blocks, and each unit is connected to its ground with a wire, then you simply take a resistor to connect faster, the resistance is relatively easy to grasp by us, the wire is complicated, so high frequency The grounding of the circuit is often avoided by using wires, the circuit units are each looking for the nearest large-area ground, directly connected by the shortest path, and multiple points are generated,

The high frequency is very particular about the grounding material, but in principle, the area must be large enough and the casing is large enough, but in fact, there must be something to declare here that your connection must be as short as possible, remember It is best to connect as many points as possible to ensure that each point is connected to the same plane to be effective, but it is still an old saying. This is only one of the means. The methods of high-frequency grounding are different for many purposes. This needs to be studied!

This article is good, I also talk about my own views.

1. The choice of cable is very important, so choose some big brands. The cables produced by manufacturers with certain influence in the industry can avoid this problem. At present, many brands of cables are still very good, and they are also used in many monitoring projects. The quality of the SYV-75-5 produced by Aipu, Diyi, Beidian, Tiancheng, etc. is good, and they are very good to use, but there is one thing to note. There are 3 similar SYV-75-5 models below Different models represent 64 turns, 96 turns, and 128 turns respectively. Of course, the larger the number of turns, the better. Of course, the higher the number of turns, the more expensive the price. Currently, 96 turns are commonly used.

2. At present, the most frequent interference is low-frequency signal interference. The frequency of 220V 50HZ rushes into the video signal, which is manifested as rolling double stripes on the screen. The test method is as follows (described earlier, no more details here) There are two solutions The video wiring pipeline requires the use of iron pipes to prevent the low-frequency signals of nearby lights and fans from entering, and also requires a good connection between the iron pipes. In addition, the UPS can be used. The UPS design should take into account the margin. Generally, the place where the monitoring requirements are not high, the UPS power-off time can be about 15 minutes. One purpose is to stabilize the voltage. In the previous project, I encountered the failure of frequent restart of the monitoring host due to the unstable voltage.

3. The grounding is generally completed by decoration. There is no problem with the grounding itself. The key is that sometimes too many devices use the same grounding, causing mutual signal interference. In addition, as mentioned below, although the ground is an equipotential carrier, but We can see that, in fact, this carrier is not strictly equipotential. It inevitably has a certain voltage difference. Although it is very small, it has greatly affected the video signal, so note: the monitoring system may be single Point grounding. In addition, if the single point grounding cannot solve the problem, there is a way to try it. It is unsolvable. As we all know, the interference signal needs to come in and needs a complete loop. Once the ground is unsolved, the interference signal cannot be formed. The loop will not interfere with the monitoring signal (of course, this method of not grounding is generally not used as much as possible. If there is leakage or large current in the system, equipment damage and electric shock accidents are easy to occur)

4. When there is a black and white interference signal, pay attention to the problem of the camera transformer. There are two situations (I have encountered it in the project). First, when the voltage is too low, for example, a transformer has been dragged. Too many cameras cause the transformer to overheat, work unstable during the internal period, and cause interference. Second, there is a problem with the filter of the transformer. It should be noted that the problem with the filter must be detected using an oscilloscope. The filter problem cannot be checked with an ordinary multimeter. , The above two solutions are to use a good transformer to connect separately, you can judge the problem.

5. If the above method can not solve the problem, there is currently a video isolator on the market. Its working principle is to modulate the low-frequency signal at the front end to high frequency, and then to the low frequency at the back end. This method is generally It is suitable for places where there is unresolved interference or there are strong interference sources at the front end, such as substations and other large-scale electrical equipment.

The above are the various interference situations that I personally encountered in the construction. Frankly speaking, the difficulty of the monitoring system is largely due to the complexity and diversity of the interference problems. The interference problems in each project seem to be different, so they need to be According to the on-site situation detection, corresponding measures are taken, so what I said is also the problem I have encountered and solved, and there are many kinds of problems that need to be solved in practical applications. If the expression is not clear, please forgive me, welcome Everyone discuss together.