There are many kinds of interference in monitoring, one is the spatial radiation interference introduced by the transmission line. This interference phenomenon is mostly caused by strong, high-frequency spatial radiation sources 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 pipeline of the transmission line adopts steel pipe and is well grounded.
In addition to the grounding work of the front-end and back-end equipment and equipment-to-ground noise isolation, the grounding work can also use steel pipes for the pipelines of the transmission lines and be well grounded and grounded at the pipe joints. However, the grounding of noise and interference can not be completely unlawful, but must be carried out step by step in order to achieve the elimination effect. First, the general circuit grounding is no potential, the grounding is stable, but in reality it will have potential, so we must pay attention to the grounding area size and resistance.
How to eliminate noise interference in the monitoring system
Single-point grounding: Single-point grounding is a method of providing a common potential reference point for many circuits together so that signals can be transmitted between different circuits. Single point grounding requires that each circuit be grounded only once and connected at the same point. Since there is only one reference point, it is believed that there is no loop to ground and therefore there is no interference problem;
Multi-point grounding: The internal circuit of the monitoring equipment is based on the chassis ground reference point, and the chassis of each device is also referenced to the ground. This kind of grounding structure can provide a low grounding impedance, because when grounding multiple points, each grounding wire can be very short; and connecting multiple wires in parallel can reduce the total inductance of the grounding conductor;
Hybrid grounding: Hybrid grounding includes both the characteristics of single-point grounding and the characteristics of multiple-point grounding. For example, the power supply in the system needs a single point of grounding, and the RF signal requires multiple points of ground. For DC, the capacitor is open circuit. The circuit is single-point grounded. For RF, the capacitor is conductive and the circuit is multi-point grounded.
When many interconnected devices are large (the physical dimensions of the device and the connecting cables are large compared to the wavelengths of any interfering signals present), there is the possibility of interference through the chassis and cables. When this happens, the path of the interference current usually exists in the ground loop of the system. Therefore grounding must pay attention to the following:
Safety grounding: The equipment that uses AC power must be grounded through the yellow-green safety ground wire. Otherwise, when the insulation resistance between the power supply and the chassis in the equipment is small, it will cause electric shock damage.
Lightning protection grounding: The facility's lightning protection system is an independent system consisting of a lightning rod, a lower conductor, and a connection to a grounding system.
Electromagnetic Compatibility Grounding: Grounding required for electromagnetic compatibility design, including isolation grounding to prevent mutual interference between circuits due to the presence of parasitic capacitances, must be necessary to ground isolation;
Filter Grounding: Filters typically include bypass capacitors for the signal line or power line to ground. When the filter is not grounded, these capacitors are in a floating state and do not function as a bypass.
Inventory of the most common interferences and solutions
1, the most common: wood grain interference in our use, wood grain interference is probably one of the most common interference. More than just a monitor, TVs in some old-style non-wired video transmissions are also disturbed by these lines. Under light conditions, we can affect our visual fatigue, and even affect our normal viewing. Therefore, this is often regarded as our number one interference situation.
There are many reasons for the disturbance presented this week, and the most common one is the conspiracy of video transmission cables. Because in the construction of the monitoring system, the cables are often the simplest part we ignore, and thus the quality of the cables is simply different. When the quality of the copper wire inside the cable is not high, and the outer screen may be too thin, it will have a very significant impact on the video display of the terminal. This is mainly due to the fact that the quality of the cable is not the same, causing the resistance of the entire line to be too large and the end of the cable to affect the quality of the signal transmission. Regarding the disposal of this line interference, the exchange of cables is probably the only method.
However, because there are many reasons for similar disturbances, the determination of defects is undoubtedly the most important task, and it is advocated by experienced people to end and prevent unnecessary mistakes.
In fact, in addition to the circuit difficulties, wood grain interference is sometimes formed due to the strong current interruption in the vicinity of the transmission line. This is mainly because during the transmission of the line, there is a superposition of normal power supply interference, perhaps constituted by the thyristor equipment in its own power grid, and these will also have a very large impact on the transmission of the line. Of course, the treatment of such interference is significantly simpler than the above method. Only in the entire system, we can use the method of purifying the power supply or online power supply, and we can pass this small line pattern out.
In addition, the passage of strong currents in the surrounding area will also affect our circuit. With regard to this problem, we have chosen to strengthen the shielding of the front-end equipment, and perhaps to ground the video cable.
2, the largest area: moire In addition to our most common wood grain interference, there is also a kind of interference will occasionally affect our viewing vision, that is, interference patterns. In the common disturbance appearances, the area of ​​influence of such interference can be regarded as one of the largest. However, despite its relatively large area, it has not been seen as "dominant" by its appearance.
The appearance of this problem is usually due to the short circuit of the transmission cable core of the video cable and the surrounding protection network, or the appearance of a broken circuit. Therefore, such problems are often presented mostly in BNC connectors or in other types of connectors. And when we deal with similar problems, we only need to look at the corresponding connectors one by one, and exchange difficult equipment, and we can easily settle the confusion of such interference.
3, the most regular: vertical bar interference This type of interference, although it is not very common, but its characteristics are very significant. The main cause of this interference is still because the quality of the cable is not in conformity with the specification, and the end results in abnormal characteristics of the associated data in the transmission process.
Therefore, in dealing with the onset of such interference, most of us will use "start resistance series" or "terminal resistance resistors" approach to disposal. Of course, the best way to prevent this kind of interference is still to carry out the necessary product inspections when we are in charge of the circuit, to find out the difficulties ahead of time, and to prevent a rework mess after the completion of the project.
These three types of interference basically cover one of the most common types of interference in video surveillance. From the above analysis, it is not difficult to find that there is a corresponding reason is due to equipment or construction of the embarrassment. Therefore, this also reminds us that in reality, interference is not a big problem. We only need to repeat and thoroughly test the equipment when constructing parts or accessories. We must master the quality of the products at the source and use them at the end. In the left to prevent the trouble.
In fact, monitoring the interference of the display device is not a waste of effort, there are many times just a little simple treatment can make our monitor "full of blood", understand the above common interference, you are also worried about our Is video surveillance unstable?
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