1. Wood grain interference

This kind of interference will not drown the normal image when it is slight, but it will not be able to view the image when it is serious (even destroy the synchronization). The causes of this fault phenomenon are many and complex. There are roughly the following reasons:

(1) The quality of the video transmission line is poor, especially the shielding performance is poor (the shielding network is not a copper network with good quality, or the shielding network is too thin to play a shielding role). At the same time, the line resistance of this kind of video line is too large, resulting in large signal attenuation, which is also the reason for aggravating the fault. In addition, the characteristic impedance of this kind of video line is not 75 Ω and the parameter exceeds the specification is also one of the reasons for the fault. Since the above interference phenomenon is not necessarily the fault caused by poor video line, the cause of this fault should be judged accurately and carefully. Only when other possibilities are excluded can it be considered from the perspective of poor video line. If it is a cable quality problem, of course, the best way is to replace all such cables with qualified cables, which is the best way to completely solve the problem.

(2) It is caused by the unclean power supply of the power supply system. The "unclean" power supply here refers to the superposition of interference signals on the normal power supply (50 cycle sine wave). Most of the interference signals on this power supply come from the equipment using thyristor in this power grid. In particular, the thyristor equipment with high current and high voltage pollutes the power grid very seriously, which leads to the unclean power supply in the same power grid. For example, the power grid has high-power thyristor frequency modulation and speed regulation device, thyristor rectifier device, thyristor AC / DC conversion device, etc., which will pollute the power supply. The solution to this situation is relatively simple. It can be basically solved as long as the whole system is powered by purified power supply or online ups.

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

2. Deep and chaotic large-area moire interference
In serious cases, all the images are damaged, and the image and synchronization signal cannot be formed. This fault is caused by the short circuit and open circuit between the core wire of the video cable and the shielding network. This often occurs on BNC connectors or other types of video connectors. That is, when this fault phenomenon occurs, it often does not happen to all signals of the whole system, but only to the number of channels with poor connectors. It can be solved by carefully checking these connectors one by one.

3. Interference of several vertical bars with equal spacing
The frequency of the interference signal is basically an integral multiple of the line frequency, which is caused by impedance mismatch because the characteristic impedance of the video transmission line is not 75 Ω. It can also be said that this interference phenomenon is caused by the fact that the characteristic impedance and distribution parameters of the video cable do not meet the requirements. The solution generally depends on the method of "series resistance at the beginning" or "parallel resistance at the end". In addition, it should be noted that when the video transmission distance is very short (generally within 150m), the above interference phenomenon may not occur when using the video cable with the above impedance mismatch and too large distribution parameters. The fundamental way to solve the above problems is to ensure the quality when purchasing video cables. If necessary, the cables shall be sampled and tested.