1. In video transmission, the most common failure phenomenon is that a black or white bar appears on the screen of the monitor, and slowly scrolls up or down.
When analyzing this kind of failure phenomenon, it is necessary to distinguish between two different causes of the failure. To distinguish between a power supply problem and a ground loop problem, a simple method is to connect the output signal of a camera with no problem to the nearest power source on the control host. If the above-mentioned interference phenomenon does not appear on the monitor , It means that there is no problem with the control host. Next, a portable monitor can be connected to the video output of the front-end camera, and each camera can be checked one by one. If so, proceed. If not, the interference is caused by other reasons such as the ground loop.
2. Wood grain interference appears on the monitor.
The appearance of such interference will not overwhelm the normal image when it is slight, and the image cannot be viewed when it is severe ( or even break the synchronization ) . There are many and complicated reasons for this failure phenomenon. There are roughly the following reasons:
⑴The 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 serve the shielding function ) At the same time, the line resistance of this type of video cable is too large, which causes a large attenuation of the signal and also aggravates the failure. In addition, the characteristic impedance of this type of video cable is not 75Ω and the parameters are beyond the specifications. Because the above-mentioned interference phenomenon is not necessarily a failure caused by a bad video cable, the cause of this failure must be accurate and cautious when judging. Only after excluding other possibilities can we consider it from the perspective of poor video lines. If it is really a cable quality problem, the best way is of course to replace all such cables with cables that meet the requirements. This is the best way to completely solve the problem.
(2) 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- cycle sine wave ) . And most of the interference signals on this power source come from the silicon controlled equipment in this power grid. In particular, high-current, high-voltage thyristor equipment has a very serious pollution to the power grid, which has caused the power supply in the same power grid to be “ clean ” . For example, there are high-power thyristor frequency and speed regulating devices, thyristor rectifiers, thyristor AC-DC converters, etc. in this power grid, which will pollute the power supply. The solution to this situation is relatively simple, as long as the entire system is powered by purified power or online UPS .
(3) There are strong interference sources near the system. This can be judged through investigation and understanding. If this is the reason, the solution is to strengthen the shielding of the camera and ground the pipeline of the video cable.
3. Failure caused by short circuit or open circuit of the video cable's core wire and the shielding network.
This failure manifests itself in the form of deep and chaotic large-area moire interference on the monitor, so that all the images are destroyed, and images and synchronization signals cannot be formed. This situation often occurs on BNC connectors or other types of video connectors. That is to say, when this kind of failure phenomenon occurs, it is often not that all the signals of the entire system are out of order, but only appear on those that have bad connections. Just check these connectors carefully one by one and you can solve it.
4. Failure phenomenon caused by transmission line characteristic impedance mismatch.
The manifestation of this phenomenon is that a number of vertical bars with equal spacing are generated on the monitor screen, and the frequency of the interference signal is basically an integer multiple of the horizontal frequency. This is due to impedance mismatch caused by the characteristic impedance of the video transmission line is not 75Ω . It can also be said that the occurrence of this interference phenomenon is caused by the combination of the characteristic impedance and distribution parameters of the video cable. The solution usually depends on the method of " starting series resistance " or " terminal parallel resistance " . In addition, it is worth noting that when the video transmission distance is short ( usually within 150 meters ) , using the video cable with the above impedance mismatch and excessive distribution parameters does not necessarily cause the interference phenomenon described above.
5. Space radiation interference introduced by transmission lines.
Most of this interference is caused by strong, high-frequency spatial radiation sources near the transmission system, the front end of the system, 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 center equipment when the radiation source cannot be avoided Pipeline of transmission line uses steel pipe