According to different interference sources, the generation of interference signals can be divided into the following three types:
First, the cause of the interference signal
According to different interference sources, the generation of interference signals can be divided into the following three types:
Interference caused by the front-end equipment: interference from the power supply of the front-end camera, interference caused by the quality of the camera itself, the judgment method is to directly observe the monitor at the front end. If the interference caused by the power supply can be replaced by the power supply, the power supply is switched on. 220V AC circuit plus AC filter and other solutions;
Terminal equipment interference: mainly the power supply of the monitoring room, the interference generated by the equipment itself, the interference caused by the grounding, the interference caused by the connection of the equipment and the equipment, etc. The simple judgment method is directly connected to the camera in the monitoring room;
Interference in the transmission process: mainly caused by interference caused by transmission cable damage, electromagnetic radiation interference and ground interference (ground potential difference), etc. For the transmission cable, it can be solved by replacing the cable or adding anti-interference equipment.
Second, the formation and solution of electromagnetic radiation interference and ground potential difference interference during signal transmission
1. Causes and methods of elimination of electromagnetic radiation interference
1.1 Transmission line eliminates the principle of external electromagnetic interference
Coaxial cable is shielded against electromagnetic interference. The coaxial cable is composed of an outer conductor and an inner conductor, and an insulating material is used as a filler between the inner and outer conductors. The outer conductor is usually a wire woven from copper wire, which has a good shielding effect on external electromagnetic interference. The inner conductor is under the tight protection of the outer conductor, so the coaxial cable has good anti-interference ability.
1.2 Interference and elimination of strong electromagnetic radiation on the line
As described above, the transmission line has the ability to withstand external electromagnetic interference, and can effectively transmit signals. However, when the interference source is too strong, it will interfere with the image signal. There are two main types of these strong electromagnetic interferences:
Strong electromagnetic radiation source nearby;
Improper line design, interference from strong lines to the transmission line.
Strong electromagnetic radiation sources are generated by high-power stations or electrical equipment with electromagnetic radiation. The interference caused by strong electromagnetic radiation on the image is mesh ripple interference. For such interference, the following methods can be used to eliminate interference:
Avoid interference sources as much as possible, and system equipment and circuits must be separated from the radiation source by a certain distance;
Choose a cable with good shielding performance. The braiding density of the outer shielding net of the coaxial cable directly affects the anti-interference performance of the cable, and the greater the weaving density, the stronger the anti-interference ability;
Increase anti-jamming equipment.
The main cause of the interference caused by the wiring is that the transmission cable and the high-voltage line are long-distance and nearly parallel to each other, and electromagnetic coupling is generated. The anti-interference ability of the coaxial cable is lower in the low frequency band, and the strong electrical interference component is mainly 50 Hz alternating current and its harmonics, so the threat to the coaxial cable is large. Therefore, it is necessary to avoid the short-distance parallel wiring of the signal line and the strong electric line. The strong electric line and the signal transmission line should be laid in the dividing line slot, and the distance between the line slots should be kept; of course, the short distance between the transmission cable and the strong electric line will not cause large interference. In both ends of the system and in the equipment cabinet, it is inevitable that short-distance parallel wiring of the strong electric line and the signal line will occur, which will not cause large interference.
2. Causes and methods of elimination of ground potential difference interference
Ground potential difference interference is a frequent interference of the system. The reason for the ground potential difference interference is that there are two or more conflicting grounds in the system. There is a certain voltage difference between the ground and the ground. The voltage is outside the signal cable. The shield mesh forms an interference current that creates interference with the image. The main component of the ground current is the 50 Hz AC and the interference pulse generated by the electrical equipment. The image is characterized by horizontal black stripes, distortion, and cumbersome horizontal clutter, and it is possible to move slowly in the vertical direction.
Solution:
Isolate the front-end equipment from the ground, but avoid the risk of possible lightning strikes or electric shocks;
Use anti-jamming devices with isolation, such as video isolator.
Third, the principle of common anti-interference and the solution of interference problems
1, the principle of video amplification
The video signal outputted by the camera is directly amplified, and then compressed at the terminal. At the same time as compressing the video signal, the interference signal is also compressed to make the video signal reach an acceptable level; from the above principle, the front end video signal can be seen. The higher the amplification, the better the anti-interference effect; the anti-interference using this principle has a good effect on the electromagnetic field interference in the transmission, but if the interference is relatively heavy, the residual of the interference signal is relatively large, and the user may not be acceptable.
2, frequency shift principle
The frequency of the video signal is 0-6 MHz. When the interference frequency falls within the frequency range, interference will occur in the image. The principle of frequency shift is to directly convert the video signal output from the camera to the high frequency. Avoiding the interference frequency and then converting it to a standard video signal of 0-6 MHz at the receiving end. In fact, this principle is the modulation and demodulation principle of cable TV. The anti-interference can effectively resist various interferences, no residue, due to The method of front and rear isolation can effectively resist the interference of ground potential difference; the anti-interference effect depends on the quality of the equipment (how to restore the signal in modulation and demodulation), and whether the interference frequency falls after modulation. Within the frequency range.
3, anti-interference selection
The interference situation in the system is very complicated, and the interference phenomenon is also varied. Generally speaking, if there is such interference as moire, the following two types of anti-interference can be used to solve the problem.
1. For the video line is fine, the transmission distance is far caused by the moire interference (it is generally recommended that the 75-3 video cable transmission distance does not exceed 300 meters, the 75-5 video cable transmission distance does not exceed 500 meters), you can choose the amplification principle. Anti-interference, can not only anti-interference and improve image quality; of course, the frequency shift principle can also be used, but you need to consult the manufacturer to see if you can meet your transmission distance requirements.
2, due to the interference caused by the damage of the video cable, if the interference is snow or moire interference, the replacement of the cable is the best way, if it can not be replaced, you can choose the anti-interference of the amplification principle, if the image distortion occurs in the interference phenomenon, The interference bar scrolls up and down, and the frequency shift principle can be selected to resist interference.
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