[ Huaqiang Security Network News ] In the closed-circuit monitoring system equipment that is widely used in the society, the interference of electronic equipment is increasing. The reasonable wiring, shielding technology application, good grounding, and signal SPD use are all better anti-electromagnetic. The method of interference. However, only a combination of electromagnetic surge protection methods can fundamentally solve the electromagnetic interference problem. 
From the analysis of the interference source, it is understood that there is no special interference source, and there are many theoretical discussions to eliminate or reduce the above interference. How to solve the interference problem for the CCTV monitoring project. The following is a discussion of common interference and solutions in CCTV surveillance.
1. The interference of the frequency signal is represented by the ground point and the 50Hz horizontal scroll on the image. The interference of the snowflake point is caused by the signal attenuation on the transmission line and the coupling of the high frequency interference. This interference is easier to eliminate, in the camera and control. Adding a video amplifier at a reasonable position between the matrices increases the signal-to-noise ratio of the signal, or changes the path of the video cable to avoid high-frequency interference sources, and the problem of high-frequency interference can be basically solved.
2. It is difficult to solve the situation of 50Hz horizontal grain rolling and further high frequency interference. In order to suppress the above interference, first analyze the cause of the above problem. There are three types of power supply required by the camera: DC 12V, AC 24V or 220V. In most engineering applications, the power supply is additionally powered by the power supply. The camera output image is sent from the pipeline via a soft video cable. The cable and the power supply cable are bundled with other power lines. When other powers are running, the electromagnetic field generated by the traction motor runs along the illumination power line, which obviously affects the camera power supply cable and the video cable. When the shielding layer of the video cable is not tight enough, High frequency interference is transmitted back to the monitor via the video cable. For 50Hz horizontal scrolling, it is known from the electromagnetic theory that the shielding layer of the video cable can completely eliminate 50Hz power frequency interference. It can be inferred that this part of the interference is not coupled through the video cable, but from the power line and the unreasonable video line connection.
3. For high-frequency interference in the image, because its frequency band is still within 8MHz, the shielding net with a void ratio of about 50% can basically fire high-frequency interference, but to achieve a 50% void ratio, the number of shielded nets needs to be There are more than 60 wavelength lengths, and this high density will reduce the flexibility of the cable. It is recommended to install the corresponding signal SPD (SC series signal lightning protector) at the port of the device.
4, the video cable shielding layer is grounded, if the video signal "ground" and the display "ground" relative to the "grid ground" potential is different, that is, the two grounding points are different from the grid "ground" voltage difference, then through the power supply A power supply loop is formed between the camera and the display, so that 50 Hz power frequency interference enters the display. From its electrical connection, it can be seen that there are two ways to eliminate 50 Hz power frequency interference. One is to find ways to make the "ground" potential everywhere. The potential difference of the "grid ground" is exactly the same, or the path forming the ground circulation is cut off. Due to the complicated engineering environment, it is difficult to completely equipotentially “ground†everywhere. Only by increasing the wire diameter of the camera power supply cable, the resistance of the ground circuit can be reduced as much as possible. Or the method of cutting off the ground loop circuit, one end of the camera or the display end is not grounded, usually the ground of the power supply is not connected to the display end, so although the interference can not be completely eliminated, the interference of 50 Hz can be greatly reduced. From the above analysis, if the high-frequency noise is coupled to the power line, even if the shielded cable of the video cable is shielded, the noise will be sent to the display, so the power supply line of the camera should also be shielded. If the interference is found during system debugging, the noise can be filtered by modulation and demodulation. A modulator is placed on the camera to move the video signal to a frequency segment of several megahertz, and a low-pass filter is set on the display. The signal is filtered out of the signal below 8NHZ, and then the video image is restored by demodulation.
5. Monitoring system power supply mode The monitoring system has only two power supply modes: one is centralized power supply mode, that is, the power supply is taken from one place, and the other is distributed power supply. The camera takes power near the installation position, and the anti-interference effect From the point of view, the centralized power supply method is better, and the situation that the reference potentials are not equal can be basically eliminated.
1. The interference of the frequency signal is represented by the ground point and the 50Hz horizontal scroll on the image. The interference of the snowflake point is caused by the signal attenuation on the transmission line and the coupling of the high frequency interference. This interference is easier to eliminate, in the camera and control. Adding a video amplifier at a reasonable position between the matrices increases the signal-to-noise ratio of the signal, or changes the path of the video cable to avoid high-frequency interference sources, and the problem of high-frequency interference can be basically solved.
2. It is difficult to solve the situation of 50Hz horizontal grain rolling and further high frequency interference. In order to suppress the above interference, first analyze the cause of the above problem. There are three types of power supply required by the camera: DC 12V, AC 24V or 220V. In most engineering applications, the power supply is additionally powered by the power supply. The camera output image is sent from the pipeline via a soft video cable. The cable and the power supply cable are bundled with other power lines. When other powers are running, the electromagnetic field generated by the traction motor runs along the illumination power line, which obviously affects the camera power supply cable and the video cable. When the shielding layer of the video cable is not tight enough, High frequency interference is transmitted back to the monitor via the video cable. For 50Hz horizontal scrolling, it is known from the electromagnetic theory that the shielding layer of the video cable can completely eliminate 50Hz power frequency interference. It can be inferred that this part of the interference is not coupled through the video cable, but from the power line and the unreasonable video line connection.
3. For high-frequency interference in the image, because its frequency band is still within 8MHz, the shielding net with a void ratio of about 50% can basically fire high-frequency interference, but to achieve a 50% void ratio, the number of shielded nets needs to be There are more than 60 wavelength lengths, and this high density will reduce the flexibility of the cable. It is recommended to install the corresponding signal SPD (SC series signal lightning protector) at the port of the device.
4, the video cable shielding layer is grounded, if the video signal "ground" and the display "ground" relative to the "grid ground" potential is different, that is, the two grounding points are different from the grid "ground" voltage difference, then through the power supply A power supply loop is formed between the camera and the display, so that 50 Hz power frequency interference enters the display. From its electrical connection, it can be seen that there are two ways to eliminate 50 Hz power frequency interference. One is to find ways to make the "ground" potential everywhere. The potential difference of the "grid ground" is exactly the same, or the path forming the ground circulation is cut off. Due to the complicated engineering environment, it is difficult to completely equipotentially “ground†everywhere. Only by increasing the wire diameter of the camera power supply cable, the resistance of the ground circuit can be reduced as much as possible. Or the method of cutting off the ground loop circuit, one end of the camera or the display end is not grounded, usually the ground of the power supply is not connected to the display end, so although the interference can not be completely eliminated, the interference of 50 Hz can be greatly reduced. From the above analysis, if the high-frequency noise is coupled to the power line, even if the shielded cable of the video cable is shielded, the noise will be sent to the display, so the power supply line of the camera should also be shielded. If the interference is found during system debugging, the noise can be filtered by modulation and demodulation. A modulator is placed on the camera to move the video signal to a frequency segment of several megahertz, and a low-pass filter is set on the display. The signal is filtered out of the signal below 8NHZ, and then the video image is restored by demodulation.
5. Monitoring system power supply mode The monitoring system has only two power supply modes: one is centralized power supply mode, that is, the power supply is taken from one place, and the other is distributed power supply. The camera takes power near the installation position, and the anti-interference effect From the point of view, the centralized power supply method is better, and the situation that the reference potentials are not equal can be basically eliminated.
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