Today, China Security Network Xiaobian brings you familiar with the signal interference problem of video surveillance. The interference of video signal is represented by the ground point and 50Hz horizontal scroll on the image. The interference of the snowflake point is due to the signal attenuation on the transmission line and the coupling of the high frequency. Due to interference, this interference is easier to eliminate. Add a video amplifier at a reasonable position between the camera and the control matrix to increase the signal-to-noise ratio of the signal, or change the path of the video cable to avoid high-frequency interference sources, high-frequency interference. The problem can be basically solved. More difficult to solve is the 50Hz horizontal scrolling and further high frequency interference, such as the output image of the camera inside the elevator car. In order to suppress the above interference, first analyze the cause of the above problem. There are three kinds of power supply required by the camera: DC 12V, AC 24V or 220V. In most engineering applications, the power supply of the elevator car is not taken. Instead, the power supply is additionally supplied to the camera. The camera output image passes through a soft one. The video cable is sent from the stop or the bottom of the hoistway. The video cable and the power supply cable are bundled with the power line of the car. When the elevator is running, the electromagnetic field generated by the traction motor runs along the illumination power line, which obviously affects the camera power supply cable and 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. The video cable shielding layer is grounded. If the video signal "ground" is different from the "ground" of the display relative to the "grid ground", then a power supply loop is formed between the camera and the display through the power supply, so that 50 Hz power frequency interference enters the display. There are two ways to eliminate 50Hz power frequency interference. One is to find a way to make the potential difference between the "ground" potential and the "grid ground" everywhere, or to cut off the path of the formation of the local circulation. 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.
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For the 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 each wavelength. More than 60 lengths, such high density will reduce the flexibility of the cable. A better method is to use a video cable with double shielding. The interference analysis of video monitoring signals is used in our daily life. Frequently encountered problems.
From the above analysis of the signal interference problem of the surveillance video, if the high-frequency noise is coupled to the power line, even if the shielding of the shielded cable of the video cable is good, the noise will be sent to the display, so the power supply of the camera The line should also be shielded. The above measures need to be fully considered in engineering design and construction. If the interference is found during system debugging, modulation and demodulation methods can be used to filter the noise, and a modulation is set at the camera end. The device moves the video signal to a frequency segment of several tens of megahertz, and a low-pass filter is provided on the display end to filter out all signals below 8 MHz, and then demodulate to restore the video image.
Analysis of interference problems of video surveillance signals
[ Huaqiang Security Network News ]