[ Huaqiang Security Network News ] Video surveillance systems are used in a wider and wider range of applications, causing different interferences in different environments and different installations. These interferences will enter the CCTV monitoring system through the transmission cable, resulting in video image quality degradation, system control failure, unstable operation and other phenomena, which directly affect the quality of the entire system. Therefore, understanding the interference of the video surveillance system helps to take corresponding measures according to different situations, which is very beneficial to improve the quality of the monitoring system and ensure the stable operation of the system.
1, wood grain interference This kind of interference appears, it will not flood the normal image when it is slight, but the image cannot be viewed (or even destroy the synchronization) when it is serious. The causes of such failures are more complicated and complicated. There are roughly the following reasons:
(1) The quality of the video transmission line is not good, especially the shielding performance is poor (the shielding net is not a good quality copper wire mesh, or the shielding mesh is too thin to prevent shielding). At the same time, the line resistance of such video lines is too large, which causes a large attenuation of the signal, which is also a cause of aggravation. In addition, the characteristic impedance of such video lines is not 75Ω and the parameter is out of specification is also one of the causes of failure. Since the above-mentioned interference phenomenon is not necessarily a failure caused by a bad video line, the cause of such a failure is accurate and prudent in judging. Only when the other possibilities are excluded can we consider from the perspective of the video line. If it is really a cable quality problem, the best way is to replace all of this cable and replace it with a cable that meets the requirements. This is the best way to solve the problem completely.
(2) Due to the fact that the power supply of the power supply system is not "clean". The power source referred to here is not "clean", which means that an interference signal is superimposed on a normal power source (a 50-sine sine wave). The interference signals on such power sources are mostly from devices using thyristors in the power grid. Especially for high-current, high-voltage thyristor equipment, the pollution of the power grid is very serious, which leads to the power supply in the same power grid is not "clean." For example, there are high-power thyristor frequency modulation speed regulation devices, thyristor rectifier devices, thyristor AC/DC converters, etc. in the 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 a clean power supply or an online UPS.
(3) There is a strong source of interference 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. When the deep and chaotic large-area reticulation is severe, the image is completely destroyed, and the image and the synchronization signal are not formed. This failure is caused by the short circuit and open circuit of the core wire of the video cable and the shielding net. This is often the case with BNC connectors or other types of video connectors. That is to say, when such a fault phenomenon occurs, it is often not a problem of the signals of the entire system, but only the number of paths in which the joints are not good. As long as you carefully check these connectors one by one, you can solve them.
3. The frequency of several equal-distance vertical bars interfering with the interference signal is basically an integer multiple of the horizontal frequency, which is caused by the impedance mismatch caused by the characteristic impedance of the video transmission line being 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 are not in compliance with the requirements. The solution is generally solved by the method of “starting series resistor†or “terminal paralleling resistorâ€. In addition, it is worth noting that when the video transmission distance is short (usually within 150 meters), the above-mentioned interference phenomenon may not occur with the video cable having the above-mentioned impedance mismatch and excessive distribution parameters. The fundamental solution to the above problem is to ensure quality when purchasing video cables. The cable should be sampled if necessary.
4. Space radiation interference introduced by the transmission line This interference phenomenon is mostly caused by a strong, high-frequency spatial radiation source in the vicinity of the transmission system, the system front end or the central control room. One solution to this situation is to understand the surrounding environment when the system is established, try to avoid or keep away from the radiation source; another method is to strengthen the shielding of the front end and the center equipment when the radiation source cannot be avoided. The pipeline of the transmission line is made of steel pipe and well grounded.
How can surveillance cameras improve anti-jamming capability? The most common transmission method used by surveillance cameras is video baseband transmission (baseband transmission refers to the way to directly transmit TV signals without any processing such as frequency conversion). The advantage of this transmission mode is that the transmission system has a simple structure, and within a certain range, the distortion is small, the noise is low (the system signal to noise ratio is high); the disadvantage is that the transmission distance cannot be too far, and must be transmitted within the range required by the cable characteristics, and A video coaxial cable can only transmit one TV signal at a time.
Because of the high stability of the transmission method, the equipment used in the system is simple, and the wiring is convenient, it has been widely used in real life. However, the use of high bandwidth and low frequency of the cable causes the amplitude and phase of the signal to be transmitted in the cable to be very different between the low frequency band and the high frequency band, especially when the phase distortion is too large. The circuit is compensated. And the low frequency part of the baseband transmission is easily interfered by interference sources such as strong electricity, transmission towers, base stations, motors, and inverters. For example, common sources of interference in engineering:
1. Broadcast interference:
When the cable is erected in the air, the cable itself is equivalent to a long antenna. As a result of the antenna effect, a considerable amount of broadcast interference voltage is generated in the cable, and an interference current is generated on the cable sheath. This current forms a loop through the grounding point at both ends of the cable and the ground, so that broadcast interference occurs at the terminal load. The voltage of the signal causes the interfering signal to mix into the video signal. This kind of interference signal appears as dense horizontal stripes, vertical stripes, twill, etc. on the image, and even floods the entire video image in severe cases.
2, high frequency interference:
The cable shield has a poorer shielding effect on signals with lower frequencies. The high-frequency interference signals introduced for this reason include carrier phones, radio signals, and the like. They cause horizontal streaks on the image.
3. Power interference:
When the system needs to be grounded at the same time as the beginning and end, due to the different ground potentials at both ends and the presence of the cable sheath resistance, a ground potential difference of 50 Hz is caused between the two places, thereby generating an interference signal voltage. When the interference signal is superimposed on the video signal, a wide horizontal dark band or the like appears on the normal image.
4, harmonic interference:
Harmonic interference is mainly manifested around a high-current or high-voltage power line. It is a radiation signal to the surrounding of the power cable. Its frequency is 2500 Hz and 125000 Hz, which mainly interferes with the low frequency band of the video signal.
5. Transmission line interference:
The quality of the video cable is not good, the shielding performance is poor (the shielding layer is sparse or the non-copper dielectric shielding layer, etc.), the cable resistance is too large, and the video signal is seriously attenuated.
6, unclean power interference:
For example, the power grid has high-power thyristor frequency modulation speed regulation device, thyristor rectifier device, thyristor AC/DC converter device, etc., which will pollute the power supply. Unclean power sources can make cameras and other active devices unstable, creating interference.
The above parts make the video images on the scene subject to strong interference. We use the SHWIT brand of video anti-jamming devices to solve the above various interferences. The specific solution is as follows:
1. Frequency shift: The frequency signal is used to shift the video signal (0-6MHZ) to a range of 49-300MHZ; the video signal is transmitted remotely through the anti-jamming transmitter, and the signal is restored to the normal video at the receiving end. Signal output. Due to the increased frequency, a variety of interfering signals are effectively avoided while transmitting over long distances.
2. Encoding: After the video signal enters the anti-jamming device, it is digitally encoded and processed before reaching the monitoring center device. Thereby, the interference of the interference source to the video signal and the crosstalk and attenuation of the video signal itself are avoided.
3. Enhancement: Since the signal transmission distance is larger, the signal attenuation is larger, so we must strengthen the signal carrier capability at the transmitting end so that it can transmit farther.
4. Adaptive: The device automatically adjusts and adapts the signal strength. Therefore, the device has the ability to automatically adapt to the transmission from 0 to its longest distance.
5, free of debugging: a good device should not have a lot of debugging parts, so our equipment has 90% market adaptability at the factory, basically can be used on the connection, no need for redundant debugging.
6. Safety: The design concept of wide voltage (12V-24V) avoids the sudden increase or sudden drop of the power supply voltage to protect the equipment and the safety of the operator.
7. Stability: The long-term aging and test test before the factory leaves the factory, as well as the statistics of the monitoring results of the project, this product can achieve continuous use around the clock, and can meet the long-term stable work requirements.
(1) The quality of the video transmission line is not good, especially the shielding performance is poor (the shielding net is not a good quality copper wire mesh, or the shielding mesh is too thin to prevent shielding). At the same time, the line resistance of such video lines is too large, which causes a large attenuation of the signal, which is also a cause of aggravation. In addition, the characteristic impedance of such video lines is not 75Ω and the parameter is out of specification is also one of the causes of failure. Since the above-mentioned interference phenomenon is not necessarily a failure caused by a bad video line, the cause of such a failure is accurate and prudent in judging. Only when the other possibilities are excluded can we consider from the perspective of the video line. If it is really a cable quality problem, the best way is to replace all of this cable and replace it with a cable that meets the requirements. This is the best way to solve the problem completely.
(2) Due to the fact that the power supply of the power supply system is not "clean". The power source referred to here is not "clean", which means that an interference signal is superimposed on a normal power source (a 50-sine sine wave). The interference signals on such power sources are mostly from devices using thyristors in the power grid. Especially for high-current, high-voltage thyristor equipment, the pollution of the power grid is very serious, which leads to the power supply in the same power grid is not "clean." For example, there are high-power thyristor frequency modulation speed regulation devices, thyristor rectifier devices, thyristor AC/DC converters, etc. in the 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 a clean power supply or an online UPS.
(3) There is a strong source of interference 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. When the deep and chaotic large-area reticulation is severe, the image is completely destroyed, and the image and the synchronization signal are not formed. This failure is caused by the short circuit and open circuit of the core wire of the video cable and the shielding net. This is often the case with BNC connectors or other types of video connectors. That is to say, when such a fault phenomenon occurs, it is often not a problem of the signals of the entire system, but only the number of paths in which the joints are not good. As long as you carefully check these connectors one by one, you can solve them.
3. The frequency of several equal-distance vertical bars interfering with the interference signal is basically an integer multiple of the horizontal frequency, which is caused by the impedance mismatch caused by the characteristic impedance of the video transmission line being 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 are not in compliance with the requirements. The solution is generally solved by the method of “starting series resistor†or “terminal paralleling resistorâ€. In addition, it is worth noting that when the video transmission distance is short (usually within 150 meters), the above-mentioned interference phenomenon may not occur with the video cable having the above-mentioned impedance mismatch and excessive distribution parameters. The fundamental solution to the above problem is to ensure quality when purchasing video cables. The cable should be sampled if necessary.
4. Space radiation interference introduced by the transmission line This interference phenomenon is mostly caused by a strong, high-frequency spatial radiation source in the vicinity of the transmission system, the system front end or the central control room. One solution to this situation is to understand the surrounding environment when the system is established, try to avoid or keep away from the radiation source; another method is to strengthen the shielding of the front end and the center equipment when the radiation source cannot be avoided. The pipeline of the transmission line is made of steel pipe and well grounded.
How can surveillance cameras improve anti-jamming capability? The most common transmission method used by surveillance cameras is video baseband transmission (baseband transmission refers to the way to directly transmit TV signals without any processing such as frequency conversion). The advantage of this transmission mode is that the transmission system has a simple structure, and within a certain range, the distortion is small, the noise is low (the system signal to noise ratio is high); the disadvantage is that the transmission distance cannot be too far, and must be transmitted within the range required by the cable characteristics, and A video coaxial cable can only transmit one TV signal at a time.
Because of the high stability of the transmission method, the equipment used in the system is simple, and the wiring is convenient, it has been widely used in real life. However, the use of high bandwidth and low frequency of the cable causes the amplitude and phase of the signal to be transmitted in the cable to be very different between the low frequency band and the high frequency band, especially when the phase distortion is too large. The circuit is compensated. And the low frequency part of the baseband transmission is easily interfered by interference sources such as strong electricity, transmission towers, base stations, motors, and inverters. For example, common sources of interference in engineering:
1. Broadcast interference:
When the cable is erected in the air, the cable itself is equivalent to a long antenna. As a result of the antenna effect, a considerable amount of broadcast interference voltage is generated in the cable, and an interference current is generated on the cable sheath. This current forms a loop through the grounding point at both ends of the cable and the ground, so that broadcast interference occurs at the terminal load. The voltage of the signal causes the interfering signal to mix into the video signal. This kind of interference signal appears as dense horizontal stripes, vertical stripes, twill, etc. on the image, and even floods the entire video image in severe cases.
2, high frequency interference:
The cable shield has a poorer shielding effect on signals with lower frequencies. The high-frequency interference signals introduced for this reason include carrier phones, radio signals, and the like. They cause horizontal streaks on the image.
3. Power interference:
When the system needs to be grounded at the same time as the beginning and end, due to the different ground potentials at both ends and the presence of the cable sheath resistance, a ground potential difference of 50 Hz is caused between the two places, thereby generating an interference signal voltage. When the interference signal is superimposed on the video signal, a wide horizontal dark band or the like appears on the normal image.
4, harmonic interference:
Harmonic interference is mainly manifested around a high-current or high-voltage power line. It is a radiation signal to the surrounding of the power cable. Its frequency is 2500 Hz and 125000 Hz, which mainly interferes with the low frequency band of the video signal.
5. Transmission line interference:
The quality of the video cable is not good, the shielding performance is poor (the shielding layer is sparse or the non-copper dielectric shielding layer, etc.), the cable resistance is too large, and the video signal is seriously attenuated.
6, unclean power interference:
For example, the power grid has high-power thyristor frequency modulation speed regulation device, thyristor rectifier device, thyristor AC/DC converter device, etc., which will pollute the power supply. Unclean power sources can make cameras and other active devices unstable, creating interference.
The above parts make the video images on the scene subject to strong interference. We use the SHWIT brand of video anti-jamming devices to solve the above various interferences. The specific solution is as follows:
1. Frequency shift: The frequency signal is used to shift the video signal (0-6MHZ) to a range of 49-300MHZ; the video signal is transmitted remotely through the anti-jamming transmitter, and the signal is restored to the normal video at the receiving end. Signal output. Due to the increased frequency, a variety of interfering signals are effectively avoided while transmitting over long distances.
2. Encoding: After the video signal enters the anti-jamming device, it is digitally encoded and processed before reaching the monitoring center device. Thereby, the interference of the interference source to the video signal and the crosstalk and attenuation of the video signal itself are avoided.
3. Enhancement: Since the signal transmission distance is larger, the signal attenuation is larger, so we must strengthen the signal carrier capability at the transmitting end so that it can transmit farther.
4. Adaptive: The device automatically adjusts and adapts the signal strength. Therefore, the device has the ability to automatically adapt to the transmission from 0 to its longest distance.
5, free of debugging: a good device should not have a lot of debugging parts, so our equipment has 90% market adaptability at the factory, basically can be used on the connection, no need for redundant debugging.
6. Safety: The design concept of wide voltage (12V-24V) avoids the sudden increase or sudden drop of the power supply voltage to protect the equipment and the safety of the operator.
7. Stability: The long-term aging and test test before the factory leaves the factory, as well as the statistics of the monitoring results of the project, this product can achieve continuous use around the clock, and can meet the long-term stable work requirements.
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