Due to the cable material, transmission distance, and location and environment factors, the image signal will inevitably have some electromagnetic noise, power harmonics, or interference with the ground noise. It is difficult to avoid, but we still must understand these. Causes of interference sources and solutions.
Coaxial cable is the most widely used video transmission medium and is generally used for transmitting short and medium distance image signals. The electrical characteristics of the coaxial cable make it ideal for transmitting full video signals from the camera to the monitor (CCTV video signals are composed of widely distributed low frequency signals and high frequency signals). Almost any kind of wire can be used when transmitting low-frequency signals (20Hz to several KHz). In practical applications, almost all wires can be used as telephone lines. But to transmit video signals in the frequency range of 20Hz to 6MHz, and you don't want any attenuation, you need to use the coaxial cable.
The use of video baseband (BoardBand) transmission in television monitoring systems is the most common method of transmission. Basic frequency transmission refers to a method of directly transmitting a full television signal without any processing such as frequency conversion. The advantage of this transmission method is that the transmission system is simple; within a certain distance range, the distortion is small; the additional noise is low (the system signal-to-noise ratio is high); additional equipment such as a modulator, a demodulator, etc. need not be added. The disadvantage is that the transmission distance cannot be too far; a video coaxial cable can only transmit one TV signal.
Because this transmission method has the advantages of stable operation and simple equipment, it has been widely used in practice. However, the frequency band of the video signal is very wide, and the start frequency is very low. Therefore, when the cable is transmitted in the cable, its amplitude and phase will be greatly different between the low frequency band and the high frequency band. Especially when the phase distortion is too large, it is difficult to compensate with a simple circuit. At the same time, the low-frequency part of the fundamental frequency transmission is easily disturbed by low-frequency interference sources such as power, telephone, and broadcasting.
1, broadcast interference:
Because of the actual application needs, the cable must be erected in the air. In this case, the cable itself is equivalent to a long antenna (ANT). As a result of the antenna effect, a considerable 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 on the terminal load. The voltage of the signal causes the interfering signal to mix in the video signal. This type of interfering signal appears in the image as a tightly slanted screen, even when the image is overwhelmed. If the cable is buried underground, or using lead-sheathed cable, balanced symmetrical cable, etc. can better overcome this interference.
2, low-frequency signal interference:
The shielding effect of the cable shielding layer is lower for signals with lower frequencies. For this reason, the interference signals that are easily introduced by the cables are probably caused by carrier phone sounds, transmission signals from broadcasting stations, and the like. They are easy to cause horizontal striped fringes on the image.
3, 50/60Hz power interference:
When the system needs to be grounded at the beginning and end of the line at the same time, due to the difference in the grounding potential at both ends and the presence of the cable sheath resistance, a 50- or 60-Hz ground potential difference is caused between the two grounds, and thus an interference signal voltage is generated. When the interference signal is superimposed on the video signal, a wide dark band appears on the normal image. In this case, when there is a serious situation, a potential difference is generated between the two ends of the coaxial cable, and the voltage is accumulated. If the discharge fails through the grounding, the discharge will occur when the engineer contacts the connector, and the discharge will occur when the device is connected to the device. Or the equipment burned out.
4, second harmonic and third harmonic interference:
Harmonic interference mainly in the power line around the high current or high voltage, is the radiation signal to the surrounding of the power cable, its frequency is 2500Hz and 125000Hz, mainly interfere with the low frequency band of the video signal.
Interference caused by the transmission line: The quality of the video transmission line is not good, especially the shielding performance is poor (if the shielding network is not a copper network with good quality, or if the shielding network is too sparsely weaved, it will not play the role of shielding). At the same time, the line resistance of such image lines will be too large, which will cause a large attenuation of the signal, which is also the cause of the increase in failure. At the same time, the characteristic impedance of this type of image line is not 75Ω and the parameter is out of specification as one of the causes of the malfunction. The manifestation of this phenomenon is that there are several vertical stripe interferences with equal spacing on the screen of the monitor. The frequency of the interfering signal is basically an integral multiple of the horizontal frequency. 5. Interference from unclean electric power:
The power source referred to here is not "clean" and means that an interference signal is superimposed on a normal power source (50/60 Hz sine wave). The interference signals on the power supply are mostly from the equipment used in the power supply, especially the control equipments with large current and high voltage, and the pollution to the power is very serious, which leads to the fact that the power supply in the same power source is not “clean†. For example, in this power source, there are high-power adjustable frequency devices, controllable current devices, controllable AC/DC converter devices, etc., all of which will cause pollution to the power supply. Unclean power makes the camera and other active devices unstable, resulting in power harmonic interference.
Coaxial cable is the most widely used video transmission medium and is generally used for transmitting short and medium distance image signals. The electrical characteristics of the coaxial cable make it ideal for transmitting full video signals from the camera to the monitor (CCTV video signals are composed of widely distributed low frequency signals and high frequency signals). Almost any kind of wire can be used when transmitting low-frequency signals (20Hz to several KHz). In practical applications, almost all wires can be used as telephone lines. But to transmit video signals in the frequency range of 20Hz to 6MHz, and you don't want any attenuation, you need to use the coaxial cable.
The use of video baseband (BoardBand) transmission in television monitoring systems is the most common method of transmission. Basic frequency transmission refers to a method of directly transmitting a full television signal without any processing such as frequency conversion. The advantage of this transmission method is that the transmission system is simple; within a certain distance range, the distortion is small; the additional noise is low (the system signal-to-noise ratio is high); additional equipment such as a modulator, a demodulator, etc. need not be added. The disadvantage is that the transmission distance cannot be too far; a video coaxial cable can only transmit one TV signal.
Because this transmission method has the advantages of stable operation and simple equipment, it has been widely used in practice. However, the frequency band of the video signal is very wide, and the start frequency is very low. Therefore, when the cable is transmitted in the cable, its amplitude and phase will be greatly different between the low frequency band and the high frequency band. Especially when the phase distortion is too large, it is difficult to compensate with a simple circuit. At the same time, the low-frequency part of the fundamental frequency transmission is easily disturbed by low-frequency interference sources such as power, telephone, and broadcasting.
1, broadcast interference:
Because of the actual application needs, the cable must be erected in the air. In this case, the cable itself is equivalent to a long antenna (ANT). As a result of the antenna effect, a considerable 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 on the terminal load. The voltage of the signal causes the interfering signal to mix in the video signal. This type of interfering signal appears in the image as a tightly slanted screen, even when the image is overwhelmed. If the cable is buried underground, or using lead-sheathed cable, balanced symmetrical cable, etc. can better overcome this interference.
2, low-frequency signal interference:
The shielding effect of the cable shielding layer is lower for signals with lower frequencies. For this reason, the interference signals that are easily introduced by the cables are probably caused by carrier phone sounds, transmission signals from broadcasting stations, and the like. They are easy to cause horizontal striped fringes on the image.
3, 50/60Hz power interference:
When the system needs to be grounded at the beginning and end of the line at the same time, due to the difference in the grounding potential at both ends and the presence of the cable sheath resistance, a 50- or 60-Hz ground potential difference is caused between the two grounds, and thus an interference signal voltage is generated. When the interference signal is superimposed on the video signal, a wide dark band appears on the normal image. In this case, when there is a serious situation, a potential difference is generated between the two ends of the coaxial cable, and the voltage is accumulated. If the discharge fails through the grounding, the discharge will occur when the engineer contacts the connector, and the discharge will occur when the device is connected to the device. Or the equipment burned out.
4, second harmonic and third harmonic interference:
Harmonic interference mainly in the power line around the high current or high voltage, is the radiation signal to the surrounding of the power cable, its frequency is 2500Hz and 125000Hz, mainly interfere with the low frequency band of the video signal.
Interference caused by the transmission line: The quality of the video transmission line is not good, especially the shielding performance is poor (if the shielding network is not a copper network with good quality, or if the shielding network is too sparsely weaved, it will not play the role of shielding). At the same time, the line resistance of such image lines will be too large, which will cause a large attenuation of the signal, which is also the cause of the increase in failure. At the same time, the characteristic impedance of this type of image line is not 75Ω and the parameter is out of specification as one of the causes of the malfunction. The manifestation of this phenomenon is that there are several vertical stripe interferences with equal spacing on the screen of the monitor. The frequency of the interfering signal is basically an integral multiple of the horizontal frequency. 5. Interference from unclean electric power:
The power source referred to here is not "clean" and means that an interference signal is superimposed on a normal power source (50/60 Hz sine wave). The interference signals on the power supply are mostly from the equipment used in the power supply, especially the control equipments with large current and high voltage, and the pollution to the power is very serious, which leads to the fact that the power supply in the same power source is not “clean†. For example, in this power source, there are high-power adjustable frequency devices, controllable current devices, controllable AC/DC converter devices, etc., all of which will cause pollution to the power supply. Unclean power makes the camera and other active devices unstable, resulting in power harmonic interference.
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