In the conventional security monitoring system, we often find that there is more or less such image interference, which does not meet the acceptance criteria, which brings a lot of trouble for the engineering acceptance work. This paper will elaborate on the outstanding effect of the common cable first-line application on anti-interference from the generation and solution of interference.
In the conventional security monitoring system, we often find that there is more or less such image interference, which does not meet the acceptance criteria, which brings a lot of trouble for the engineering acceptance work. This paper will elaborate on the outstanding effect of the common cable first-line application on anti-interference from the generation and solution of interference.
First, the generation of image interference
There are many causes of interference, but basically it can be attributed to three parts: joints, equipment, and transmission.
The interference generated by the joint basically represents "drawing", "crossing", image distortion, flicker, etc. This type of interference can basically be solved by remaking the BNC head.
When the interference generated by the device, such as a matrix or a distributor, is poorly isolated, image alignment, insufficient signal, etc. may occur, which can basically be solved by replacing the device.
The interference generated during the transmission process is the most difficult to handle. Several common interferences are listed: electromagnetic interference, potential difference interference, AC interference, and cable damage interference.
1, electromagnetic interference
This kind of interference is the most common interference in video surveillance. The video line passes through some large-scale power equipment, or high-voltage substation, motor, etc. This type of equipment will generate strong electromagnetic interference, which will cause deviations in the transmission on the video line. Intuitively, it is to mix a lot of clutter on the image, such as messy "crossing", "corrugation", "web" or a burst of flying spots, thorns, lines and so on.
Such interference most often occurs in industrial plants, elevators, substations, stations, launch pads, homes, buildings and other monitoring environments.
2, potential difference interference
Some monitoring systems use the nearest power connection in the power design. It is possible that there are different potential differences in the same area. For example, the power of the A transformer is used in the camera position, while the power in the control room is used. The power output from the other B transformer is not synchronized with the potential of the two parts. The transmitted video line has a large potential difference, which also causes raceway interference to the image.
Such interference occurs most often in areas with a wide area, such as campuses, industrial areas, streets, and urban road monitoring.
3, AC power interference
In order to solve the potential difference and ensure the stability of power consumption, the power supply design of the monitoring system is recommended to adopt a centralized power supply mode: that is, pull a power line from the control room to each monitoring point. Centralized power supply has DC12V, AC24V, AC220V, etc. Because DC12V, AC24V voltage is low, fast voltage is reduced, and transmission distance is close, so it is used relatively less, and AC220V is the most commonly used output voltage, when passing to each camera point, The transformer converts the voltage required by the device.
However, the AC220V is an AC variable frequency power supply, but it is easy to interfere with the video signal. Especially in some environments, after the power cable and the multiple video cables are twisted into a pipe, the power cable and the video cable are twisted together. The farther the distance is, the more obvious the impact of the communication is, mainly reflected in the "waveform" stripes stored in the image, and it is moving up and down regularly.
4, cable damage interference
During the wiring construction process, if there are many video cables connected and twisted together, when the selected casing is just suitable, it is often at the elbow, and it is necessary to pull the wire with force. If the frequency of the pulling is high, it may be The cortex and shielding net of the wire cause certain damage. Sometimes even the outer skin of the video cable is worn out, and the internal shielding nets are in contact with each other, which will cause these several image signals to affect each other, reflecting the "drawing" and "slanting point". And so on, if this process, along with the power cord damage, it is a dangerous thing.
This situation generally occurs in a monitoring environment with more monitoring points and denser.
Second, the basis of anti-interference
The common cable transmission method is a low-cost effective method for monitoring image interference. As shown below, let's see how to solve the above kinds of interference through the common cable.
1, anti-electromagnetic interference
The common cable transmission system transmits various signals (such as video, audio, control signals, etc.) to a high frequency band (47 MHz to 860 MHz) through a specific filter resonance circuit to form a radio frequency signal for transmission. The advantage of this transformation is that it completely avoids the interference area of ​​electromagnetic interference and plays a role of anti-interference.
As can be seen from the above figure, electromagnetic interference and motor interference are particularly serious for interference below 20MHz. Conventional point-to-point video transmission is exactly in the range of 0-6MHz, so the interference between electromagnetic and motor is obvious. When the common cable is used for transmission, the video signal is modulated to a frequency after 50 MHz, which greatly improves the anti-interference ability, and basically can avoid the interference of electromagnetic and motor.
2, resistance to potential interference
The reason why the conventional point-to-point video transmission has a potential difference occurs because the shielded network of the cable and the copper shaft form a loop during the entire transmission process, so that the potential difference between the two places can be transmitted in the same cable.
The use of common cable one-line transmission, the key transmission is the copper shaft part of the cable, and the shielding network is the grounding end, which protects the signal, the potential difference can not be transmitted normally at both ends (unless normal conduction), so The potential of the video signal restored by all receiving ends is the same, and has nothing to do with the voltage of the transmitting end, so it can normally resist the potential difference interference.
Look at the cable TVs in various districts, cities, provinces, and countries. When will the potential difference be disturbed because of the reception? This is a strong proof.
3, anti-AC power interference
It can be seen that the interference level generated by AC220V or AC380V has a large value at 0-6MHz, and the conventional point-to-point video information transmission mode is just at 0-6M baseband transmission, so Explains why AC power can cause significant interference with conventional images.
In the same way, after adopting the common cable one-line transmission mode, the signal transmission is fully adjusted to above 50MHz. It can be found from the above figure that the interference generated by the AC power supply is basically negligible in this area, thus effectively avoiding Interference from AC power.
4, to avoid damage to the cable due to cable
This part mainly solves the problem by reducing the number of cables and reducing human error.
Let's make a comparison. When transmitting 16 channels of images, conventional point-to-point transmission requires 16 cables. PVC tubes for cables are required. At the very least, 2 1 inch tubes are required, and one tube needs to be squeezed into 8 For the wire above the strip, the process of laying and wiring will inevitably lead to artificially pulling the wire, which may easily cause the above-mentioned damage to the cable.
After adopting the common cable one-line transmission mode, 16 images are transmitted by only one cable, and there is no need to use a 1-inch PVC pipe. Even if 4-tube is used, it is more than enough, and there is no dead force in the wiring process. The event of pulling the cable occurs, so the process of artificially damaging the cable is fundamentally reduced, thereby avoiding the occurrence of interference.
Third, the conclusion
In summary, the common cable-to-wire transmission method effectively solves the occurrence of multiple types of interference in video surveillance, mainly to fundamentally eliminate the interference generation conditions or avoid the interference area, which is a radical method and worth promoting. application.
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