Each monitoring project has its own characteristics and particularities, so it is necessary to fully consider these specific situations when setting up a monitoring network and select the most appropriate image and signal transmission method. In view of coaxial cable, twisted pair and optical fiber are the three most widely used transmission media in the current monitoring system, we can do some analysis and comparison of them from several aspects.
1. Analysis of characteristics and transmission characteristics
1. Coaxial cable
Generally, in a small-scale monitoring system, since the transmission distance is very short, the direct transmission of the monitoring image using a coaxial cable will not cause much damage to the image quality, and can meet the actual requirements. However, according to the analysis of the coaxial cable's own characteristics, when the signal is transmitted in the coaxial cable, the attenuation is related to the transmission distance and the frequency of the signal itself. Generally speaking, the higher the signal frequency, the greater the attenuation. Therefore, the coaxial cable is only suitable for transmitting image signals at close range. When the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the color becomes dark and distorted.
In engineering practice, in order to extend the transmission distance, coaxial amplifiers are used. The coaxial amplifier has a certain amplification of the video signal, and can also compensate for different sizes of different frequency components through equalization adjustment, so that the distortion of the video signal output by the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded without limit. Generally, in a point-to-point system, the coaxial amplifiers can only be cascaded up to 2 or 3, otherwise the video transmission quality cannot be guaranteed, and adjustment is also difficult. Therefore, when using coaxial cables in the monitoring system, in order to ensure a better image quality, the transmission distance range is generally limited to about four or five hundred meters.
In addition, there are some shortcomings in the transmission of image signals by the coaxial cable in the monitoring system:
1) The coaxial cable itself is greatly affected by climate change, and the image quality is affected to a certain extent;
2) The coaxial cable is thick, and the wiring is not convenient for intensive monitoring applications;
3) Coaxial cables can generally only transmit video signals. If the control data and audio signals need to be transmitted simultaneously in the system, additional wiring is required;
4) Coaxial cables have limited anti-interference ability and cannot be used in strong interference environments;
5) The coaxial amplifier also has the disadvantage of difficult adjustment.
2. Twisted pair
Twisted-pair cable has been used for a long time. Twisted-pair cable has been used in many industrial control systems, places with large interference, and long-distance transmission. The LAN that we widely use today also uses twisted-pair cable. The reason why twisted pair is so widely used is because it has many advantages such as strong anti-interference ability, long transmission distance, easy wiring, and low price. The twisted pair signal also has a large attenuation. If the video signal is directly transmitted in the twisted pair, it will also be greatly attenuated. Therefore, to achieve long-distance transmission of the video signal on the twisted pair, it must be amplified and compensated. Twisted pair video transmission equipment is to complete this function. With the addition of a pair of twisted pair video transmission equipment, the image can be transmitted to 1 to 2km. Twisted pair and twisted pair video transmission equipment are very cheap, not only does not increase the cost of the system, but as the distance increases, its cost has dropped a lot compared to coaxial cable. Therefore, the use of twisted pair transmission in the monitoring system has obvious advantages:
1) Long transmission distance and high transmission quality. Due to the use of advanced processing technology in the twisted pair transceiver, the attenuation of the twisted pair to the video signal amplitude and the attenuation difference between different frequencies are excellently compensated, maintaining the brightness and color of the original image and real-time, When the transmission distance reaches 1km or more, the image signal is basically free of distortion. If the relay method is adopted, the transmission distance will be farther.
2) Easy wiring and high cable utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, any one of the 5 types of unshielded twisted pair widely laid in the building can transmit one video signal without additional wiring. Even if the wiring is re-wired, the 5 type cable is easier than the coaxial cable. In addition, there are 4 pairs of twisted pairs in a category 5 cable. If you use one pair to transmit video signals, the other pairs can also be used to transmit audio signals, control signals, power supply or other signals, which improves the cable. The utilization rate, while avoiding the trouble caused by the separate wiring of various signals, reduces the project cost.
3) Strong anti-interference ability. Twisted pair can effectively suppress common mode interference, even in a strong interference environment, the twisted pair can also transmit excellent image signals. Moreover, several pairs of twisted pairs in a cable are used to transmit different signals without interference.
4) High reliability and easy to use. To use twisted pair to transmit video signals, a dedicated transmitter should be connected at the front end and a dedicated receiver should be connected at the control center. This kind of twisted pair transmission equipment is cheap and easy to use. It does not require professional knowledge or much operation. It can be installed once and work stably for a long time.
5) The price is cheap and the material is convenient. Because it is a common type 5 unshielded cable or common telephone line that is widely used at present, it is easy to purchase and the price is also very cheap, which brings great convenience to engineering applications.
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