On the twisted pair, coaxial cable, optical fiber and use the difference – twist
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1, coaxial cable Coaxial cable with cheaper, more convenient installation advantages (relative to the optical case), so, usually in small-scale monitoring system, due to close distance, use the coaxial cable directly monitor the images sent little damage to the image quality can meet the actual requirements.
However, based on its characteristics of coaxial cable, coaxial cable when the signal within the transmission and the transmission of its attenuation by distance and the frequency of the signal itself. Typically, the higher the frequency, the greater the attenuation. Large bandwidth of video signals, to 6MHz, and, part of the modulated color images in the frequency of high-end, so that the video signal in the coaxial cable transmission only within the overall range of the signal by the attenuation, and attenuation of various frequency components very different large, especially in color part of the attenuation maximum. Therefore, the coaxial cable is only suitable for short distance transmission image signal, when the transmission distance of 200 meters, the image quality will be significantly decreased, especially the color becomes dull, with a sense of distortion.
In engineering practice, in order to extend the transmission distance, to use the coaxial amplifier. Coaxial amplifier has a certain video signal amplification, and also through a balanced adjustment of different frequencies, separate compensation of different sizes, so that the receiver output of video signal distortion as small as possible. However, the coaxial amplifier cascade is not unlimited, usually in a point-to-coaxial amplifier system can cascade up to 2 or 3, or can not guarantee the quality of video transmission and very difficult to adjust them. Therefore, in the monitoring system using coaxial cable, in order to ensure better image quality, the general will limit the transmission distance in four or five hundred meters or so.
In addition, the coaxial cable to transmit the image signal control system, there are still some disadvantages:
1) coaxial cable itself is great impact of climate change, the image quality affected to some extent;
2) thick coaxial cable, wiring in the intensive monitoring application is not easy;
3) coaxial cable generally can only transfer video signals, if the system that need to simultaneously transmit control data, audio and other signals, additional wiring is required;
4) the limited capacity of coaxial cable immunity can not be used in strong interference environment;
5) coaxial amplifiers, there are still shortcomings of adjustment difficulties.
2, twisted pair The use of long twisted pair, twisted pair telephone transmission is used in many industrial control systems and large areas disturbances and long-distance transmission have used twisted pair, widely used today LAN is to use twisted pair right. The reason why so widely used twisted-pair, because of its strong anti-interference, transmission distance, cabling is easy, inexpensive, and many other advantages. As the twisted pair of signal attenuation there are larger, so the long transmission distance, the signal's frequency can not be too high, and high-speed signals such as Ethernet can only be limited to less than 100m. For video signal, the bandwidth of 6MHz, if directly within the twisted pair transmission, attenuation will be large, so the video signal on the twisted pair to achieve the long-distance transmission, the need for amplification and compensation, twisted pair video transmission equipment is complete this function. Plus a pair of twisted pair video transceiver equipment, images can be transferred to the 1 to 2km. Twisted pair and twisted-pair transmission equipment prices are very cheap video, did not increase the system cost, but the distance increases the cost compared with coaxial cable down a lot. Therefore, the monitoring system for transmission using twisted pair has obvious advantages:
1) transmission distance transmission of high quality. As in the twisted pair transceiver uses advanced processing technology, excellent compensate the twisted pair video signal amplitude attenuation and the attenuation difference between different frequencies, maintaining the original image brightness and color as well as real-time, the transmission distance of up to 1km or longer, the image signal almost no distortion.
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