Coaxial cable has the advantages of cheaper price and convenient laying (relative to optical fiber). Therefore, in a small-scale monitoring system, because of the short transmission distance, the coaxial image is used to directly transmit the surveillance image to the image. The quality damage is not large enough to meet the actual requirements.
However, according to the analysis of the characteristics of the coaxial cable itself, the attenuation of the signal as it travels within the coaxial cable is related to the transmission distance and the frequency of the signal itself. In general, the higher the signal frequency, the greater the attenuation. The bandwidth of the video signal is very large, reaching 6MHz, and the color portion of the image is modulated at the high end of the frequency, so that when the video signal is transmitted in the coaxial cable, not only the overall amplitude of the signal is attenuated, but also the attenuation of each frequency component is greatly different. In particular, the color portion is most attenuated. Therefore, the coaxial cable is only suitable for transmitting image signals at a short distance. When the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the color becomes dim and has a sense of distortion.
In engineering practice, in order to extend the transmission distance, a coaxial amplifier is used. The coaxial amplifier has a certain amplification of the video signal, and can also compensate different frequency components by different adjustments by equalization adjustment, so that the distortion of the video signal outputted by the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded without limitation. Generally, a coaxial amplifier can only cascade up to 2 to 3 in a point-to-point system. Otherwise, the quality of video transmission cannot be guaranteed and it is difficult to adjust. Therefore, when using a coaxial cable in a monitoring system, in order to ensure good image quality, the transmission distance is generally limited to about four or five hundred meters.
In addition, coaxial cable has some disadvantages in transmitting image signals in the monitoring system:
1) The coaxial cable itself is greatly affected by climate change, and the image quality is affected to some extent;
2) The coaxial cable is thicker, and the wiring is not convenient in dense monitoring applications;
3) Coaxial cable can only transmit video signals. If the system needs to transmit control data, audio and other signals at the same time, additional wiring is required.
4) Coaxial cable has limited anti-interference ability and cannot be applied to strong interference environment;
5) Coaxial amplifiers also have the disadvantage of difficulty in adjustment.
2, twisted pairTwisted pair has been used for a long time. The telephone transmission uses twisted pair. In many industrial control systems and in places with high interference and long distance transmission, twisted pair is used. The LAN that we use today is also used. Twisted pair. Twisted pair is so widely used because of its many advantages such as strong anti-interference ability, long transmission distance, easy wiring, and low price. Since the twisted pair signal also has a large attenuation, the frequency of the signal cannot be too high when the transmission distance is long, and the high speed signal such as Ethernet can only be limited to 100 m. For video signals, the bandwidth reaches 6MHz. If it is transmitted directly in the twisted pair, it will also attenuate greatly. Therefore, the video signal must be transmitted over long distances on the twisted pair. It must be amplified and compensated. The device is to complete this function. With a pair of twisted pair video transceivers, the image can be transferred to 1 to 2 km. Twisted pair and twisted pair video transmission equipment are very cheap, not only do not increase the system cost, but the cost is much lower than the coaxial cable when the distance increases. Therefore, the use of twisted pair transmission in the monitoring system has obvious advantages:
1) The transmission distance is long and the transmission quality is high. Due to the advanced processing technology used in the twisted pair transceiver, the attenuation of the amplitude of the video signal of the twisted pair and the attenuation difference between different frequencies are well compensated, and the brightness and color of the original image and the real-time performance are maintained. When the transmission distance reaches 1 km or more, the image signal is substantially free of distortion. If you use the relay method, the transmission distance will be farther.
2) Convenient wiring and high cable utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, any one of the five types of unshielded twisted-pair cables widely installed in a building can transmit one video signal without additional wiring. Even if it is re-routed, Category 5 cable is easier than coaxial cable. In addition, there are 4 pairs of twisted pairs in a Category 5 cable. If a pair of wires is used to transmit video signals, the other pairs of wires can also be used to transmit audio signals, control signals, power supplies or other signals, and the cable is improved. The utilization rate avoids the troubles caused by separate wiring of various signals and reduces the construction cost.
3) Strong anti-interference ability. Twisted pair can effectively suppress common mode interference, and even in a strong interference environment, the twisted pair can transmit excellent image signals. Moreover, several pairs of twisted pairs in one cable are used to transmit different signals, and no interference occurs between them.
4) High reliability and easy to use. The video signal is transmitted by twisted pair, and the dedicated transmitter is connected to the front end, and the dedicated receiver is connected to the control center. This twisted pair transmission equipment is cheap and easy to use, requires no professional knowledge, and does not have much operation, one installation, and long-term stable work.
5) The price is cheap and the material is convenient. Due to the use of the common 5 types of unshielded cables or ordinary telephone lines that are widely used at present, the purchase is easy and the price is also very cheap, which brings great convenience to engineering applications.
3, fiberOptical fiber and optical transceiver applications in the field of monitoring are mainly to solve two problems: one is the transmission distance, and the other is environmental interference. Twisted pair and coaxial cable can only solve the problem of surveillance image transmission in short distance and small range. If it is necessary to transmit image signals of several kilometers or even hundreds of kilometers, optical transmission is required. In addition, for some super-interfering places, in order to be free from environmental interference, optical fiber transmission is also required. Because the optical fiber has many advantages such as wide transmission bandwidth, large capacity, no electromagnetic interference, and little influence from the external environment, one fiber can transmit all the signals needed in the monitoring system, and the transmission distance can reach hundreds of kilometers. The optical transceiver can provide one-way and multi-channel image interfaces, and can also provide two-way audio interface, one-way and multiple-way various types of bidirectional data interfaces (including RS232, RS485, Ethernet, etc.), and integrate them into one optical fiber for transmission. The optical transceiver provides a flexible transmission and networking mode for the monitoring system, with good signal quality and high stability. In recent years, due to the rapid development of optical fiber communication technology, the price of optical fiber and optical devices has dropped rapidly, and the cost of optical fiber monitoring systems has been greatly reduced. Therefore, the application of optical fibers and optical transceivers in monitoring systems is becoming more and more popular.
However, the use of fiber optics and optical transceivers requires certain expertise and specialized equipment, which poses certain difficulties for engineering construction and user use. In addition, for short-distance, small-scale surveillance systems, the use of fiber-optic transmission is also not economical.
Fiber Optic Cable,Fiber Cable,Optical Cord,Optic Cable
Dongguan Tuojun Electronic Technology Co., Ltd , https://www.fibercablessupplier.com