At present, there are many ways to transmit video signals in security monitoring industry, but the main transmission medium is coaxial cable, twisted pair and fiber. The corresponding coaxial video transmission equipment, amplifiers, respectively, twisted pair video transmission equipment and Optical. View of the coaxial cable, twisted pair and fiber optic monitoring system is currently the most widely used of the three transmission media, following are the analysis of their performance advantages.
1. Coaxial cable, usually in small-scale monitoring system, due to close distance, much of image quality was damaged by using of coaxial cable directly monitor the image. When the transmission distance of 200 meters, the image quality will be significantly decreased, especially the color becomes dark and there is a sense of distortion. In engineering practice, in order to extend the transmission distance, coaxial amplifier will be used. Coaxial amplifier has a certain video signal amplification, and also through a balanced composition of different frequencies were adjusted to compensate for different sizes, but the transmission distance is limited. Although 2,000 meters coaxial video transmission equipment have been appear on the market now, the individual can solve the problem of long-distance image transmission. But the cost is slightly higher, not suitable for large-scale monitoring system widely used. In addition, the coaxial cable to transmit the image signal control system, there are still some shortcomings:
1) Coaxial impact of climate change itself, the image quality will be affected;
2) Thick coaxial cable, wiring in the intensive monitoring of applications is not easy;
3) Coaxial cable is currently only used to transmit video signals, if the system has other signals to be transmitted, additional wiring is required;
4) The limited capacity of coaxial cable immunity cannot be used in strong interference environment;
5) Coaxial adjustment difficulties in the amplifier, there are still shortcomings.
2. The use of twisted-pair, in many industrial control systems and greater interference in both places, and long-distance transmission using twisted pair, we widely used local area network is also twisted pair. Twisted pair had used so widely because it has a strong anti-interference, transmission distance, wiring easy, inexpensive, and many other advantages. Twisted pair of signal attenuation there are large, if the video signal is transmitted directly to the twisted pair will decay much, so the video signal on the twisted pair to achieve the long-distance transmission, the need for amplification and compensation, Twisted pair video transmission devices that accomplish this function. Plus a pair of twisted pair video transceiver device, the image can be transferred to 2km. Twisted pair and twisted pair video transmission equipment prices are very cheap, not only does not increase the system cost, but the distance increases the cost compared with coaxial cable down a lot. Therefore, the monitoring system using twisted pair transmission has obvious advantages：
1) Transmission distance, transmission of high quality. Since the twisted pair transceiver uses advanced processing techniques to compensate for the twisted pair an excellent 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 2km or further away, almost no distortion of the image signal. If using relay mode, the transmission distance will be longer.
2) To facilitate wiring, cable high utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, the laying of building a wide range of building Category 5 unshielded twisted-pair to take one pair in any way you can transmit video signals without additional wiring, even the re-wiring, Category 5 cable is also easier than coaxial cable. In addition, a Category 5 cable has four twisted pairs inside, if you use a pair of lines to carry video signals, while a few of the lines can also be used to transmit audio signals, control signals, power supply, or other signals, to improve line cable utilization, while avoiding a variety of signals alone the hassle of wiring, reducing the project cost.
3) Anti-interference ability. Twisted pair can effectively suppress common mode interference, even in strong interference environment, the twisted pair is also capable of delivering excellent image signal. Moreover, the use of a twisted pair cable in a few of the different signals were sent, does not interfere with each other.
4) High reliability, ease of use. The use of twisted pair transmission video signal, the front end to access special transmitter in the control center to access special receiver. This twisted pair transmission equipment cheap, very simple to use, no expertise, no too many operations, once installed, the long-term stability.
5) The cheap, easily obtained. The use of the now widely used as an ordinary Category 5 unshielded cable or ordinary telephone lines, purchase easy and the price is very cheap, to engineering applications has brought great convenience.
3, Optical fiber and optical fiber applications in the field of monitoring are mainly to solve two problems: First, the transmission distance, one environmental interference. Twisted pair and coaxial cable can only solve the short-range, small area of the monitor image transmission problems, if the need to transfer a few kilometers or even hundreds of kilometers away from the image signal will need to use fiber optic transmission. In addition, the interference of some super place to interference from environmental impact, but also the use of optical fiber transmission. Because the fiber has a wide bandwidth, large capacity, free from electromagnetic interference by the external environmental impact, and many other advantages, an optical fiber can transmit monitoring system.