1. Coaxial cable
Coaxial cable has the advantages of cheaper price and more convenient laying (compared to optical fiber). Therefore, generally in a small-scale monitoring system, due to the short transmission distance, the use of coaxial cable to directly transmit monitoring images to images. The quality of the damage is not large, can meet the actual requirements.
However, according to the analysis of the characteristics of the coaxial cable itself, the attenuation of the signal when it is transmitted in 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 part of the image is modulated at the high frequency. In this way, 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 very different. , especially the color part is attenuated the most. 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 will become dim and there will be a sense of distortion.
In engineering practice, in order to extend the transmission distance, a coaxial amplifier is used. The coaxial amplifier can amplify the video signal to a certain extent, and can also compensate different frequency components in different sizes through equalization adjustment, so as to minimize the distortion of the video signal output by the receiving end. However, coaxial amplifiers cannot be cascaded indefinitely. Generally, in a point-to-point system, only 2 to 3 coaxial amplifiers can be cascaded at most. Otherwise, the video transmission quality cannot be guaranteed, and it is difficult to adjust. Therefore, when using a coaxial cable in a monitoring system, in order to ensure better image quality, the transmission distance is generally limited to about four or five hundred meters.
coaxial cable
In addition, there are some disadvantages in the transmission of image signals by 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 thicker, and the wiring is inconvenient in intensive monitoring applications;
3) Coaxial cables generally 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 used in strong interference environment;
5) The coaxial amplifier also has the disadvantage of being difficult to adjust.
2. Twisted pair
Twisted pair has been used for a long time. Telephone transmission uses twisted pair. Twisted pair is used in many industrial control systems and in places with large interference and long-distance transmission. The local area network that we widely use today is also used. twisted pair. The reason why twisted pair is so widely used is that it has many advantages such as strong anti-interference ability, long transmission distance, easy wiring and low price. Because the twisted pair signal also has a large attenuation, when the transmission distance is long, the frequency of the signal cannot be too high, and high-speed signals such as Ethernet can only be limited to within 100m. For the video signal, the bandwidth reaches 6MHz. If it is directly transmitted in the twisted pair, it will also be attenuated greatly. Therefore, to achieve long-distance transmission of the video signal on the twisted pair, it must be amplified and compensated. The twisted pair video transmission The device does just that. After adding a pair of twisted pair video transceiver equipment, the image can be transmitted to 1 to 2km. The price of twisted pair and twisted pair video transmission equipment is very cheap, not only does not increase the cost of the system, but its cost is much lower than that of coaxial cable when the distance increases. 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 advanced processing technology used in the twisted pair transceiver, the attenuation of the video signal amplitude of the twisted pair pair and the attenuation difference between different frequencies are perfectly compensated, and the brightness and color of the original image and the real-time performance are maintained. When the transmission distance reaches 1km or more, the image signal is basically free of distortion. If the relay method is used, the transmission distance will be longer.
2) The wiring is convenient and the cable utilization rate is high. A pair of ordinary telephone lines can be used to transmit video signals. In addition, any pair of Category 5 unshielded twisted-pair cables widely laid in buildings can transmit one video signal without additional wiring. Even re-wiring, Category 5 cables are easier than coaxial cables. In addition, there are 4 pairs of twisted pairs in a Category 5 cable. If one 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 supply or other signals, improving the cable performance. Utilization rate, while avoiding the trouble caused by the separate wiring of various signals, reducing the engineering cost.
3) Strong anti-interference ability. The 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 respectively, and there will be no interference with each other.
4) High reliability and easy to use. Using twisted pair to transmit video signal, a dedicated transmitter should be connected to the front end, and a dedicated receiver should be connected to the control center. This twisted pair transmission equipment is cheap and easy to use. It does not require professional knowledge and does not require much operation. It can be installed once and can work stably for a long time.
5) The price is cheap and the materials are easy to obtain. Since the common Category 5 unshielded cables or common telephone lines that are widely used are used, it is easy to purchase and the price is very cheap, which brings great convenience to engineering applications.
3. Optical fiber
The application of optical fibers and optical transceivers in the monitoring field is mainly to solve two problems: one is transmission distance, and the other is environmental interference. Twisted pair and coaxial cable can only solve the problem of monitoring image transmission in a short distance and a small range. If you need to transmit image signals with a distance of several kilometers or even hundreds of kilometers, you need to use optical fiber transmission. In addition, for some super-interference places, in order not to be affected by environmental interference, optical fiber transmission should also be used. Because the optical fiber has many advantages such as wide transmission bandwidth, large capacity, no electromagnetic interference, and little influence by the external environment, one optical fiber can transmit all the signals needed in the monitoring system, and the transmission distance can reach hundreds of kilometers. Optical transceivers can provide one-way and multi-channel image interfaces, and can also provide two-way audio interfaces, one-way and multi-channel various types of two-way 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 prices of optical fibers and optical components have dropped rapidly, which has greatly reduced the cost of optical fiber monitoring systems. Therefore, the application of optical fibers and optical transceivers in monitoring systems has become more and more popular.
However, the use of optical fibers and optical transceivers requires certain professional knowledge and special equipment, which brings certain difficulties to engineering construction and user use. In addition, for short-distance, small-scale monitoring systems, the use of optical fiber transmission is not economical.
