In the monitoring system, monitoring the transmission of images of a whole system a key link to choose which media and equipment transfer images and other control signal monitoring system will be directly related to the quality and reliability. Currently, the monitoring system used to transmit video signals mainly coaxial media, twisted pair and fiber, the corresponding coaxial video transmission equipment, amplifiers, respectively, twisted pair video transmission devices, and Optical.
Early use of coaxial cable is also used the longest transmission. Later, with images far and wide to monitor needs and people's demands to improve the image quality control, monitoring network began to use optical fiber to transmit large video signals. The twisted pair is used to monitor the network image is a recent thing, it appears Zhuyao solves two problems: one hand, it settled 200 to 2000 m distance within the scope of high-quality map as the signal transmission of the problem, because in this distance range coaxial cable transmission without Shiyong amplifier had a hard enough Dadaoyaoqiu, Er using coaxial video Fangda Qi and Guang Qian transmission You seem Butai economy; other hand, it resolved the large-scale intensive monitoring network routing problem, twisted pair their size and flexibility to overcome a lot of wiring problems when using coaxial cable. Of course, the twisted pair also have strong anti-interference ability, low cost advantages. It is precisely because twisted pair well 解决 long plagued the people of the Wen Ti, Suo Yi Ta Ying Yong in monitoring the network immediately aroused the industry's widespread concern, in short of time, had been widely used practice of Zhong Dao Gong Cheng, Bing Ju achieved very good application of the results.
Each monitoring project has its own characteristics and particularities, and therefore set up monitoring networks need to consider these circumstances, choose the most appropriate image and signal transport. View of the coaxial cable, twisted pair and fiber optic monitoring system is the most widely used three kinds of transmission media, we can make some aspects of their analysis and comparison.
1, characteristics and transmission characteristics of
1, coaxial cable
General monitoring system in a small area, due to the short distance transmission using coaxial cable transmission surveillance image directly on the image quality is not the damage, can meet the actual requirements. However, based on its characteristics of coaxial cable, coaxial cable inside the transmission when the signal attenuation by the time of its transmission distance and signal with its own frequency. In general, the higher the frequency, the greater the attenuation. Therefore, the coaxial cable is only suitable for over 0 to 300 meters transmission image signal, when the transmission distance of more than 300 m, 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 video Xinhaojuyou zoom function, and also through a balanced Tiaozheng on Butong different frequencies Chengfenfenbie Jin Xing Da Xiao compensation so that output video 接收 end distortion Xiao, transmission distance. Coaxial amplifier cascade can not be unlimited, usually in a point-system, coaxial amplifiers can only cascade 1-2, or can not be guaranteed video transmission quality, and also very difficult to adjust them. Therefore, in monitoring systems using coaxial cable, the better the image in order to ensure the quality, generally limits the scope of transmission distance in the 200 meters Zuo You, Ji save Chengben You do not need to enlarge Buchang.
Coaxial cable to transmit the image signal control system, there exist some shortcomings:
1) coaxial cable itself is great impact of climate change, the image quality will be affected to some extent;
2) thick coaxial cable, control-intensive applications at the wiring is not convenient;
3) The 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) The relatively high cost coaxial, coaxial amplifiers are not cheap. 2, twisted pair
The use of twisted-pair long, in many industrial control systems and interference as well as large areas of the transmission distance and use a twisted pair, we also used widely used LAN twisted pair right. The reason why such a broad use of twisted pair, because of its strong anti-interference, wiring is easy, cheap, and zoom compensation after transmission distance, and many other advantages. Twisted pair of signal attenuation there are large, twisted pair video signal directly within the transmission if the will decay much, can only transfer 60 to 80 meters. Therefore, the video signal in the twisted pair to achieve the long-distance transmission, the need for amplification and compensation, automatic twisted pair video transmission devices that accomplish this function. Plus a pair of twisted pair video transmission devices, the images can be transferred to the 0m to 1.8km. UTP twisted pair video transmission devices and the price is very cheap, not only does not increase the system cost, but the distance increases the cost down a lot compared with the coaxial cable. Therefore, the monitoring system a more long-distance transmission using twisted pair has obvious advantages:
1) The transmission distance, transmission of high quality. As automatic twisted pair transceiver uses advanced processing techniques, to best 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 nature of the transmission distance of up to 1km or longer, the image signal distortion small.
2) wiring convenience, cable high utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, the building blocks laid in a wide range of Category 5 unshielded twisted-pair get one pair in any way you can transmit video signals without additional wiring, even the re-wiring, Category 5 cable coaxial cable and fiber than easy. In addition, a Category 5 cable has 4 twisted pair, 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 or other signals, to improve the cable utilization, separate the various signals while avoiding the hassle of wiring to reduce the project cost.
3) anti-jamming capability. Twisted pair can effectively suppress common mode interference, even in strong interference environment, the twisted pair is also an excellent image signal can be conveyed. Moreover, the use of a twisted pair cable in a few of the different signals are transmitted, interference will not occur between each other.
4) high reliability, ease of use. Use of twisted pair transmission video signals, in the front end to access UTP signaling device, the control center to access automatic twisted pair video transmission equipment. This inexpensive twisted pair transmission equipment, is also very simple to use, plug and play, no expertise, no need for other operations, once installed, long-term stability.
5) The cheap, easily obtained. As is widely used in ordinary Category 5 unshielded cable, purchase easy and the price is very cheap, for engineering applications bring great convenience.
3, optical fiber
Optical fiber and used in the field monitoring is mainly to solve two problems: First, the transmission distance, one environmental interference. Twisted pair and coaxial cable can only solve the short distance, a small area of the monitor image transmission problem, if you need a few kilometers or even hundreds of kilometers of transmission distance of video signals will need to use fiber-optic transmission. In addition, the interference of some super place to interference from environmental impact, but also with optical fiber transmission. Because the fiber has a wide bandwidth, large capacity, from electromagnetic interference by the external environmental impact of small, and many other advantages of an optical monitoring system can transmit all the signals required in the transmission distance can reach hundreds of kilometers. Optical monitoring system provides the flexibility for the transmission and networking mode, signal quality, high stability.
However, the use of Optical Fiber and need some expertise and special equipment, construction and this has caused some users problems. In addition, for a short distance, small-scale monitoring system, using optical fiber transmission also appears to lack the economy.
Second, technical performance analysis
1, coaxial cable in the monitoring system, using coaxial cable 75Ω-5 is more common. General, the distribution of this coaxial cable capacitance in 50-60pF / m or so, plus DC resistance of the cable, the signal will be transmitted by the attenuation. Tests show that the frequency of 5MHz signal in the 75Ω-5 transport within 100m of coaxial cable, it will be about 5dB attenuation, the higher the frequency, the greater attenuation suffered. Video signals is a high bandwidth signal, image color section in the frequency of high-end, when the color image signal transmission using coaxial cable, its brightness and color are subject to decay, especially with the increasing distance image transmission colors will lighten or even distortion. In the laboratory tests show that color image signals within the coaxial cable 75Ω-5 transmission is about 200m, its magnitude and color have been marked attenuation. If you want to transfer more remote, have joined the coaxial video amplifier, you can guarantee that within 2Km distance able to transmit high quality video signals.
2 twisted-pair twisted-pair as a transmission medium in use, general use commonly used Category 5 UTP cable. Its characteristic impedance is about 100Ω, distributed capacitance of about 15pF / m, with greater attenuation than coaxial cable, coaxial transmission distance is only 43%. Twisted pair video transmission equipment using automatic and with the use of twisted pair, you can ensure 1.5Km distance range can transmit high quality video signals. In the laboratory using special devices transmit video signals on twisted pair performance was tested as follows:
Test Equipment: TektronixTSG-271PAL system signal generator, TektronixVN700A video signal analyzer
DUT: ONDON (settled) twisted pair video transmission devices, Category 5 UTP cable
(1) when the transmission distance of 300m
Index: DG ≤ 1%; DP ≤ 1 °; SNR ≥ 65dB
(2) when the transmission distance of 1500m
Index: DG ≤ 2%; DP ≤ 2 °; SNR ≥ 60dB
3, optical fiber
Remote control image transmission fiber is a natural choice, it offers hundreds of kilometers of transmission distance, and can provide a good quality image transmission. In the laboratory using special instruments to settle Paul Electronics Co., Ltd. of Beijing twisted pair video transmission equipment, performance tested, results are as follows:
DG ≤ 1%; DP ≤ 1 °; SNR ≥ 70dB
Third, economic analysis
Optical transmission is the most costly of three transmission of a, now the price of single-mode optical fiber is usually in each about 2.5 yuan per meter, Optical cost about 1,000 yuan more per road image, coupled with engineering of accessories such as fiber jumper, fiber optic light flange and welding costs, etc., making the cost of fiber optic monitoring network is relatively high. However, optical fiber transmission with many outstanding advantages, 所以 transmission distance in 2, 3 kilometers or more Jiankong network, some Huanjingganrao Jiang's spaces, and the high quality and reliability requirements of the monitoring Changsuo most Haishixuanyong transmitted through optical fiber .
Coaxial cable in one, about 200 meters away from the more widely used within the scope of, the use of coaxial cable in this area do not need other equipment, easy to use, transmission quality can fully meet the requirements. However, when the transmission distance increases, the cost is coaxial and twisted pair is inferior compared to the style. General, 75Ω-5 of the coaxial cable market in the 3 yuan / m, coaxial amplifiers prices at 600 yuan / unit; Category 5 UTP cable price 1.5 yuan / m, twisted pair video transmission equipment, automatic price Active 1500 meters of 580 yuan / sets, passive 300 meters 180 yuan / set.
The following distance of 200m and 1500m on two conditions on the coaxial cable and twisted pair transmission costs for some more.
(1) assumes that a monitoring network in eight control points, each control center monitor dot pitch of about 200m or so.
When using coaxial cable:
3 yuan / m × 200m × 8 = 4800 元
When using twisted pair:
Since each image can share a four Category 5 UTP cable, so the network only two Category 5 UTP cable. 1.5 yuan / m × 200m × 2 +180 yuan / set × 8 = 2040 元
(2) assumes that a monitoring network in eight control points, each control center monitor dot pitch of about 1500m or so
When using coaxial cable:
To ensure image quality, image every way to join an amplifier. 3 yuan / m × 1500m × 8 +600 yuan / unit × 8 = 40800 元
When using twisted pair:
Each image can be shared as a four Category 5 UTP cable, so the network only two Category 5 UTP cables. 1.5 yuan / m × 1500m × 2 +580 yuan / set × 8 = 6890 元
From the above analysis we can see that, with the transmission distance increases, more and more twisted-pair transmission shows the cost advantage. If the network there are control signals and other signal transmission, twisted pair will be more obvious cost advantage.
Fourth, the proposed project
In actual engineering, needs transmission choices the specific circumstances of engineering and engineering budget in consideration.
The transmission distance of more than 2Km monitoring network should be considered optional and Optical Fiber. If the transmission distance of up to several kilometers or tens of kilometers of fiber optic only use the. In addition, strong interference in some situations, such as by industrial interference and electromagnetic interference areas, in order to ensure image quality, but also recommend the use of optical fiber transmission.
Coaxial and twisted-pair selection, you need more analysis and comparison. As the twisted pair is an emerging De technologies, and coaxial cable 使用 has very long history, there is also a need to fully explore the twisted pair of features and advantages, and construction for the Gongchengsheji facilitate Jiesheng cost . Transmission distance from 200 to 1.5 m or so, the twisted pair can undoubtedly play an excellent performance and price advantage, for the 200 meters within the monitoring network, if small, use the coaxial cable is a very reasonable cost too not high.
However, the actual works will also have its special nature, even if very close, if the network is large, such as five 60 to hundreds of monitoring points, then would be difficult to use coaxial cable wiring, and twisted pair line can make this problem solved.
Furthermore, if the environment interfere with a larger, if you use twisted-pair interference can effectively ensure the image transmission quality. In addition, the twisted pair wiring convenience and flexibility of making building blocks and some other short-range, intensive monitoring network are also beginning to use it to replace the coaxial cable, and well integrated into the project to the Cabling.
