In the monitoring system, the transmission of monitoring images is a crucial part of the entire system. The choice of media and equipment to transmit images and other control signals will directly affect the quality and reliability of the monitoring system. At present, the media used to transmit image signals in the monitoring system mainly include coaxial cable, twisted pair and optical fiber, and the corresponding transmission equipment are coaxial video amplifier, twisted pair video transmission device and optical terminal.
Coaxial cable was used earlier and is also the longest used transmission method. Later, due to the need for long-distance and large-scale image monitoring and people's increased requirements for monitoring image quality, a large number of optical fibers were used in monitoring networks to transmit image signals. As for the use of twisted-pair cable in the image monitoring network, it is a recent matter. Its appearance mainly solves two problems: on the one hand, it solves high-quality images in the range of 200 meters to 2000 meters. Like the problem of signal transmission, because the coaxial cable transmission is difficult to meet the requirements within this distance and the optical fiber transmission is not economical; on the other hand, it solves the wiring problem of large-scale intensive monitoring networks. The twisted pair itself The size and flexibility of the cable overcome a lot of wiring problems when using coaxial cables. Of course, the twisted pair also has the advantages of strong resistance to common mode interference and low price. It is precisely because the twisted pair has solved these problems that have troubled people for a long time, so its application in the monitoring network has immediately attracted widespread attention in the industry, and has been widely used in engineering practice in a short period of time, and Good application results have been achieved.
Each monitoring project has its own characteristics and particularities, so it is necessary to fully consider these specific situations when setting up a monitoring network and select the most appropriate image and signal transmission method. In view of coaxial cable, twisted pair and optical fiber are the three most widely used transmission media in the current monitoring system, we can do some analysis and comparison of them from several aspects.
1. Analysis of characteristics and transmission characteristics
1. Coaxial cable
Generally, in a small-scale monitoring system, since the transmission distance is very short, the direct transmission of the monitoring image using a coaxial cable will not cause much damage to the image quality, and can meet the actual requirements. However, according to the analysis of the coaxial cable's own characteristics, when the signal is transmitted in the coaxial cable, the attenuation is related to the transmission distance and the frequency of the signal itself. Generally speaking, the higher the signal frequency, the greater the attenuation. Therefore, the coaxial cable is only suitable for transmitting image signals at close range. When the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the color becomes dark and distorted.
In engineering practice, in order to extend the transmission distance, coaxial amplifiers are used. The coaxial amplifier has a certain amplification of the video signal, and can also compensate for different sizes of different frequency components through equalization adjustment, so that the distortion of the video signal output by the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded without limit. Generally, in a point-to-point system, the coaxial amplifiers can only be cascaded up to 2 or 3, otherwise the video transmission quality cannot be guaranteed, and adjustment is also difficult. Therefore, when using coaxial cables in the monitoring system, in order to ensure a better image quality, the transmission distance range is generally limited to about four or five hundred meters.
In addition, there are some shortcomings in the transmission of image signals by the 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 thick, and the wiring is not convenient for intensive monitoring applications;
3) Coaxial cables can generally only transmit video signals. If the control data and audio signals need to be transmitted simultaneously in the system, additional wiring is required;
4) Coaxial cables have limited anti-interference ability and cannot be used in strong interference environments;
5) The coaxial amplifier also has the disadvantage of difficult adjustment.
2. Twisted pair
Twisted-pair cable has been used for a long time. Twisted-pair cable has been used in many industrial control systems, places with large interference, and long-distance transmission. The LAN that we widely use today also uses twisted-pair cable. The reason why twisted pair is so widely used is because it has many advantages such as strong anti-interference ability, long transmission distance, easy wiring, and low price. The twisted pair signal also has a large attenuation. If the video signal is directly transmitted in the twisted pair, it will also be greatly attenuated. Therefore, to achieve long-distance transmission of the video signal on the twisted pair, it must be amplified and compensated. Twisted pair video transmission equipment is to complete this function. With the addition of a pair of twisted pair video transmission equipment, the image can be transmitted to 1 to 2km. Twisted pair and twisted pair video transmission equipment are very cheap, not only does not increase the cost of the system, but as the distance increases, its cost has dropped a lot compared to coaxial cable. 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 use of advanced processing technology in the twisted pair transceiver, the attenuation of the twisted pair to the video signal amplitude and the attenuation difference between different frequencies are excellently compensated, maintaining the brightness and color of the original image and real-time, When the transmission distance reaches 1km or more, the image signal is basically free of distortion. If the relay method is adopted, the transmission distance will be farther.
2) Easy wiring and high cable utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, any one of the 5 types of unshielded twisted pair widely laid in the building can transmit one video signal without additional wiring. Even if the wiring is re-wired, the 5 type cable is easier than the coaxial cable. In addition, there are 4 pairs of twisted pairs in a category 5 cable. If you use one pair to transmit video signals, the other pairs can also be used to transmit audio signals, control signals, power supply or other signals, which improves the cable. The utilization rate, while avoiding the trouble caused by the separate wiring of various signals, reduces the project cost.
3) Strong anti-interference ability. Twisted pair can effectively suppress common mode interference, even in a strong interference environment, the twisted pair can also transmit excellent image signals. Moreover, several pairs of twisted pairs in a cable are used to transmit different signals without interference.
4) High reliability and easy to use. To use twisted pair to transmit video signals, a dedicated transmitter should be connected at the front end and a dedicated receiver should be connected at the control center. This kind of twisted pair transmission equipment is cheap and easy to use. It does not require professional knowledge or much operation. It can be installed once and work stably for a long time.
5) The price is cheap and the material is convenient. Because it is a common type 5 unshielded cable or common telephone line that is widely used at present, it is easy to purchase and the price is also very cheap, which brings great convenience to engineering applications.
3. Optical fiber
The application of optical fiber and optical transceiver in the field of monitoring is mainly to solve two problems: one is the transmission distance, and the other is environmental interference. Twisted pair and coaxial cables can only solve the problem of short-distance, small-range surveillance image transmission. If you need to transmit image signals at a distance of several kilometers or even hundreds of kilometers, you need to use optical fiber transmission. In addition, in some places with strong interference, in order not to be affected by environmental interference, it is necessary to use optical fiber transmission. Because the optical fiber has many advantages such as wide transmission bandwidth, large capacity, no electromagnetic interference, and little influence from the external environment, a single optical fiber can transmit all the signals required in the monitoring system, and the transmission distance can reach hundreds of kilometers. The optical transceiver provides a flexible transmission and networking method for the monitoring system, with good signal quality and high stability.
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 enough.
2. Technical performance analysis
1. Coaxial cable In the monitoring system, the use of 75Ω, -5 coaxial cable is more common. Generally, the distributed capacitance of this coaxial cable is about 50-60pF / m, and the DC resistance of the cable will attenuate the transmitted signal. Tests show that when a signal with a frequency of 5MHz is transmitted 100m in a 75Ω, -5 coaxial cable, it will be attenuated by about 5dB. The higher the signal frequency, the greater the attenuation. The image signal is a high-frequency broadband signal. The color part of the image is located at the high end of the frequency. When the color image signal is transmitted by the coaxial cable, its brightness and color will be attenuated, especially with the increase of the transmission distance. Will fade or even distort. Testing in the laboratory found that when the color image signal is transmitted for about 200m in a 75Ω, -5 coaxial cable, its amplitude and color have been significantly attenuated. If you want to transmit a longer distance, only add a coaxial video amplifier.
2. When twisted-pair cable is used as the transmission medium, generally used Category 5 UTP cable is generally used. Its characteristic impedance is about 100Ω, and the distributed capacitance is about 15pF / m. Unlike coaxial cables, signals are transmitted in a balanced manner in twisted pairs. With the use of twisted pair video transmission equipment and twisted pairs, you can ensure that The video signal can be transmitted with high quality within a distance of 1.5Km. In the laboratory, the performance indicators of the image signals transmitted by twisted pairs were tested using special instruments. The results are as follows:
Test instruments: Tektronix TSG-271 PAL signal generator, Tektronix VN700A video signal analyzer
Device under test: Wuhan Weichuang Optoelectronics Technology Co., Ltd. (027-87462996) WTCS-03 C10 twisted pair video transmission equipment, Category 5 UTP cable
(1) When the transmission distance is 300m
Index: DG≤1%; DP≤1 °; SNR≥65dB
(2) When the transmission distance is 1200m
Index: DG≤2%; DP≤2 °; SNR≥60dB
3. Optical fiber
Optical fiber is an inevitable choice for long-distance monitoring image transmission. It can provide a transmission distance of hundreds of kilometers, and can provide very good image transmission quality. Using special instruments in the laboratory, the performance indexes of the WTOS-02 S4000 uncompressed digital video transmission equipment of Wuhan MicroPort Optoelectronic Technology Co., Ltd. were tested as follows:
DG≤1%; DP≤1 °; SNR≥70dB
3. Economic analysis
Optical fiber transmission is the most expensive of the three transmission methods. The price of single-mode fiber is generally about 2 yuan per meter per fiber, and the price of the optical transceiver is about 2,000 yuan per image, plus the auxiliary materials in the project. For example, the cost of optical fiber patch cords, optical flanges and optical fiber fusion splicing, etc., makes the cost of optical fiber monitoring network relatively high. However, optical fiber transmission has many outstanding advantages, so in the monitoring network with a transmission distance of more than two or three kilometers, some places with strong environmental interference, and monitoring places with high requirements for quality and reliability still choose fiber as the transmission medium .
Coaxial cables are widely used within a distance of about one or two hundred meters. Using coaxial cables within this range does not require other equipment. It is easy to use and the transmission quality can basically meet the requirements. However, when the transmission distance increases, the coaxial cable is inferior to the twisted pair in cost performance. In general, the market price of 75Ω and -5 coaxial cable is 1.5 yuan / m, the price of coaxial amplifier is 300 yuan / unit; the price of category 5 UTP cable is 0.8 yuan / m, and the price of twisted pair video transmission equipment is 600 yuan / m In the following, the transmission costs of coaxial cables and twisted pairs are compared for the distances of 300m and 600m.
(1) Assume that there are 8 monitoring points in a monitoring network, and each monitoring point is about 300m away from the control center.
When using coaxial cable:
1. 5 yuan / m × 300m × 8 = 3600 yuan
When using twisted pair:
Since each four-way image can share one Category 5 UTP cable, only two Category 5 UTP cables are needed in the network. 0.8 yuan / m × 300m × 2 + 600 yuan / set × 8 = 5280 yuan
(2) Assume that there are 8 monitoring points in a monitoring network, each monitoring point is about 600m away from the control center
When using coaxial cable:
In order to ensure the image quality, a first-level amplifier is added to each image. 1.5 yuan / m × 600m × 8 + 300 yuan / unit × 8 = 9600 yuan
When using twisted pair:
Since each four-way image can share one Category 5 UTP cable, only two Category 5 UTP cables are needed in the network. 0.8 yuan / m × 600m × 2 + 600 yuan / set × 8 = 5760 yuan
It can be seen from the above analysis that as the transmission distance increases, the cost advantage of twisted pair transmission is increasingly shown. If there are other signal transmissions such as control signals in the network, the cost advantage of twisted pair transmission will be more obvious. In addition, when the transmission distance is further increased, such as reaching about 1 km, the coaxial cable transmission has been difficult to meet the transmission quality requirements, and the twisted pair can still ensure better image quality when transmitting about 1.5 km.
4. Engineering proposal
In the actual project, the choice of transmission method needs to be considered in combination with the specific conditions of the project and the project budget.
For the monitoring network with a transmission distance exceeding 2Km, optical fiber and optical transceiver should be considered. If the transmission distance reaches several kilometers or tens of kilometers, only the optical fiber is used. In addition, in some places with strong interference, such as industrial interference and electromagnetic interference, in order to ensure the image quality, it is also recommended to use optical fiber transmission.
The selection of coaxial cable and twisted pair requires more analysis and comparison. Since twisted pair transmission is a newly emerging technology, and the use of coaxial cables has a long history, it is necessary to fully explore the characteristics and advantages of twisted pair transmission to provide convenience and cost savings for engineering design and construction. . When the transmission distance is between 400 meters and 500 meters to 1.5 kilometers, twisted pairs can undoubtedly exert excellent performance and price advantages. For monitoring networks within 200 meters and 300 meters, if the scale is small, it is reasonable to use coaxial cables The cost is lower. However, the actual project will also have its particularity. Even if the distance is very close, if the network is very large, for example, there are hundreds of monitoring points, it will be difficult to use coaxial cable wiring, and twisted pair can make This problem is solved. Furthermore, if the environmental interference is large, if you use twisted pair, you can effectively resist interference and ensure the image transmission quality. In addition, the convenience and flexibility of the twisted pair wiring make some short-distance, dense monitoring networks such as buildings and buildings gradually begin to use it to replace coaxial cables, and are well integrated into the integrated wiring project.
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