1. Overview
In the monitoring system, the transmission of the monitoring image is a crucial link of the whole system, and the choice of media and equipment to transmit images and other control signals will be directly related to the quality and reliability of the monitoring system. At present, the media used to transmit image signals in the monitoring system mainly include coaxial cables, twisted pair cables and optical fibers, and the corresponding transmission equipment are coaxial video amplifiers, twisted pair video transmission equipment and optical transceivers. To build a high-quality monitoring network, it is necessary to understand the characteristics and usage environment of these three main transmission methods, so as to adopt appropriate transmission media and equipment for actual engineering needs.
2. Coaxial cable and coaxial video amplifier
When it comes to image transmission, people always think of coaxial cables first, because coaxial cables are the earliest and longest-used transmission methods. At the same time, coaxial cable has the advantages of cheaper price and more convenient laying, so generally in a small range of monitoring systems, due to the very close transmission distance, the use of coaxial cable to directly transmit monitoring images does not damage the image quality to the image quality, which 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 within the coaxial cable is related to the transmission distance and the frequency of the signal itself. Generally speaking, 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 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 very different, especially the color part is the largest.
Therefore, coaxial cables are only suitable for transmitting image signals at close range, and when the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the colors become dull and distorted.
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 sizes through equalization adjustment, so that the distortion of the video signal output at the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded indefinitely, generally only cascading up to 2 to 3 coaxial amplifiers in a point-to-point system, otherwise the quality of video transmission cannot be guaranteed, and it is also difficult to adjust. Therefore, when using coaxial cables in monitoring systems, in order to ensure good image quality, the transmission distance range is generally limited to about four or five hundred meters.
In addition, there are some disadvantages of coaxial cable 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 a certain extent;
2) The coaxial cable is thick, which is not convenient to wiring in intensive monitoring applications;
3) Coaxial cables can generally only transmit video signals, if the system needs to transmit control data, audio and other signals at the same time, it needs to be wired separately;
4) The anti-interference ability of coaxial cables is limited and cannot be used in strong interference environments;
5) Coaxial amplifiers also have the disadvantage of difficulty in adjustment.
3. Twisted pair and twisted pair video transmission equipment
Due to the disadvantages of traditional coaxial cable monitoring systems, especially the limited transmission distance. In the early days, multi-mode optical fibers and multi-mode optical transceivers were generally used in monitoring systems with transmission distances of more than five or six hundred meters, which solved the problem of long-distance transmission, but the cost of the system increased a lot.
Recently, a twisted pair video transmission device has emerged, through the use of such equipment, twisted pair can be applied to surveillance image transmission, which solves the above problem well. However, like coaxial cables, this method faces the same interference and maintenance problems, and the maturity of the technology has not yet been widely used.
4. Optical fiber and optical transceiver
The application of optical fibers and optical transceivers 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 cable can only solve the problem of monitoring image transmission over short distances and small ranges, and if the image signal needs to be transmitted over a distance of several kilometers or even hundreds of kilometers, it needs to be transmitted by optical fiber. In addition, for some places with super interference, in order not to be affected by environmental interference, optical fiber transmission method should also be used. Because optical fiber has many advantages such as wide transmission bandwidth, large capacity, no electromagnetic interference, and less influence by 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 can provide one and multiple image interfaces, as well as bidirectional audio interfaces, one and multiple types of bidirectional data interfaces (including RS232, RS485, Ethernet, etc.), and integrate them into a single optical fiber for transmission. The optical transceiver provides flexible transmission and networking methods 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, which has greatly reduced the cost of optical fiber monitoring systems, so the application of optical fiber and optical transceivers in monitoring systems has become more and more popular.
Optical fibers are divided into two types: multimode fiber and singlemode fiber. Due to the large dispersion and attenuation of multimode optical fibers, their maximum transmission distance generally cannot exceed 5Km, so except for places where multimode optical fibers have been laid before, multimode optical fibers are generally no longer used in new projects, but mainly use single-mode optical fibers.
The transmission of monitoring signals in optical fiber uses optical transceivers, which mainly realizes electricity-optical and photo-electric conversion.
Since digital technology has obvious advantages compared with traditional analog technology in many aspects, just as digital technology has replaced analog technology in many fields, the digitization of optical transceivers is also an inevitable trend. At present, there are two main technical ways of digital image optical transceivers: one is MPEG II image compression digital optical transceiver, and the other is non-compressed digital image optical transceiver.
Image compression digital optical transceivers generally use MPEG II image compression technology, which can compress the active image into N×2Mbps data streams transmitted through standard telecommunications interfaces or directly through optical fibers. Due to the use of image compression technology, it can greatly reduce the signal transmission bandwidth to facilitate the transmission of image signals using fewer resources. At the same time, due to the use of the N×2Mbps standard interface, the surveillance image can be transmitted using the rich channels of existing telecommunications transmission equipment, which brings convenience to engineering applications. However, image compression digital optical transceivers also have their inherent drawbacks. Its fatal weakness is that it cannot guarantee the real-time transmission of images. Because image compression and decompression take a certain amount of time, there is generally a delay of 1-2s for the transmitted image. Therefore, this equipment is only suitable for use in places with low real-time requirements, and is subject to some restrictions in engineering use. In addition, the image will produce a certain distortion after compression, and the price of this optical transceiver is also high.
The principle of uncompressed digital image optical transceiver is to convert analog video signals into A/D and then multiplex them with voice, audio, data and other signals, and then transmit them through optical fibers. It uses high data rates to ensure the transmission quality and real-time performance of video signals, and because the bandwidth of optical fibers is very large, this high data rate does not put forward excessive requirements for transmission channels. The uncompressed digital image transceiver can provide good image transmission quality (signal-to-noise ratio greater than 60dB, differential phase distortion less than 2°, differential gain distortion less than 2%), achieve broadcast-level transmission quality, and the image transmission is fully real-time. Due to the use of digital technology, mature communication technologies such as multiplexing technology, optical transceiver technology, etc. can be used in the equipment, which improves the reliability of the equipment and reduces the cost. The advantages of non-compressed digital image optical transceivers are reflected in:
a) The use of digital technology greatly improves the quality of image transmission;
b) The use of digital technology and large-scale integrated circuits ensures the stability and reliability of equipment work and overcomes the shortcomings of analog optical transceivers.
c) There will be no transmission delay, which ensures the real-time performance of the monitoring image;
d) It can easily integrate multiple images and audio, data switching, telephone, network and other signals through a single optical fiber (such as Tianwei Telecom Technology Company's one-fiber communication multi-service digital optical transceiver).
The technical content of digital image optical transceivers is high, and their use time in monitoring engineering is not long, and most of them are currently used in multi-channel image transmission, mainly because there are not many manufacturers that can provide this kind of optical transceiver, and the price is slightly higher than that of analog optical transceivers. However, due to the outstanding advantages of digital image optical transceivers, especially non-compressed digital image optical transceivers, coupled with the cost reduction after heavy use, analog optical transceivers will soon be replaced by digital image optical transceivers.
5. Conclusion
In addition to the three main methods introduced above, some projects also use point-to-point wireless transmission methods and multi-channel subcarrier multiplexed radio frequency transmission methods used on cable television. Wireless transmission is too affected by the environment and climate, the work is unstable, and the equipment installation and adjustment are difficult; The multi-channel subcarrier multiplexing RF transmission method requires a lot of equipment, low stability, poor image quality, and difficult equipment installation and adjustment. So, both devices are rarely used and are not recommended for users. For the three transmission methods of coaxial cable, twisted pair and optical fiber, users can choose according to the actual situation of the project. Generally speaking, cables can be considered in places where the distance is within two or three hundred meters, and there is no environmental interference and large wiring space; When the transmission distance is within two kilometers, or in places with large environmental interference and compact wiring requirements, it is recommended to use twisted pair cables; When the distance reaches a few kilometers or more, fiber optics are the inevitable choice. Of course, in the actual project, many users use optical fiber in the same project, or twisted pair cables in the same project, regardless of distance, which is completely determined by the actual needs of the project.
