In the current security industry, the surveillance image transmission method has always been a concern of people. This article gives you an in-depth analysis of the monitoring image transmission method, and hopes to help everyone.
1. Overview In the monitoring system, the transmission of surveillance images is a vital part of the whole system. The choice of which medium and equipment to transmit images and other control signals will directly affect the quality and reliability of the monitoring system. At present, the medium used for transmitting image signals in the monitoring system mainly includes a coaxial cable, a twisted pair cable and an optical fiber, and the corresponding transmission devices are a coaxial video amplifier, a twisted pair video transmission device, and an optical transceiver. In order 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. When the coaxial cable and the coaxial video amplifier lift up the image transmission, people will always think of the coaxial cable first, because the coaxial cable is used earlier and is the longest transmission method. At the same time, coaxial cable has the advantages of cheaper price and convenient laying. Therefore, in a small-scale monitoring system, because the transmission distance is very close, direct transmission of the monitoring image using the coaxial cable has little damage to the image quality. Can meet the actual requirements.
However, according to the analysis of the characteristics of the coaxial cable itself, the attenuation of the signal as it travels within 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 portion 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 greatly different. In particular, the color portion is most attenuated.
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 becomes dim and has a sense of distortion.
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 adjustments by equalization adjustment, so that the distortion of the video signal outputted by the receiving end is as small as possible. However, coaxial amplifiers cannot be cascaded without limitation. Generally, a coaxial amplifier can only cascade up to 2 to 3 in a point-to-point system. Otherwise, the quality of video transmission cannot be guaranteed and it is difficult to adjust. Therefore, when using a coaxial cable in a monitoring system, in order to ensure good image quality, the transmission distance is generally limited to about four or five hundred meters.
In addition, coaxial cable has some disadvantages in 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 some extent;
2) The coaxial cable is thicker, and the wiring is not convenient in dense monitoring applications;
3) Coaxial cable 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 applied to strong interference environment;
5) Coaxial amplifiers also have the disadvantage of difficulty in adjustment.
3. Twisted pair and twisted pair video transmission equipment Due to the shortcomings of the traditional coaxial cable monitoring system, the transmission distance is limited. In the early days, multimode fiber and multimode optical transceivers were generally used in monitoring systems with transmission distances exceeding five or six hundred meters, which solved the problem of long-distance transmission, but the system cost increased a lot.
Recently, a twisted pair video transmission device has emerged, by which a twisted pair cable can be applied to monitor image transmission, which solves the above problems well. However, like coaxial cable, this approach also faces the same interference and maintenance problems, and the maturity of the technology, there is not a lot of applications.
4, fiber optic and optical transceiver fiber and optical transceiver applications in the monitoring field is mainly to solve two problems: one is the transmission distance, one is the environmental interference. Twisted pair and coaxial cable can only solve the problem of surveillance image transmission in short distance and small range. If it is necessary to transmit image signals of several kilometers or even hundreds of kilometers, optical transmission is required. In addition, for some super-interfering places, in order to be free from environmental interference, optical fiber transmission is also required. Because the optical fiber has many advantages such as wide transmission bandwidth, large capacity, no electromagnetic interference, and little influence from the external environment, one fiber can transmit all the signals needed in the monitoring system, and the transmission distance can reach hundreds of kilometers. The optical transceiver can provide one-way and multi-channel image interfaces, and can also provide two-way audio interface, one-way and multiple-way various types of bidirectional 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 price of optical fiber and optical devices has dropped rapidly, and the cost of optical fiber monitoring systems has been greatly reduced. Therefore, the application of optical fibers and optical transceivers in monitoring systems is becoming more and more popular.
The optical fiber is divided into two types: multimode fiber and single mode fiber. Due to the large dispersion and attenuation of multimode fiber, the maximum transmission distance can't exceed 5Km. Therefore, in addition to the place where multimode fiber has been laid out before, multimode fiber is generally not used in new construction, but mainly Use single mode fiber.
Optical transmission is used to transmit the monitoring signal in the optical fiber. Its main function is to realize electro-optical and optical-electrical conversion. Optical transceivers are divided into analog optical transceivers and digital optical transceivers:
1) Analog optical transceiver The analog optical transceiver uses PFM modulation technology to transmit image signals in real time, which is currently used more. The transmitting end first performs PFM modulation on the analog video signal, and then performs electro-optical conversion. After the optical signal is transmitted to the receiving end, optical-electrical conversion is performed, and then PFM demodulation is performed to recover the video signal. Thanks to the PFM modulation technology, the transmission distance can easily reach 30 Km, and some products can reach 60 Km or even hundreds of kilometers. Moreover, the image signal has little distortion after transmission, and has a high signal-to-noise ratio and a small nonlinear distortion. By using wavelength division multiplexing technology, it is also possible to realize bidirectional transmission of image and data signals on one optical fiber to meet the actual needs of monitoring engineering.
2) Digital optical transceivers Because digital technology has obvious advantages in many aspects compared with traditional analog technologies, just as digital technology has replaced analog technology in many fields, the digitization of optical transceivers is also an inevitable trend. At present, digital image optical transceivers mainly have two technical methods: one is MPEG II image compression digital optical transceiver, and the other is uncompressed digital image optical transceiver.
Image compression digital optical transceivers generally use MPEG II image compression technology, which can compress moving images into N × 2 Mbps data streams through standard telecommunication communication interfaces or directly through optical fibers. Due to the use of image compression technology, it can greatly reduce the signal transmission bandwidth, so that it can transmit image signals with less resources. At the same time, due to the adoption of the N x 2 Mbps standard interface, it is possible to transmit surveillance images using the rich channels of the existing telecommunication transmission equipment, which brings convenience to engineering applications. However, image compression digital optical transceivers also have their inherent disadvantages. Its fatal weakness is that the real-time performance of image transmission cannot be guaranteed. Since image compression and decompression require a certain amount of time, a delay of 1~2S is generally generated for the transmitted image. Therefore, this kind of equipment is only suitable for use in places where the real-time performance is not high, and there are some restrictions on the use of the project. In addition, the image will produce a certain distortion after compression, and the price of such an optical transceiver is also high.
The principle of the uncompressed digital image optical transceiver is to perform analog A/D conversion on the analog video signal and multiplex the signals such as voice, audio, and data, and then transmit through the optical fiber. It uses high data rate to ensure the transmission quality and real-time performance of the video signal. Because of the very large bandwidth of the optical fiber, this high data rate does not impose excessive requirements on the transmission channel. The uncompressed digital image optical 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-quality transmission quality, and image transmission is Fully real time. Due to the adoption of digital technology, the already mature communication technologies such as multiplexing technology and optical transceiver technology can be utilized in the device, thereby improving the reliability of the device and reducing the cost. The advantages of uncompressed digital image optical transceivers are:
a) The use of digital technology has greatly improved the image transmission quality;
b) The use of digital technology and large-scale integrated circuits ensures the stability and reliability of the equipment, and overcomes the shortcomings of analog optical transceivers;
c) no transmission delay will occur, ensuring the real-time performance of the surveillance image;
d) It is convenient to integrate multiple signals, audio, data and other signals into one optical fiber. Currently, this uncompressed digital image optical transceiver can transmit dozens of channels or even in one direction. Hundreds of images (such as Yingqiao Intelligent Systems Co., Ltd.'s uncompressed digital optical transceiver can transmit 64 images on a single fiber).
The digital image optical transceiver has high technical content, and its use time in monitoring engineering is not long. At present, it is mostly used in multi-channel image transmission. The main reason is that there are not many manufacturers that can provide such optical transceivers at present, and the price is relatively simulated. The optical transceiver is also slightly higher. However, due to the outstanding advantages of digital image optical transceivers, especially uncompressed digital image optical transceivers, and the increased cost after extensive use, analog optical transceivers will soon be replaced by digital image optical transceivers.
5. Concluding remarks Transferring image monitoring signals In addition to the three main methods described above, some projects use point-to-point wireless transmission methods and multi-channel subcarrier multiplexed RF transmission methods used on cable television. Wireless transmission is affected by environment and climate too much, work is unstable, and equipment installation and adjustment is difficult. Multi-channel subcarrier multiplexed RF transmission requires more equipment, low stability, poor image quality, and equipment installation adjustment. difficult. Therefore, these two devices are used very little and are not recommended for users. For coaxial cable, twisted pair and fiber transmission, users can choose according to the actual situation of the project. Generally speaking, if the distance is within two or three hundred meters, and there is no environment interference or wiring space, consider using cables; when the transmission distance is within two kilometers, or where the environmental interference is large and the wiring requirements are compact, it is recommended to use Twisted pair; when the distance reaches a few kilometers or more, fiber optics is an inevitable choice. Of course, in engineering practice, many users use fiber optics in the same project regardless of distance, or use twisted pair cables in projects with close distances, which is completely determined by the actual needs of the project.
Bread basket,stainless wire basket,stainless wire bread basket,Metal Wire Bread Basket,steel bread basket,etc. we offered that you can trust. Welcome to do business with us.
Kichen Rack is made of high quality 304 stainless steel, This kind of material steel luxury, never rust, resist corruption, easily clean, safe, healthy and durable. Prevent rust or chemicals from contaminating food and damaging health
stainless wire basket,stainless wire bread basket,Metal Wire Bread Basket,steel bread basket,versatile basket
Shenzhen Lanejoy Technology Co.,LTD , https://www.grill-grid.com