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Video image transmission
Send time:2014-06-10 Popularity:
1, An overview of the

  In the monitoring system, monitoring image transmission is a vital link in the whole system, choose what kind of media and transmit images and other equipment control signals will be directly related to the quality and reliability of monitoring and control system. At present, in the monitoring system of the main medium used to transmit image signal are coaxial cable, twisted pair and fibre, and the corresponding transmission equipment are coaxial video amplifier, twisted pair video transmission equipment and optical transceiver. To form a high quality monitoring network, we must understand the characteristics of the three major transmission mode and using the environment, so that in view of the actual engineering needs to take proper transmission medium and equipment.

  2, Coaxial cable and coaxial video amplifier

  About the image transmission, people first, always think of coaxial cable, because earlier coaxial cable is used, is also used for the longest transmission mode. At the same time, the coaxial cable laid with cheaper price, the advantages of convenient, so, usually in a small range of monitoring system, due to the transmission distance is very near, directly using coaxial cable transmission monitoring image on image quality of damage is not big, can meet the practical requirements.

  However, according to the analysis of the characteristics of the coaxial cable itself when the signal in a coaxial cable transmission attenuation and 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, 6 MHZ, and part of the color of the image is in high frequency modulation, in this way, the video signal transmission in the coaxial cable of the signal not only the overall amplitude attenuation, and each frequency component attenuation vary widely, especially the color part of the attenuation is the largest.

  So, coaxial cable is only suitable for close range image signal transmission, when the transmission distance of 200 meters, image quality will be significantly lower, especially the color is pale, have a sense of distortion.

  In engineering practice, in order to extend the transmission distance, to use the coaxial amplifier. Coaxial amplifier for video signal is amplified, and it also can be adjusted through equilibrium of different frequency components respectively for different sizes of compensation, in order to make the receiver output video signal distortion as far as possible. Coaxial amplifier, however, cannot unlimited cascade, generally in a point-to-point system coaxial amplifier cascade can be at most 2 to 3, otherwise unable to guarantee the quality of video transmission, and adjustment is also very difficult. Accordingly, when used in the monitoring system of coaxial cable, in order to ensure good image quality, the transmission distance range are limited to four, about five hundred meters.

  In addition, coaxial cable transmission image signal in the monitoring and control system is still there are some disadvantages:

  1), A coaxial cable is affected by climate change is big, the image quality is affected by a certain;

  2), The coaxial cable of the coarser, wiring is not very convenient when it is applied in intensive monitoring;

  3), Normally only pass the coaxial cable of the video signal, if the system needs, such as audio signal transmission control data at the same time, you need to other wiring;

  4), The coaxial cable of the anti-interference ability is limited, cannot be used in strong jamming environment;

  5),The shortcomings of coaxial amplifier adjustment difficulties still exist.

  3, Twisted pair and twisted pair video transmission equipment

  Due to the coaxial cable of the traditional monitoring system has some disadvantages, especially the transmission distance is restricted, so seek an economic and high quality of transmission distance of the solution is necessary. Early in the transmission distance of more than 5, six hundred m monitoring systems generally use multimode optical fiber and multimode optical transceiver, it solved the problem of long distance transmission, but the system cost has increased a lot, and the construction of the fiber complex, need professional and special equipment. So, for this distance is not far from the monitoring system using optical fiber and optical transceiver or lack of economy.

  Recently, there is a twisted pair video transmission equipment, through the use of such equipment, twisted pair can be applied to monitor the image transmission, it is good to solve the above problem, in the future will be extensively used in monitoring and control system.

  In fact, the use of twisted pair has a long history, the phone use is twisted pair transmission, in many industrial control systems and interfere with larger space and long-distance transmission USES twisted-pair cable, we today is also widely used LAN using twisted pair. Twisted-pair cable is used so widely, because it has strong anti-jamming capability, long transmission distance, easy wiring, low cost and many other advantages. Due to twisted pair also exist large attenuation of signal, so the transmission distance, the frequency of the signal cannot be too high, and high speed signal such as Ethernet is only limited within 100 m. Achieve 6 MHZ bandwidth for video signal, if within the twisted-pair transmission directly, also attenuation is very big, when the transmission distance is about 150 m video signal attenuation curve as shown in the figure below.

  As a result, the video signal on twisted pair to realize long distance transmission, must be amplified and compensation, twisted pair video transmission equipment is to complete this function. Combined with a twisted pair video transceiver equipment, image can be transmitted to a 1 to 2 km, if use trunking, transmission distance, also can multiply and image transmission quality can be comparable to optical transceiver. Twisted-pair and twisted pair video transmission equipment prices are very cheap, not only did not increase the system cost, rather than in distance increases its cost down many compared with coaxial cable. So, monitoring and control system using twisted-pair transmission has obvious advantages:

  1) Transmission distance, high quality. Because in the twisted pair transceiver is installed with advanced processing technology, offers an excellent compensation to the twisted pair of video signal amplitude attenuation as well as the difference between the different frequency attenuation, keep the original image brightness and color and real time, when the transmission distance reach 1 km or more, basic image signal without distortion. If using relay, transmission distance will further.

  2) To improve the utilization of convenient wiring, cables. A pair of ordinary telephone line can be used to transmit video signals. In addition, building buildings widely laying category 5 unshielded twisted pair take a pair of can send a video signal, no additional wiring, even rewiring, five types of cable is easier than the coaxial cable. In addition, a five kind of cable has 4 of twisted pair, if you use a pair of lines transmit video signals, the other a few on the line can also be used to transmit audio signals and control signals, power supply, or other, improve the utilization rate of the cable at the same time to avoid the various signal wiring trouble alone, reduces the project cost.

  3) Strong anti-interference ability. Twisted-pair cable can effectively inhibit common-mode interference, even under strong interference environment, twisted pair has excellent image can be transmitted signal. And, using a cable pairs twisted pair transmission respectively in different signal, each other interference will not occur.

  4) high reliability, easy to use. Using twisted pair transmission video signal, on the front end to access special transmitter, special access to the receiver in the control center. This kind of twisted pair transmission equipment cheap, it is very easy to use, without professional knowledge, also do not have too much *, once installed, long-term and stable work.

  5) price cheap, conveniently. Due to the use of which is widely used ordinary five types of shielded cable or ordinary telephone line and buy easily, and the price is very cheap, to bring great convenience for engineering applications.

  4, Optical fiber and optical transceiver

  Optical fiber and optical transceiver used in monitoring field is mainly to solve two problems: one is the transmission distance, one is the environmental interference. Twisted pair or coaxial cable can only solve the short distance, small scale monitoring image transmission problem, if you need to several kilometers or even hundreds of kilometers of distance image signal transmission requires using optical fiber transmission. For some places strong interference, in addition, in order to not affected by the environmental interference, also want to use optical fiber transmission mode. Because the optical fiber transmission with Eric, large capacity, small is not affected by electromagnetic interference, the external environment, and many other advantages, a piece of optical fiber can transmit need all the signals in the monitoring system, transmission distance can reach hundreds of kilometers. Optical transceiver can provide all the way and multi-channel image interface, you can also provide two-way audio interface, all the way and multiplex various types of two-way data interface (RS232, RS485, Ethernet, etc.), and integrate them into a single fiber transmission. Optical transceiver for monitoring system provides a flexible way of transmission and networking, signal quality is good, high stability. In recent years, due to the rapid development of optical fiber communication technology, optical fiber and optical device prices fell sharply, make optical fiber monitoring system at a much lower cost, so the application of optical fiber and optical transceiver in the monitoring system is more and more popular.

  Optical fiber can be divided into multimode optical fiber and single mode fiber. Multimode fiber because of its high dispersion and attenuation, the maximum transmission distance is not more than 5 km, so, in addition to the previously has been rolling out the multimode fiber place outside, generally no longer used in the process of construction engineering of multimode optical fiber, the main use single mode fiber.

  Optical fiber transmission of control signal optical transceiver is used, it mainly is to implement the action of electricity - light and light - electric conversion. Optical transceiver is divided into analog optical transceiver and digital optical transceiver:

  1) To simulate the optical transceiver

  Analog optical transceiver used the PFM modulation technique for real-time transmission image signal, is one of the currently used. Transmitter to analog video signal after PFM modulation first and then the electric - light switch, light signals to the receiving end, light - electric conversion, then PFM demodulation, restore the video signal. The adoption of PFM modulation technology, the transmission distance can be easily reached around 30 km, the transmission distance with some products can reach 60 km, or even hundreds of kilometers. After transmission and image signal distortion is small, has a high signal to noise ratio and small nonlinear distortion. Through the use of the WDM technology, also can be realized on a single fiber two-way transmission of images and data signal meet the practical needs of monitoring and control engineering. However, the simulation of optical transceiver also has some disadvantages:

  A) Production debugging more difficult;

  B) The multi-channel image transmission on the single fiber is difficult, the performance will decline, the current analog optical transceiver can only do it on the single fiber transmission commonly 4 road image;

  C) As a result of the analog demodulation technology, its stability is not high, with the increase of using time or the change of environmental features, the performance of the optical transceiver also change, some inconvenience for engineering use.

  2) Digital optical transceiver

  Because digital technology compared with traditional simulation technology has obvious advantages in many ways, so as the digital technology replaced the simulation technology in many fields, the digital optical transceiver is also a kind of inevitable trend. At present, there are two main types of optical transceiver digital image technology: one is MPEGII image compress digital optical transceiver, another is compressed digital image optical transceiver.

  Digital optical transceiver image compression using MPEGII image compression technology, it can active images compressed into N x 2 MBPS data flow through standard telecom communication interface transfer or directly through the optical fiber transmission. With the image compression technology, it can greatly reduce the signal transmission bandwidth, so take up less resources can transmit video signal. At the same time, the adoption of N x 2 MBPS standard interface, can use the existing rich channel transmission monitoring images of telecommunication transmission equipment, brought convenient for engineering application. However, image compressed digital optical transceiver also has its inherent shortcomings. The fatal weakness is can't guarantee real-time performance of image transmission. Because it takes time to image compression and decompression, so often on the image transmission time delay 1 ~ 2 s. Therefore, this equipment is only suitable for use in place of real-time demand is not high, is limited by some in the engineering application. In addition, after compressed images will produce a certain amount of distortion, and the price of this optical transceiver is also high.

  The digital image compression, the principle of optical transceiver is the analog video signal after A/D conversion and voice, audio, data and other signal multiplex, again through the optical fiber transmission. It with high data rate to ensure the quality of video signal transmission and real-time performance, due to the bandwidth of the fiber is very large, so this kind of high data rate transmission channels is not to request too high. The compression of digital images of the light transmission end function to provide good image quality (such as wuhan minimally invasive photoelectric technology co., LTD., the compression of digital optical transceiver signal-to-noise ratio greater than 60 db, differential phase distortion is less than 2 °, differential gain distortion is less than 2%), reached the broadcast transmission quality, and image transmission in real time. Due to the digital technology, can use in the equipment has been very mature communication technologies such as multiple connection technology, optical transceiver, etc., to improve the reliability of the equipment, also reduces the cost. The digital image compression optical transceiver advantage embodies in:

  A) using the digital technology, greatly improve the image transmission quality.

  B) the use of digital technology and large scale integrated circuit, ensure the stability and reliability of the equipment, and overcome the disadvantages of analog optical transceiver.

  C) won't produce transmission delay, to ensure the real-time monitoring image;

  D) can be easily to multi-channel image and audio, data and other signal together by a piece of optical fiber transmission, at present, this kind of compressed digital image optical transceiver can be done in a single direction transmission dozens or even hundreds of road images

  Digital image optical transceiver of high technical content, its use in the monitoring project of time is not long, are largely used in the multi-channel image transmission, the main reason is that can provide this kind of optical transceiver manufacturer also is not much, the price is slightly higher than the level of analog optical transceiver. However, due to the digital image optical transceiver is a special digital image compression optical transceiver prominent advantages, coupled with a large number of after use can reduce costs, to simulate the optical transceiver will soon replaced by the digital image optical transceiver.

  5, The conclusion

  Transmit image monitoring signal in addition to the above three main ways, also used in some engineering point to point wireless transmission mode, and on cable television USES multiplex subcarrier multiplexing rf transmission mode. Wireless transmission environment and climate influence is too big, work is not stable, and difficult to install equipments; Multiplexed subcarrier multiplexing rf transmission mode requires more equipment, stability is not high, the poor quality of image, the equipment is also very difficult to install. So, the two devices use very little, also do not recommend users to use. For fiber coaxial cable, twisted pair, and three kinds of transmission mode, user can choose according to actual engineering. In general, the distance within the 2, three hundred m, and no environmental interference, wiring space is large, can consider to use the cable; When the transmission distance within two kilometres, or compact environmental interference, wiring requirements, recommend the use of twisted pair; Distance to several kilometers or more, the optical fiber is the inevitable choice. Engineering practice, of course, many users regardless of distance, the use of optical fiber, all in the same project or in the engineering of close all use twisted pair, this is completely determined by of the practical need of engineering.


  In the PSTN online, using the user's existing telephone line for multimedia (especially) video signal transmission can use several different ways:

  The first is to use cable-modem, with low data rate of h. 263 conference television video compression standard, data flow through dozens of K 28.8 Kbps v. 34 cable-modem PSTN network, transmission CIF or QCIF image 5 ~ 15 frames per second. Now 33.5 Kbps to 56 Kbps Modem has been very popular, this mode of transmission is advantageous to the low rate of video transmission, frame rate can be further improved;

  The second is to use XSDL access, mainly including ASDL (descending rate 1.5 ~ 9 MBPS, uplink rate of 16 ~ 640 KBPS, transmission distance of 5.5 KM), is mainly used for video on demand and video broadcasting; HSDL USES a pair of two pairs of twisted pair, two-way rate is 1.5 ~ 2 MBPS, transmission distance is about 5 km, video conference or two-way video control.

  DDN way:

  DDN is the use of digital channels provide a permanent connection electric circuit, give priority to in order to transmit data signals of digital transmission network. It mainly provides, high speed, high quality point to point and point to multipoint digital dedicated links, so that the leased circuit should be provided to the business. The lines of communication rate is 2.4 ~ 19.2 Kbps, N * 64 Kbps (N = 1 ~ 32). It can also provide VPN business. Post and telecommunications departments in our country has built throughout the country and open the DDN business, communications bandwidth is 64 k ~ 2.048 M (E1), if the user does not have own remote data communication network, can apply to the local post and telecommunications departments for DDN business. By the way, the user can according to their own requirements for bandwidth, video terminals can use g. 703 or v. 35 mouth of multimedia business access DDN network.

  ISDN mode:

  The channels of ISDN is classified into information channel and control channels, information channel and B and H channel, channel is also included for the D channel control channel. B channel bandwidth of 64 KBPS is used to transmit voice, video, data, or bit stream images; H channel bandwidth is 384 KBPS or 1536 KBPS and 1920 KBPS, used to transmit high speed data or high flow image; For 16 KBPS D channel bandwidth for transmission control signal to control the B channel (H) channel calls, sometimes D channel can also be used to transmit data at low speed. ISDN user/network interface has two kinds of structure: the basic rate interface (BRI) and the base rate interface (PRI), the basic rate interface is ordinary users of existing network line as ISDN subscriber line interface, it consists of a 2 B and D channels, become 2 B + D, transfer rate of 144 KBPS. PRI interface is composed of 30 B channel and a D channel become 30 B + D, transfer rate of 2 MBPS is equivalent to a E1. In ISDN BRI transfer audio and video signals when there are three options to choose from: one is to focus on the images and sound a B (64 KBPS) in the transmission channel, such as image with 48 KBPS, voice with 16 KBPS; 2 it is to use two B channel, a transmit images (64 KBPS), another voice transmission (64 KBPS); 3 it is to mix two B channel is used as a 128 KBPS link, image with 112 KBPS, voice with 16 KBPS. ISDN interface can be achieved by dedicated ISDN communication card will video monitoring terminal access multimedia.

  Fibre channel:

  High quality of optical fiber channel transmission, channel stability is a well known advantages. Optical transceiver including PCM group after receiving equipment, the second group, group of three times, four times group and dance group of multiplex equipment. The PCM group multiplex equipment provided to the user in accordance with g. 703 standard 64 KBPS interface, low rate of video data can be transmitted directly into the PCM terminal. Multiplex equipment directly to the user to provide a accord with g. 703 standard E1 interface, the rate of 2.048 Mbps transmission channel bandwidth, the g. 703 / v. 35 E1 communication card will video monitoring terminal access optical fiber links.

  Wireless transmission:

  Wireless transmission is mainly index of radio and wireless spread spectrum transmission, at present most enterprises have their own wireless private network, wireless private network can support more remote access, they can generally provide 64 k ~ 42 MBPS bandwidth channel. At the same time, these wireless private network construction planning, mostly has set aside for the bandwidth of the transmission of video image, as long as the video terminal to access IP way and with the original data and voice services for seamless connection, multimedia communication can be realized.

  VSAT satellite link:

  Satellite transmission system wide geographical coverage, less construction, is can't replace other transmission system, especially the VSAT station to the mobile, with mobility, is an important means of military defense department communication, very small satellite ground station is also the main means of communication in remote areas, general users can to satellite operating company satellite lines, such as the serial data is converted to 64 KBPS v. 35 interface video connections.


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