Monitoring image transmission method
1 Overview 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. To build a high-quality monitoring network, it is necessary to understand the characteristics and use environment of these three main transmission methods, so as to adopt the appropriate transmission medium and equipment for the actual project.
2 Coaxial cable and coaxial video amplifier When mentioning image transmission, people always think of coaxial cable first, because coaxial cable is used earlier and is also the longest used transmission method. At the same time, the coaxial cable has the advantages of cheaper price and easier laying. Therefore, in a small-scale monitoring system, because the transmission distance is very short, the use of coaxial cable to directly transmit the monitoring image does not damage the image quality. , 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. 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. In this way, when the video signal is transmitted in the coaxial cable, not only the overall amplitude of the signal is attenuated, but the attenuation of each frequency component varies greatly. , Especially the color part has the greatest 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, 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) Coaxial cable is relatively thick, which is not convenient for wiring in intensive monitoring applications;
3) Coaxial cables 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) The coaxial cable has limited anti-interference ability and cannot be used in a strong interference environment;
5). The coaxial amplifier also has the disadvantage of difficult adjustment.
3 Twisted Pair and Twisted Pair Video Transmission Equipment Because traditional coaxial cable monitoring systems have some shortcomings, especially the transmission distance is limited, it is necessary to find an economical, high transmission quality, long transmission distance solution. In the early days, multi-mode optical fibers and multi-mode optical transceivers were generally used in monitoring systems with transmission distances exceeding five or six hundred meters. Although this solved the problem of long-distance transmission, the cost of the system increased a lot, and the construction of optical fibers was complicated and required Professionals and special equipment. Therefore, it is still not economical to use optical fibers and optical transceivers for such monitoring systems that are not too far away.
Recently, a kind of twisted pair video transmission equipment has appeared. By using this kind of equipment, twisted pair cable can be used for monitoring image transmission. It solves the above problems very well and will surely be used in future monitoring systems. Used heavily.
In fact, twisted pair has been used for a long time. Telephone transmission uses twisted pair. Twisted pair is used in many industrial control systems and in places with large interference and long-distance transmission. The LAN we use today is widely used. Twisted pairs are also used. 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. Due to the large attenuation of the signal of the twisted pair, the frequency of the signal cannot be too high when the transmission distance is long, and high-speed signals such as Ethernet can only be limited to 100m. For the video signal, the bandwidth reaches 6MHz. If it is transmitted directly in the twisted pair, it will also be greatly attenuated. When the transmission distance is about 150m, the attenuation curve of the video signal is shown in the following figure.
Therefore, in order to achieve long-distance transmission of video signals on twisted pairs, it must be amplified and compensated. Twisted pair video transmission equipment accomplishes this function. With the addition of a pair of twisted pair video receiving and sending equipment, the image can be transmitted to 1 to 2km. If the relay method is adopted, the transmission distance can also be doubled, and the quality of the transmitted image can be comparable to that of the optical transceiver. 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, in the monitoring system
Twisted pair transmission 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 and too much work. Once installed, it works stably for a long time.
5) The price is cheap and the materials are 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.
4 Optical fiber and optical transceiver 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 can provide one-way and multi-way image interfaces, as well as two-way audio interfaces, one-way and multi-way various types of two-way data interfaces (including RS232, RS485, Ethernet, etc.), and integrate them into a single optical fiber for transmission. The optical transceiver provides a flexible transmission and networking method 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 prices of optical fibers and optical devices have fallen 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 has become more and more popular.
Optical fiber is divided into two types of multimode fiber and single mode fiber. Due to the large dispersion and attenuation, the maximum transmission distance of multimode optical fiber generally cannot exceed 5Km. Therefore, except for the place where the multimode optical fiber has been paved previously, the multimode optical fiber is generally not used in new construction projects. Use single-mode fiber.
The transmission of monitoring signals in optical fibers requires the use of optical transceivers, whose main function is to realize electrical-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 adopts PFM modulation technology to transmit image signals in real time, which is one of the most widely used currently. The transmitting end performs PFM modulation on the analog video signal first, and then performs electrical-optical conversion. After the optical signal is transmitted to the receiving end, it performs optical-electrical conversion, and then performs PFM demodulation to recover the video signal. Due to the use of PFM modulation technology, its transmission distance can easily reach about 30 Km, and some products can reach a transmission distance of 60 Km, or even hundreds of kilometers. In addition, the image signal has little distortion after transmission, has a high signal-to-noise ratio and little nonlinear distortion. Through the use of wavelength division multiplexing technology, two-way transmission of image and data signals can also be achieved on a single fiber to meet the actual needs of monitoring projects. However, this analog optical transceiver also has some disadvantages:
a) Production debugging is more difficult;
b) It is difficult to realize multi-channel image transmission with a single fiber, and the performance will be degraded. At present, such analog optical transceivers can generally only transmit 4 channels of images on a single fiber;
c) Because the analog modulation and demodulation technology is used, its stability is not high enough. With the increase of use time or the change of environmental characteristics, the performance of the optical transceiver will also change, which will bring some inconvenience to the project.
2) Digital optical transceivers have obvious advantages in many aspects compared with traditional analog technologies. Just as digital technologies replace analog technologies in many fields, the digitization of optical transceivers is also an inevitable trend. At present, there are mainly two technical methods for 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 moving images into a data stream of N × 2Mbps and transmit them 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 to facilitate the transmission of image signals with less resources. At the same time, since the standard interface of N × 2Mbps is adopted, the rich channel of the existing telecommunication transmission equipment can be used to transmit the monitoring image, which brings convenience to the engineering application. However, image compression digital optical transceivers also have their inherent shortcomings. Its fatal weakness is that it cannot guarantee the real-time nature of image transmission. Because image compression and decompression require a certain amount of time, it generally produces a delay of 1 to 2 seconds to the transmitted image. Therefore, this kind of equipment is only suitable for use in places that do not require high real-time performance, and is limited in engineering use. In addition, after compression, the image will produce some distortion, and the price of this optical transceiver is also high.
The principle of the uncompressed digital image optical transceiver is to A / D convert the analog video signal and multiplex the voice, audio, data and other signals, and then transmit it through the optical fiber. It uses a high data rate to ensure the transmission quality and real-time performance of the video signal. Because the bandwidth of the optical fiber is very large, this high data rate does not put too high requirements on the transmission channel. The uncompressed digital image optical transceiver can provide very good image transmission quality (such as Wuhan Weichuang Optoelectronics Technology Co., Ltd.'s uncompressed digital optical transceiver's signal-to-noise ratio is greater than 60dB, differential phase distortion is less than 2 °, differential gain distortion is less than 2%) The broadcast-quality transmission quality is achieved, and the image transmission is fully real-time. Due to the use of digital technology, already mature communication technologies such as multiplexing technology and optical transceiver technology can be used in the device, which improves the reliability of the device and reduces costs. The advantages of the uncompressed digital image optical transceiver 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 ensure the stability and reliability of the equipment and overcome the disadvantages of analog optical transceivers;
c) There will be no transmission delay, ensuring the real-time nature of the monitoring image;
d) It can easily integrate multiple channels of images, audio, data and other signals together to transmit through a single fiber. At present, this uncompressed digital image optical transceiver can achieve dozens of channels in one direction, even on The technical content of the 100-channel image digital image optical transceiver is high, and its use time in the monitoring project is not long. At present, most of them are used in multi-channel image transmission. The main reason is that there are not many manufacturers that can provide this optical transceiver. The price is slightly higher than that of the analog optical transceiver. However, due to the outstanding advantages of digital image optical transceivers, especially non-compressed digital image optical transceivers, coupled with a large number of uses will reduce costs, analog optical transceivers will soon be replaced by digital image optical transceivers.
5 Concluding remarks In addition to the three main methods described above, the point-to-point wireless transmission method and the multi-channel subcarrier multiplexing radio frequency transmission method used on cable TV are also used in some projects. The wireless transmission is greatly affected by the environment and climate, the work is unstable, and the installation and adjustment of the equipment is difficult; the multi-channel subcarrier multiplexing radio frequency transmission method requires more equipment, the stability is not high, the image quality is poor, and the equipment installation and adjustment are also very difficult. Therefore, these two 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, if the distance is within two or three hundred meters, and there is no environmental interference, and the wiring space is large, you can 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 several kilometers or more, the optical fiber is the inevitable choice. Of course, in practice, many users use fiber optic cables in the same project regardless of distance, or use twisted pair cables in projects that are close to each other. This is entirely determined by the actual needs of the project.
Telephone line method:
On the PSTN network, the user's existing telephone line can be used for multimedia (especially video signal) transmission in several different ways:
The first is to use MODEM access, using the low data rate H.263 conference TV video compression standard, the tens of K data stream through 28.8Kbps V.34 MODEM access PSTN network, transmission CIF, QCIF 5 ~ per second 15 frames of images. At present, Modems from 33.5Kbps to 56Kbps have become very popular. This transmission method is conducive to low-rate video transmission, and the frame rate can be further increased;
The second is to use XSDL access, mainly including ASDL (downlink rate 1.5 ~ 9Mbps, upstream rate 16 ~ 640Kbps, transmission distance 5.5KM), mainly used for video on demand and video broadcasting; HSDL uses a pair of two pairs of twisted pair, bidirectional The rate is 1.5 ~ 2Mbps, the transmission distance is about 5KM, can be used for video conference or two-way video control.
DDN method:
DDN is a digital transmission network that uses digital channels to provide semi-permanent connection circuits and mainly transmits data signals. It mainly provides medium and high speed, high-quality point-to-point and point-to-multipoint digital dedicated links to provide users with leased circuit services. The communication rate of its line is 2.4 ~ 19.2Kbps, N × 64Kbps (N = 1 ~ 32). It can also provide VPN services. China's post and telecommunications department has built and opened DDN services nationwide, with a communication bandwidth of 64K to 2.048M (E1). If users do not have their own remote data communication network, they can apply for DDN services from the local post and telecommunications department. In this way, users can apply for bandwidth according to their own requirements. Video terminals can use G.703 or V.35 ports to connect multimedia services to the DDN network.
ISDN method:
The channel type of ISDN is divided into information channel and control channel, and the information channel includes B channel and H channel, and the control channel is D channel. B channel bandwidth is 64Kbps for transmitting various voice, data or bit stream images; H channel bandwidth is 384Kbps or 1536Kbps or 1920Kbps for transmitting high-rate data or high bit stream images; D channel bandwidth is 16Kbps for transmitting control signals To control calls on the B channel (H channel), sometimes the D channel can also be used to transmit low-speed data. The ISDN user / network interface has two structures: basic rate interface (BRI) and group rate interface (PRI). The basic rate interface is an interface that uses ordinary subscriber lines in the existing telephone network as ISDN subscriber lines. It is composed of 2 B and one D channel, which becomes 2B + D port, and the transmission rate is 144Kbps; PRI interface is composed of 30 B channels and one D channel, which becomes 30B + D port, and the transmission rate of 2Mbps is equivalent to an E1 port . When transmitting audio and video signals in ISDN's BRI, there are three options to choose from: one is to focus the image and sound on a B channel (64Kbps) for transmission, such as 48Kbps for images and 16Kbps for sound; two is to use two B Channel, one transmits image (64Kbps), the other transmits sound (64Kbps); the third is to mix two B channels as a 128Kbps line, the image uses 112Kbps, and the sound uses 16Kbps. The ISDN interface can access the video multimedia monitoring terminal through a dedicated ISDN communication card.
Fibre Channel:
Fiber channel transmission quality is high, and channel stability is a well-known advantage. The optical communication terminal includes PCM group multiplexing equipment, secondary group, tertiary group, quaternary group and other hop group multiplexing equipment. Among them, the PCM group multiplexing device provides users with a 64Kbps interface conforming to the G.703 standard, which can directly send low-rate video data to the PCM terminal for transmission. The multiplexing equipment directly provides users with an E1 interface conforming to the G.703 standard with a transmission channel bandwidth of 2.048Mbps. The video monitoring terminal is connected to the optical fiber line via the G.703 / V.35 E1 communication card.
Wireless transmission:
Wireless transmission is mainly exponential transmission radio and wireless spread spectrum transmission. At present, most enterprise units have their own wireless private network. The wireless private network can support multi-point remote access, and they can generally provide a channel bandwidth of 64K ~ 42Mbps. At the same time, most of these wireless private networks have been reserved for the transmission of video images during the construction planning. As long as the video terminal is accessed by IP and seamlessly connected to the original data and voice services, multimedia can be achieved Communication.
VSAT satellite line:
The satellite transmission system covers a wide area and has a small amount of construction. It is irreplaceable by other transmission systems. Especially for mobile VSAT stations, it is mobile and is an important means of communication for the military defense department. Satellite small-caliber ground stations are also in remote areas. As the main communication method, general users can rent satellite lines from satellite operating companies, such as converting 64Kbps serial data to V.35 interfaces to establish video connections.
Frequently Asked Questions for Surveillance System Frequently Asked Questions for Surveillance System-Splitter
1. The power supply is not working properly, causing the splitter to lock. Replace the power supply.
2. The video cable connected to the BNC connector has poor contact, causing the screen to jump.
3. As a result of the misprogramming, the splitter works in chaos and resets.
4. When the video is used, the wrong playback port is connected and cannot be played back.
5. Using simplex splitter can only record but not playback. Duplex, half-duplex.
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