Analysis and Comparison of Monitoring Image Transmission Methods

In the monitoring system, the transmission of monitoring images is a crucial link in the whole 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 cables, twisted pairs and optical fibers, and the corresponding transmission equipment are coaxial video amplifiers, twisted pair video transmission equipment and optical transceivers.

Coaxial cable is an earlier and longest-used transmission method. Later, due to the need for long-distance and large-scale image monitoring and the increasing demand for monitoring image quality, optical fiber began to be used in large quantities in monitoring networks to transmit image signals. As for the use of twisted pair in image monitoring networks, it is a recent thing. Its appearance mainly solves two problems well: on the one hand, it solves the problem of high-quality image signal transmission within a distance range of 200 meters to 2000 meters, because coaxial cable transmission is difficult to meet the requirements within this distance range and optical fiber transmission is not very economical; on the other hand, it solves the wiring problem of large-scale intensive monitoring networks. The size and flexibility of twisted pair overcome the wiring difficulties when coaxial cable is used in large quantities. Of course, twisted pair also has the advantages of strong anti-common mode interference ability and low price. It is precisely because twisted pair solves these problems that have long plagued people that its application in monitoring networks immediately attracted widespread attention in the industry. In a relatively short period of time, it has been widely used in engineering practice and has achieved good application results.

Each monitoring project has its own characteristics and particularities, so when setting up a monitoring network, it is necessary to fully consider these specific situations and choose the most appropriate image and signal transmission method. Given that coaxial cable, twisted pair and optical fiber are the three most widely used transmission media in monitoring systems, we can analyze and compare them from several aspects.

1. Characteristics and transmission characteristics analysis

1. Coaxial cable

Generally, in a small-scale monitoring system, since the transmission distance is very short, using coaxial cable to directly transmit monitoring images will not cause much damage to the image quality and can meet the actual requirements. However, according to the analysis of the characteristics of the coaxial cable itself, when the signal is transmitted in the coaxial cable, the attenuation it suffers 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, coaxial cable is only suitable for short-distance transmission of image signals. When the transmission distance reaches about 200 meters, the image quality will be significantly reduced, especially the color becomes dim and distorted.

In actual engineering, in order to extend the transmission distance, a coaxial amplifier is used. The coaxial amplifier has a certain amplification effect on the video signal, and can also compensate for different frequency components in different amounts through equalization adjustment to minimize the distortion of the video signal output by the receiving end. However, coaxial amplifiers cannot be cascaded indefinitely. Generally, in a point-to-point system, only 2 to 3 coaxial amplifiers can be cascaded at most, otherwise the video transmission quality cannot be guaranteed, and it is also difficult to adjust. Therefore, when using coaxial cables in monitoring systems, in order to ensure better image quality, the transmission distance is generally limited to about four or five hundred meters.

In addition, there are some disadvantages in transmitting image signals through coaxial cables in monitoring systems:

1) The coaxial cable itself is greatly affected by climate change, and the image quality is affected to a certain extent;

2) Coaxial cables are relatively thick, which makes wiring in intensive monitoring applications inconvenient;

3) Coaxial cable can generally 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 used in strong interference environments;

5) Coaxial amplifiers also have the disadvantage of being difficult to adjust.

2. Twisted pair

The use of twisted pair cables has a long history. They are used in many industrial control systems, places with large interference, and long-distance transmission. The local area networks that we widely use today also use twisted pair cables. The reason why twisted pair cables are so widely used is that they have many advantages such as strong anti-interference ability, long transmission distance, easy wiring, and low price. There is also a large attenuation of the signal of the twisted pair cable. If the video signal is transmitted directly in the twisted pair cable, it will also be greatly attenuated. Therefore, if the video signal is to be transmitted over a long distance on the twisted pair cable, it must be amplified and compensated. The twisted pair video transmission equipment completes this function. With a pair of twisted pair video transceiver equipment, the image can be transmitted to 1 to 2 km. The price of twisted pair cables and twisted pair video transmission equipment is very cheap. Not only does it not increase the cost of the system, but when the distance increases, its cost is much lower than that of coaxial cable. Therefore, there are obvious advantages in using twisted pair cables for transmission in monitoring systems:

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 video signal amplitude and the attenuation difference between different frequencies are extremely compensated, the brightness and color of the original image and the real-time performance are maintained. When the transmission distance reaches 1km or more, the image signal is basically free of distortion. If the relay method is used, the transmission distance will be longer.

2) Convenient wiring and high cable utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, any pair of the 5-category unshielded twisted-pair cables widely laid in buildings can transmit a video signal without additional wiring. Even if rewiring is required, 5-category cables are easier than coaxial cables. In addition, there are 4 pairs of twisted-pair cables in a 5-category cable. If one pair of cables is used to transmit video signals, the other pairs of cables can be used to transmit audio signals, control signals, power supply or other signals, which improves the cable utilization rate, avoids the trouble caused by separate wiring of various signals, and reduces the project cost.

3) Strong anti-interference ability. Twisted pair cables can effectively suppress common mode interference. Even in a strong interference environment, twisted pair cables can still transmit excellent image signals. Moreover, using several pairs of twisted pair cables in one cable to transmit different signals separately will not interfere with each other.

4) High reliability and easy to use. When using twisted pair cables to transmit video signals, a dedicated transmitter must be connected at the front end and a dedicated receiver must be connected at the control center. This 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) Cheap and easy to obtain. Since it uses the common Category 5 unshielded cables or ordinary telephone lines that are widely used, it is easy to buy and the price is also very cheap, which brings great convenience to engineering applications.

3. Fiber Optic

Optical fiber and optical transceivers are used in the field of monitoring 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 within a short distance and a small range. If you need to transmit image signals over a distance of several kilometers or even hundreds of kilometers, you need to use optical fiber transmission. In addition, for some places with extremely strong interference, in order to avoid being affected by environmental interference, optical fiber transmission should also be used. Because optical fiber has many advantages such as wide transmission bandwidth, large capacity, no electromagnetic interference, and little influence from the external environment, one optical fiber can transmit all the signals required in the monitoring system, and the transmission distance can reach hundreds of kilometers. Optical transceivers provide flexible transmission and networking methods for monitoring systems, with good signal quality and high stability.

However, the use of optical fiber and optical transceivers requires certain expertise 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.

2. Technical performance analysis

1. Coaxial cable In the monitoring system, it is common to use 75Ω, -5 coaxial cable. Generally, the distributed capacitance of this coaxial cable is about 50-60pF/m, and the DC resistance of the cable will cause the transmitted signal to be attenuated. Tests show that when a signal with a frequency of 5MHz is transmitted for 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, and the color part of the image is at the high end of the frequency. When the color image signal is transmitted by coaxial cable, its brightness and color will be attenuated, especially as the transmission distance increases, the color of the image will become lighter or even distorted. Tests in the laboratory found that when the color image signal was transmitted for about 200m in a 75Ω, -5 coaxial cable, its amplitude and color had been significantly attenuated. If you want to transmit longer distances, you can only add a coaxial video amplifier.

2. When twisted pair is used as the transmission medium, the commonly used Category 5 UTP cable is generally used. Its characteristic impedance is about 100Ω, and the distributed capacitance is about 15pF/m. Unlike coaxial cable, the signal is transmitted in a balanced manner in the twisted pair. With the use of twisted pair video transmission equipment and twisted pair, it can be guaranteed that the video signal can be transmitted with high quality within a distance of 1.5Km. The performance indicators of the image signal transmitted by the twisted pair were tested in the laboratory using special instruments, and the results are as follows:

(1) When the transmission distance is 300m

Indicators: DG≤1%; DP≤1°; SNR≥65dB

(2) When the transmission distance is 1200m

Indicators: DG≤2%; DP≤2°; SNR≥60dB

3. Fiber Optic

Optical fiber is the inevitable choice for long-distance monitoring image transmission. It can provide a transmission distance of hundreds of kilometers and can provide good image transmission quality. The performance indicators of Wuhan Micro-Invasive Optoelectronics Technology Co., Ltd. WTOS-02 S4000 non-compressed digital video transmission equipment were tested in the laboratory using special instruments, 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 optical fiber is generally about 2 yuan per fiber per meter, and the price of optical terminal is about 2,000 yuan per channel. In addition to the auxiliary materials in the project, such as optical fiber jumpers, optical flanges, and optical fiber fusion costs, the cost of optical fiber monitoring network is relatively high. However, optical fiber transmission has many outstanding advantages, so in monitoring networks 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, optical fiber is mostly used as the transmission medium.

Coaxial cables are widely used within a distance of about 100 to 200 meters. In this range, coaxial cables do not require other equipment, are easy to use, and can basically meet the transmission quality requirements. However, when the transmission distance increases, coaxial cables are inferior to twisted pair cables in terms of cost performance. Generally, the market price of 75Ω, -5 coaxial cables is 1.5 yuan/m, and the price of coaxial amplifiers is 300 yuan/unit; the price of Category 5 UTP cables is 0.8 yuan/m, and the price of twisted pair video transmission equipment is 600 yuan/set. The following is a comparison of the transmission costs of coaxial cables and twisted pair cables for 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 cables:

Since each of the four channels 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, and each monitoring point is about 600m away from the control center.

When using coaxial cable:

To ensure the image quality, each channel needs to add a first-level amplifier. 1.5 yuan/m×600m×8+300 yuan/unit×8=9600 yuan

When using twisted pair cables:

Since each of the four channels 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

From the above analysis, it can be seen that as the transmission distance increases, the cost advantage of twisted pair transmission becomes more apparent. 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, coaxial cable transmission can no longer meet the transmission quality requirements, while twisted pair can still guarantee good image quality when transmitting about 1.5 km.

4. Engineering Suggestions

In actual projects, the choice of transmission method needs to be considered comprehensively based on the specific conditions of the project and the project budget.

For monitoring networks with a transmission distance of more than 2 km, optical fiber and optical transceivers should be considered. If the transmission distance reaches several kilometers or tens of kilometers, optical fiber is the only choice. In addition, in some places with strong interference, such as places subject to industrial interference and electromagnetic interference, optical fiber transmission is also recommended to ensure image quality.

The selection of coaxial cable and twisted pair requires more analysis and comparison. Since twisted pair transmission is a newly emerging technology, and coaxial cable has a long history of use, it is even more necessary to fully explore the characteristics and advantages of twisted pair transmission to provide convenience for engineering design and construction and save costs. When the transmission distance is about four or five hundred meters to 1.5 kilometers, twisted pair can undoubtedly play an excellent performance and price advantage. For monitoring networks within two or three hundred meters, if the scale is small, it is reasonable to use coaxial cable, and the cost is also low. However, the actual project will also have its particularity. Even if the distance is very close, if the network scale is large, such as hundreds of monitoring points, it will be difficult to use coaxial cable wiring, and twisted pair can solve this problem. Furthermore, if the environmental interference is large, if twisted pair is used, it can effectively resist interference and ensure the quality of image transmission. In addition, the convenience and flexibility of twisted pair wiring have gradually made some short-distance and intensive monitoring networks such as buildings and buildings use it to replace coaxial cable, and it has been well integrated into the integrated wiring project.