How to accurately use the transmission media in the monitoring project?

Each monitoring project has its own characteristics and particularities. Therefore, many factors need to be fully considered when setting up a monitoring network. The selection of the most appropriate image and signal transmission method is very important. Given that coaxial cable, twisted pair, and fiber are the three most widely used transmission media in current surveillance systems, we can analyze and compare them in several ways.

Coaxial cable

Generally in a small-scale monitoring system, since the transmission distance is very short, the direct transmission of the monitoring image using the coaxial cable has little damage to the image quality and can meet the actual requirements. However, the coaxial cable is only suitable for transmitting image signals at a short distance. When the transmission distance reaches 200 meters, the image quality will be significantly reduced, especially the color becomes dull and there is a sense of distortion.

In engineering practice, coaxial amplifiers are used to extend the transmission distance. The coaxial amplifier has a certain amplification to the video signal, and can also compensate different magnitudes of the different frequency components separately through equalization adjustment, so that the distortion of the video signal output by the receiver is as small as possible. However, coaxial amplifiers cannot be cascaded without restriction. Generally, in a point-to-point system, coaxial amplifiers can only cascade at most 2 to 3, otherwise the quality of video transmission cannot be guaranteed, and it is difficult to adjust them. Therefore, in the use of coaxial cable in the monitoring system, in order to ensure better image quality, the transmission range is generally limited to four or five hundred meters.

In addition, there are some disadvantages in the coaxial cable transmission of image signals in the surveillance system:

1) The coaxial cable itself is greatly affected by climate change and image quality is affected.

2) The coaxial cable is relatively thick, 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 used in strong interference environments;

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

Twisted pair

The use of twisted pair has been a long time. Twisted pair has been used in many industrial control systems and in places with large interference and in long-distance transmission. The LANs we use widely today also use twisted pair cables. The use of twisted pair for transmission in surveillance systems has clear advantages:

1) The transmission distance is long and the transmission quality is high. Due to the advanced processing technology used in twisted-pair transceivers, it perfectly compensates for the attenuation of the amplitude of the video signal due to the twisted pair and the difference in attenuation between different frequencies, maintaining the brightness and color of the original image as well as real-time performance. When the transmission distance reaches 1km or more, the image signal is almost no distortion. If relay mode is used, the transmission distance will be further away.

2) Easy wiring and high cable utilization. A pair of ordinary telephone lines can be used to transmit video signals. In addition, one of the five types of unshielded twisted pairs widely installed in a building can transmit one video signal without the need for additional wiring. Even re-wiring makes Category 5 cables easier than coaxial cables. In addition, a Category 5 cable has 4 pairs of twisted pairs. If a pair of cables are used to transmit video signals, the other pairs of cables can also be used to transmit audio signals, control signals, power supplies, or other signals. The utilization rate, at the same time avoids the trouble caused by the separate wiring of various signals, and reduces the project cost.

3) Strong anti-interference ability. Twisted pair can effectively suppress common-mode interference. Even in strong interference environments, twisted-pair cables can transmit excellent image signals. In addition, several pairs of twisted pairs in one cable are used to transmit different signals, and no interference occurs between them.

4) High reliability and easy to use. Utilize twisted pair to transmit the video signal, need to insert the special transmitter in the front end, want to insert the special receiver in the control center. This type of twisted-pair transmission equipment is cheap and easy to use, requires no specialized knowledge, and does not require much operation. It can be installed once and work stably for a long time.

5) The price is cheap, easy to draw. Due to the use of common 5 types of unshielded cables or ordinary telephone lines which are widely used at present, the purchase is easy and the price is also very cheap, which brings great convenience to engineering applications.

However, the twisted pair signal also has a large attenuation. If the video signal is directly transmitted in the twisted pair, it will also be greatly attenuated. Therefore, the video signal must be amplified and compensated for long distance transmission on the twisted pair cable. Twisted pair video transmission equipment is to complete this function.

optical fiber

Optical fiber and optical transceiver applications in the field of monitoring are mainly to solve two problems: First, the transmission distance, the first is the environmental interference. Twisted pair and coaxial cables can only solve the problem of transmission of monitoring images in a short range and a small range. If it is necessary to transmit image signals of distances of several kilometers or even hundreds of kilometers, optical fiber transmission methods are required. In addition, for some super-disturbing sites, in order to be free from environmental interference, optical fiber transmission must also be used. Because the optical fiber has many advantages such as wide transmission bandwidth, large capacity, immunity to electromagnetic interference, and small influence from the external environment, an optical fiber can transmit all the signals required in the monitoring system, and the transmission distance can reach hundreds of kilometers. The optical transceiver provides a flexible transmission and networking method for the monitoring system, with good signal quality and high stability.

However, the use of optical fiber and optical transceivers requires a certain amount of expertise and special equipment, which brings certain difficulties for construction and user use. In addition, for short-range, small-scale monitoring systems, the use of optical fiber transmission is also not economical.

Project proposal

In practical projects, the choice of transmission method needs to be considered in combination with the specific conditions of the project and the project budget.

For monitoring networks with transmission distances exceeding 2 Km, optical and optical transceivers should be considered. If the transmission distance reaches several kilometers or dozens of kilometers, only the optical fiber is used. In addition, in some areas with strong interference, such as industrial interference and electromagnetic interference, in order to ensure image quality, it is also recommended to use optical fiber transmission.

When the transmission distance is between four, five hundred meters and 1.5 kilometers, the twisted pair can undoubtedly exert excellent performance and price advantages. For the monitoring network within two or three hundred meters, if the scale is small, the use of coaxial cable is Reasonably, the cost is also lower.

However, the actual project will also have its specificity. Even if the distance is very close, if the network is very large, for example, there are hundreds of monitoring points, then it will be difficult to use coaxial cable, and twisted pair can make This problem has been solved. Furthermore, if there is a large amount of interference in the environment, using twisted-pair cables can effectively prevent interference and ensure image transmission quality. In addition, the convenience and flexibility of twisted-pair cabling make some short-distance, intensive monitoring networks such as building buildings gradually use it instead of coaxial cable, and it is well integrated into the integrated cabling project.