Coaxial cable is an ultra-wideband transmission medium that can transmit from DC to microwave. The theoretical basis of coaxial transmission is electromagnetic field theory, which is an important difference from general electrical circuit theory. For cable connection, core wire and shielding net are welded and twisted separately, and "three links" are used for video signal distribution, etc. This is reasonable from the perspective of electrical circuit, but it is a principle from the perspective of coaxial transmission error. In order to reduce cable attenuation, another cable manufacturer changed the 0.75-0.8mm core wire of the SYV75-5 solid cable to 1.0mm. The reason is that the physical foam cable core wire is 1.0mm, and the solid cable can also be made 1.0mm, it seems that the diameter of the RVV cable can be selected randomly. What is wrong with the thin one and the thick one? I do n’t know. In this way, the characteristic impedance of the cable is no longer 75 ohms, but it is about 60 ohms. It is not matched and cannot be used in a 75 ohm transmission system. This is not a "pure theory" problem, but a scientific conclusion that practice and theory are completely unified.
There are two main ways of coaxial video cable transmission: baseband coaxial transmission and radio frequency coaxial transmission, and there is also a "digital video transmission", such as the Internet, which is a comprehensive transmission method. It is worth mentioning that there is already a digital TV transmission technology, will it develop into the monitoring industry, it is worth discussing and paying attention to. Here mainly talk about the technical principles of the first two categories. From an engineering application point of view, it is important to grasp the transmission characteristics of the cable and the basic performance of the transmission equipment, and more particularly pay attention to the "video transmission channel characteristics" composed of the cable and the transmission equipment.
Radio frequency transmission, that is, the mature transmission method of cable TV, is to modulate the radio frequency carrier through the video signal, and the video information is carried and hidden in the amplitude change of the radio frequency signal to form an 8M standard bandwidth channel. Different camera video signals are modulated to Different RF channels, and then use a multi-channel mixer to mix all channels into a broadband RF output, to achieve the transmission of multiple signals in the same distance with a transmission cable, at the end, then use the RF adapter to divide into multiple channels, each signal uses one The demodulator demodulates the video signal of a channel. For the frequency distortion generated by cable transmission in a channel (8M), it should be completed by the weighting circuit inside the modem. For the broadband transmission frequency distortion between each channel, the dedicated equalizer is tested and debugged at the project site; for the transmission attenuation, through calculation Completion of field strength detection and debugging on site, including remote transmission tandem amplifier, field strength level control before and after the equalizer; RF multiplexing has obvious advantages for medium and long distance video transmission within a few kilometers. The radio frequency transmission method inherits the mature transmission method of cable television, and its application in the monitoring industry, its feasibility, credibility and reliability are technically without doubt. However, the maturity of the technology does not mean that specific products and specific projects will be successful. RF network design and debugging knowledge and experience, the level of engineering and technical personnel, and the matching of testing equipment are also key factors for the success of the project. When designing the transmission system, it is necessary to pay attention to the reasonable combination with other transmission methods, and avoid the design scheme of "unifying the world". The current status quo is: many products, the pursuit of low cost, low price, reducing the technical performance of the original broadcast equipment, but the actual monitoring application is an outdoor all-weather environment. The manufacturer announced that it is overwhelming, but the application is still less. The reason is that in addition to the complexity of the system and the lack of experience of the engineering company, the manufacturer ’s publicity only emphasizes the benefits of "multi-channel common cable" and "multi-channel one line communication", which makes people feel that this transmission method It ’s good, simple, and cheap. It does n’t focus on the technical points and difficulty of designing, installing, and debugging the RF transmission network. The result is that I do n’t understand and do n’t dare to use it. Promotion and application.
Coaxial video baseband transmission: This is the most basic, the most common, the earliest application, and the most used transmission method, jokingly known as "people will do" transmission method. In fact, it is the transmission method with the most superficial understanding and the slowest technological progress. The slow technological progress is the main factor. The low-frequency attenuation of coaxial cable and the high-frequency attenuation are also understood by everyone, but radio frequency has achieved multi-channel long-distance transmission as early as more than 20 years ago, but the video baseband transmission has remained in a single channel for a long time. At the level below the meter; in the case of reducing the image quality requirements in the monitoring project, only 3 or 4 hundred meters can be used.
The difficulty of the technical progress is the problem of too severe frequency distortion of the coaxial video baseband transmission. For radio frequency transmission, the relative bandwidth (8M) of a channel is only a few percent, and the difference between high and low frequency attenuation is very small, which can generally be ignored; but in the coaxial video baseband transmission mode, low frequency 10--50 Hz and high frequency 6M, The difference between high and low frequencies is more than 100,000-hundreds of thousands of times. The attenuation of high and low frequencies (frequency distortion) is too large, and the attenuation difference of different length cables is also different. It is impossible to use a simple, fixed frequency weighting network to correct the Frequency distortion, the use of broadband and other gain video amplifiers, can not solve the problem of frequency distortion. Therefore, to achieve coaxial long-distance baseband transmission, it is necessary to solve the problem of weighted amplification technology, and the "compensation characteristics" of this frequency weighted amplification must be opposite to the attenuation and frequency distortion characteristics of the cable, complementary, continuously adjustable, to Adapting to the compensation needs of different types of projects and cables of different lengths is the historical reason for the slowest technological progress. This technology was breakthrough in the EIE laboratory of Yantai Development Zone in 2000 and obtained a national patent. After several years of productization and popularization, the technology and products have matured, and they have been successfully applied in all provinces and cities in China and Hong Kong. . Its direct engineering application effect is: greatly improve the distance and range of coaxial video surveillance-from 100 meters to 2, 3 kilometers; effectively improve the image transmission quality and reduce the cost of long-distance transmission cables; "end "Compensation" effectively simplifies the transmission system, facilitates the installation and debugging of the project, and realizes the on-site adjustable control of the image quality in the project for the first time. However, this type of video reply product can only be applied one-to-one, cannot be multi-channel common cable, nor can it be used as anti-interference equipment.
In January 2005, the emergence of the patented technology of frequency-weighted anti-interference video transmission in the EIE laboratory simultaneously realized the dual functions of suppressing interference and video reply. On the basis of solving the problem of frequency-weighted amplification technology, it further promoted the coaxial video baseband transmission The development and application of technology.
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