I. Overview
In the weak current system of modern intelligent buildings, the security system has a very important position. In traditional design and construction, network cabling and security monitoring cabling are often considered separately. Due to differences in the media used, people cannot unify them together. However, in the case of increasing security monitoring points in the building, the increase in the number of thicker coaxial cable wiring will take up a lot of pipeline resources, and the wiring is very difficult, and the length of the wiring will also cause image quality. decline.
With the extension of distance, the protagonist in traditional wiring, coaxial cable, is increasingly unable to meet the needs of construction and application. The starting point of the new wiring technology is to unify the network wiring and security wiring on one platform as much as possible, reducing the difficulty and cost of wiring. In this wiring technology, unshielded cables (such as category 5 cables) widely used in network wiring are used to transmit image signals, and the spare pairs in the unshielded cables are used to power the cameras. In order to realize this technology, two key equipments are used: twisted pair video transmission equipment and remote power supply.
Second, the basic principle of twisted pair transmission video signal
Two problems must be solved when the video signal is transmitted in the twisted pair: impedance matching and attenuation compensation.
The standard video signal interface is generally 75Ω, unbalanced mode, and the twisted pair transmission is 100Ω, balanced mode, so the use of twisted pair transmission video signal must try to solve the conversion problem of 75Ω ↠→ 100Ω and unbalanced →→ balanced. There are two ways to accomplish this conversion: one is the passive method using a transmission Transformer, which does not require power supply, but it will have a certain loss of signal, and the driving capacity is limited; the other is the active method, which uses a broadband amplifier and The special chip can not only complete the conversion of impedance and balance mode, but also provide strong driving and amplification compensation for the image signal. The disadvantage is that it needs power supply to work.
In addition, the video signal is a high-frequency broadband signal with a bandwidth of 6MHz. If it is transmitted directly in the twisted pair, the amplitude of the signal will be greatly attenuated, and the color of the image will become lighter and the brightness will become darker on the monitor. . Therefore, if the video signal is to be transmitted over a long distance on a twisted pair, it must be amplified and compensated to ensure the image quality.
Third, the advantages of twisted pair and twisted pair transmission image
1. Brief introduction of twisted pair
Twisted pair wire, Twisted Pair wire, is the most commonly used transmission medium in integrated wiring projects. The twisted pair consists of two copper wires with an insulating protective layer. Twisting two insulated copper wires with each other at a certain density can reduce the degree of signal interference. The electric wave radiated by each wire during transmission will be cancelled by the electric wave emitted by the other wire. Twisted pair is generally made of two 22 to 26 insulated copper wires intertwined with each other. If you put one or more pairs of twisted pairs in an insulating sleeve, it becomes a twisted pair cable. In twisted pair cable (also called twisted pair cable), different pairs have different twist lengths. Generally speaking, the twist length is within 38.1cm to 14cm, twisted counterclockwise, adjacent pairs The twist length is more than 12.7cm. Compared with other transmission media, twisted pairs are subject to certain restrictions in terms of transmission distance, channel width, and data transmission speed, but the price is relatively low.
Twisted pair can be divided into shielded twisted pair (STP) and unshielded twisted pair (UTP) two categories.
In addition, twisted pairs are classified into categories 3, 4, 5, 5 and 6, etc. The rate of category 3 twisted pair is 10Mb / S, the rate of category 5 twisted pair can reach 100Mb / S, and the category 5 can reach 155Mb / s or more. Only Category 5 or category 5 can be used for 100Base-TX . Shielded twisted pair because the outer layer of the cable has a layer of aluminum wrap to reduce radiation, making it more troublesome, and the price line is more expensive than unshielded twisted pair, so we are commonly used in 10Base-T or 100Base-TX network It is unshielded category 5 and super category 5 twisted pair.
2. The advantages of twisted pair transmission image
Twisted pair is used in many places where industrial control needs to be neutralized and interfered with, and in long-distance transmission. The widely used local area network also uses twisted pair, which has strong anti-interference ability, long transmission distance, easy wiring, Space saving, low price and so on. At present, through the use of advanced technology and special chips, it has been able to transmit high-quality image signals on twisted pairs.
Easy wiring and high cable utilization
Any pair of unshielded twisted pairs widely laid in an intelligent building can transmit a video signal. Since there are 4 twisted pairs in a category 5 cable, 4 video images can be transmitted, which is more for monitoring points For buildings that require dense wiring, it will undoubtedly greatly reduce the difficulty of wiring construction. In addition, one of the four pairs of twisted pairs transmits video signals, and the rest can also transmit audio signals, control signals, power supplies, etc., which provides cable utilization, avoids the trouble of separate wiring of various signals, and reduces engineering The cost overcomes the shortcomings of coaxial cable wiring difficulties and occupies more pipeline resources, making the type of integrated wiring more unified.
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. For example, there are strong interferences in the elevators and equipment rooms of buildings, and buildings in downtown areas are also subject to various interferences from the surrounding environment. The strong anti-interference ability of twisted pairs is greater than that of coaxial cables. Advantage.
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 twisted pair transmission equipment is cheap, simple to use, and stable in operation.
Lower overall cost
Super 5 types of unshielded cables are widely used in the current integrated wiring of intelligent buildings, and their prices are lower than coaxial cables. With the large number of super 5 types of cables used in communications, the price will drop further, so the use of twisted pair The economy and convenience of the line are more prominent.
Real-time transmission, real-time monitoring
The image signal transmitted by the twisted pair uses analog signal technology to ensure the real-time performance of the monitoring system, and the real-time image can be monitored in the control room.
4. Remote power supply
In response to the current needs of monitoring systems, combined with the development trend of integrated wiring systems, many manufacturers of integrated wiring products, such as CommScope, Simon Siemon, and Rihai SILK integrated wiring products, have developed a series of products based on their strong technical advantages. Products suitable for monitoring systems are ushering in the era of twisted pair monitoring systems. I believe that in the near future, the integrated wiring system will no longer only serve traditional voice and data network systems, it will increasingly occupy weaker systems for transmission. The important position of the medium.
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