This article mainly introduces the outline of the twisted pair, its basic principle of transmitting video signals, and the related knowledge points of the advantages of the twisted pair and twisted pair transmission images.
I. Overview
In the weak system of modern intelligent buildings, the security system has a very important position. In the traditional design and construction, network cabling and security monitoring cabling are often considered separately. Due to the differences in the media used, people cannot unite them together. However, in the situation where the number of security monitoring points in a building is increasing, the increase in the number of hard coaxial cables will occupy a large amount of pipeline resources, and the wiring will be very difficult. Moreover, when the length of the wiring is too large, it will cause image quality. decline.
With the extension of the distance, the coaxial cable, the protagonist of traditional wiring, is increasingly unable to meet the needs of construction and application. The starting point of the new cabling technology is to unify the network cabling and security wiring as much as possible to a single platform, reducing the difficulty and cost of cabling. In this type of cabling technology, unshielded cables (for example, Category 5 cables) widely used in network cabling are used to transmit image signals, and a vacant pair of wires in an unshielded cable is used to power a video camera. To implement this technology requires the use of two key devices: twisted-pair video transmission equipment and remote power supply.
Second, the basic principle of twisted pair transmission video signal
Video signals to be transmitted within a twisted pair solve two problems: impedance matching and attenuation compensation.
The standard video signal interface is generally 75Ω, unbalanced, and twisted-pair transmission is 100Ω, balanced, so the use of twisted pair transmission of video signals must try to solve 75Ω ↠→ 100Ω and unbalanced ↠→ balance conversion problem. There are two ways to accomplish this conversion: one is a passive method using a transfer transformer, which does not require power, but it has a certain loss of signal, and the drive capability is limited; the other is the use of active mode, through a broadband amplifier and A dedicated chip can not only achieve impedance and balanced conversion, but also provide strong driving and amplification compensation for image signals. The disadvantage is that it requires power supply to work.
In addition, the video signal is a high-frequency broadband signal, bandwidth up to 6MHz, if transmitted directly in the twisted pair, the amplitude of the signal will be greatly attenuated, on the monitor as the image color fades and the brightness is dimmed . Therefore, the video signal must be amplified and compensated for long distance transmission on the twisted pair to ensure the image quality.
Third, twisted and twisted pair transmission image advantages
1, twisted pair introduction
Twisted Pairwire, TwistedPairwire, is the most commonly used transmission medium in integrated cabling engineering. The twisted pair consists of two copper conductors with an insulating protective layer. Twisting two insulated copper conductors at a certain density can reduce the degree of signal interference, and the electric wave radiated by each conductor during transmission can be cancelled by the electric wave emitted from the other conductor. Twisted pair generally consists of two 22-26 insulated copper conductors. If one or more pairs of twisted pairs are placed in one insulating sleeve, they become twisted pair cables. In twisted pair cables (also known as twisted pair cables), different pairs have different twist lengths. Generally speaking, the twist length is within 38.1cm to 14cm, twisted in a counterclockwise direction, and the opposite line pair. The twist length is above 12.7cm. Compared with other transmission media, twisted-pair cables 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-pair cables have three types, four types, five types, five types or even six types. The rate of Category 3 twisted pair is 10Mb/S, the rate of Category 5 twisted pair is 100Mb/S, and the Category 5 is more than 155Mb/s. There can be only five categories or Super Category 5 to 100Base-TX. . Shielded Twisted Pair Because the outer layer of the cable has a layer of aluminum package to reduce the radiation, the production is more troublesome, and the price line is more expensive than the unshielded twisted pair, so we are commonly used in 10Base-T or 100Base-TX networks. The unshielded Category 5 and Super Category 5 twisted pairs.
2, the advantages of twisted pair transmission images
Twisted pair is used in many industrial control needs and places with large interference and long-distance transmission. The widely used LAN also uses twisted pair, it has strong anti-interference ability, transmission distance, easy wiring, Space saving, low price, and so on. At present, high-quality image signals can be transmitted on the twisted pair by adopting advanced technologies and dedicated chips.
Easy wiring and high cable utilization
A wide range of unshielded twisted pairs laid in an intelligent building can transmit one video signal from any pair. Since there are four twisted pairs in a Category 5 cable, four video images can be transmitted, which is more than a monitoring point. For buildings requiring dense wiring, it is undoubtedly greatly eased the difficulty of wiring construction. In addition, 1 pair of twisted pair transmission video signals, the rest can also transmit audio signals, control signals, power, etc., provide the cable utilization, to avoid the trouble caused by the separate wiring of various signals, reducing the project The construction cost has overcome the disadvantages of difficulty in co-axial cabling and the large number of pipeline resources used, making the type of integrated wiring more uniform.
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. For example, elevators, equipment rooms, etc. in buildings will have strong interference. Buildings in downtown areas will also be subject to various interference from the surrounding environment. The strong anti-interference ability of twisted pair cables is greater than that of coaxial cables. Advantage.
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 twisted pair transmission device is inexpensive, simple to use, and stable in operation.
Overall cost reduction
Super Category 5 unshielded cable is widely used in the current smart building integrated wiring, its price is lower than the coaxial cable, with the use of ultra-type 5 cable in communications, the price will further decline, so the use of twisted The economy and convenience of the line are even more prominent.
Real-time transmission, real-time monitoring
The image signal transmitted by the twisted pair adopts analog signal technology, which ensures the real-time performance of the monitoring system and can monitor the live image in real time in the control room.
Fourth, remote power supply
In response to the current demand for monitoring systems, combined with the development trend of integrated cabling systems, many manufacturers of integrated cabling products, such as CommScope COMMSCOPE, Simon SIEMON, and Rihai SILK integrated wiring products, rely on their own strong technical advantages to develop a series of Suitable for monitoring system products to meet the arrival of the monitoring system twisted pair era, I believe that in the near future integrated cabling system will no longer only serve traditional voice and data network systems, it will increasingly occupy the transmission of weak systems The important position of the media.
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