Wired transmission main media and security monitoring applications

Good video transmission design is a very important part of video surveillance systems. As an important transmission medium, wire and cable products play an indispensable role in the data, video and audio transmission and power connection of security systems.

According to the "barrel rule", the final image quality will depend on the difference in the entire system; in the monitoring system, the poor one is often the transmission system. The system designer and installer must select the appropriate transmission method, high-quality transmission cable and equipment according to actual needs, and install according to professional standards, in order to achieve the desired transmission effect.

In the video surveillance system, the transmission system is mainly composed of a control signal cable and a video transmission cable; its function is to reliably connect and transmit the front and rear devices of the video monitoring system. The transmitted signals are audio and video signals and control signals. Common transmission cables are coaxial cable, twisted pair, and fiber. At present, wired transmission is still the main transmission medium of video surveillance transmission system. It mainly includes video baseband transmission, optical fiber transmission, network transmission, twisted pair balanced transmission, and broadband common cable transmission.

Coaxial cable

In the conventional video signal transmission, direct modulation technology is generally adopted, and the common transmission medium is a coaxial cable in the form of a baseband frequency (about 8 MHz bandwidth). Coaxial cable is specially designed to transmit video signals, and its frequency loss, image distortion, and image attenuation are relatively small, which can complete the task of transmitting video signals. Generally, the special SYV75 ohm series coaxial cable is used. The common model is SYV75-5 (it has a non-relay transmission distance of 300-500m for video signals); when it is far away, it needs to use SYV75-7, SYV75-9 or even SYV75-12 coaxial cable (in actual engineering, the non-relay transmission distance of the thick cable can reach more than 1km); also increase the video amplifier to enhance the brightness, chrominance and synchronization signal of the video, but the interference signal in the line is also Will be amplified, so too many video amplifiers can not be connected in the loop, otherwise there will be saturated images, resulting in image distortion.

Among them, broadband common cable transmission is a solution to solve video transmission signal transmission from several kilometers to tens of kilometers. It adopts advanced technologies such as amplitude modulation, audio frequency modulation and FSK data signal modulation, and can monitor forty video surveillance images, sound, control and alarm signals. Integrated into a "one" coaxial cable for bidirectional transmission.

Network twisted pair

With the extension of the distance, the protagonist in the traditional wiring---coaxial cable is increasingly unable to meet the construction and application requirements. The starting point of the new wiring technology is to unify the network wiring and security wiring to one platform as much as possible, reducing the difficulty and cost of wiring. Twisted pair is used in many industrial control needs and in places with high interference and long-distance transmission. The widely used LAN also uses twisted pair, which has strong anti-interference ability, long transmission distance and easy wiring. Space saving, low price and so on. High-quality image signals have been transmitted over twisted pairs by using advanced technology and dedicated chips.

Network twisted pair transmission is a video surveillance transmission method that solves the long distance between the metropolitan areas and the extremely scattered points. A pair of unshielded twisted-pair cables that are widely deployed in an intelligent building can transmit one video signal. Since there are four twisted pairs in a Category 5 cable, four 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., providing cable utilization, avoiding the troubles caused by separate wiring of various signals, and reducing engineering. The cost has overcome the shortcomings of coaxial cable wiring and the occupation of many pipeline resources, so that the types of integrated wiring are more unified.

For video signals, the bandwidth reaches 6MHz. If it is transmitted directly in the twisted pair, it will also attenuate greatly. Therefore, the video signal must be transmitted over long distances on the twisted pair. It must be amplified and compensated, and twisted pair video transmission. The device is to complete this function. After adding a pair of twisted pair video transceivers, the image can be transmitted to 1 to 2 km. Twisted pair and twisted pair video transmission equipment has a much lower cost than coaxial cable when the distance is increased. Therefore, the use of twisted pair transmission in the monitoring system has obvious advantages.

Optical Fiber Communication

Fiber Optic Cable transmits signals in the form of light pulses, so the material is also mainly glass or plexiglass. It consists of a fiber core, a cladding and a protective sleeve. The structure of the optical fiber is similar to that of the coaxial cable. It is also an optical fiber made of glass or transparent plastic. It is surrounded by a protective material. If necessary, multiple optical fibers can be combined in one cable. Optical fibers can be classified into single mode fibers and multimode fibers depending on how the optical signals are generated. (The so-called "mode" refers to a beam of light entering the fiber at a certain angle).

Multimode fibers are typically used for network connections in the same office building or in relatively close proximity. Single-mode fiber delivers higher quality data and longer transmission distances, often used to connect between office buildings or geographically dispersed networks. If fiber optic cable is used as the network transmission medium, devices such as optical transceivers need to be added, so the cost is relatively large, and it is rarely used in general applications.

The characteristic of optical fiber is that the optical signal is transmitted, so it is not interfered by external electromagnetic signals, and the attenuation speed of the signal is very slow. Therefore, the transmission distance of the signal is much farther than the various network cables for transmitting electrical signals, and is especially suitable for electromagnetic A place with a bad environment. Due to the optical reflection characteristics of the optical fiber, multiple optical signals can be transmitted simultaneously in one fiber. Therefore, the transmission speed of the optical fiber can be very high. At present, the 1Gbps 1000Mbps optical fiber network has become the mainstream high-speed network. In theory, the optical fiber network can reach the speed of 50000Gbps 50Tbps. .

With the development of optical fiber communication technology, there are more and more ways of image transmission. The video signal transmission of optical transceivers has greater advantages than other transmissions such as twisted pair transmission and coaxial cable transmission. As is well known, optical fiber transmission mainly relies on an optical transceiver to convert an electrical signal into an optical signal at one end of the optical fiber (cable) and an optical signal back to a corresponding electrical signal at the other end. The optical transceiver provides a flexible transmission and networking mode for the monitoring system, with good signal quality and high stability.

summary

In recent years, due to the rapid development of optical fiber communication technology, the cost of optical fiber transmission monitoring system has been greatly reduced, so the application of optical fiber and optical transceiver in monitoring systems has become more and more popular. Optical fiber has been widely used in home optical fiber and office access networks, in the fields of home intelligence, office automation, industrial control networks, vehicle airborne and military communication networks. For high-definition video streams with high transmission bandwidth and high transmission distance, it is no longer a dream to realize high-definition surveillance in the era of optical fiber.