I. coaxial cable
The coaxial cable center axis is a copper conductor with a layer of insulating material, which is surrounded by a hollow cylinder of copper conductors, and the outer layer is the insulating layer. Outer conductor and the center axis core's center on the same axis, it is called a coaxial cable, coaxial cable is designed in this way, but also to prevent external electromagnetic wave interference signal transmission.
Coaxial cable is also one of the most common transmission medium in LAN. It is characterized by good anti-interference ability, stable transmission data and cheap price, so it is widely used.
However, according to the analysis of the characteristics of coaxial cable, when the signal is transmitted in coaxial cable, the higher the signal frequency, the greater the attenuation. Coaxial cable is only suitable for the near distance transmission image signal. When the transmission distance reaches about 200 meters, the image quality will decrease obviously, especially when the color becomes dim and distorted.
In the actual monitoring project, the coaxial amplifier is used to extend the transmission distance. The coaxial amplifier has a certain amplification to video signal, so that the video signal distortion is as small as possible. Coaxial amplifier, however, cannot unlimited cascade, generally in a point-to-point system coaxial amplifier cascade can be at most 2 to 3, otherwise unable to guarantee the quality of video transmission, and adjustment is also very difficult. Therefore, when using coaxial cable in the monitoring system, the transmission range is generally limited to about 400 to 500 meters in order to ensure good image quality.
In addition, there are some disadvantages of coaxial cable transmitting image signals in the monitoring system:
1. Coaxial cable itself is affected by climate change, and the image quality is affected;
2. The coaxial cable is thicker, and the wiring is not convenient in the intensive monitoring and application;
3. The coaxial cable can only transmit video signal. If the system needs to transmit control data and audio signals simultaneously, it needs to be wired.
4. Coaxial cable has limited anti-jamming capability, which cannot be applied to strong interference environment;
5. Coaxial amplifier also has the disadvantage of adjusting difficulties.
2. Twisted pair
Double stranded wire is the most commonly used transmission medium in the integrated wiring project. The twisted pair used a pair of insulated metal conductors twisted together to protect against some external electromagnetic interference. The two insulated copper wires twisted together, with each other in a certain density can reduce the degree of interference, each wire in transit radiation waves will be offset by another online from the waves.
Twisted-pair cable in the monitoring project, usually exceed five line, for the video signal, if within the twisted-pair transmission directly, also attenuation is very big, so the video signal on the twisted pair to realize long distance transmission, amplification and compensation must be conducted.
With a pair of twisted pair signal amplification equipment, the image can be transferred to 1 to 2km. Double stranded wire and double stranded wire amplification equipment price is very cheap, not only did not increase the system cost, but when the distance increases, its cost and coaxial cable are reduced a lot. Therefore, many previous simulation monitoring systems have obvious advantages in using twisted pair for transmission.
Optical fiber, a shorthand for optical fiber, is an optical transmission tool that USES the principle of total reflection in glass or plastic fibers. The optical fiber was invented by Charles kao, former President of the Chinese university of Hong Kong. The microfiber is encapsulated in a plastic sheath so that it can bend without breaking.
In daily life, the transmission loss of light in the optical fiber is much lower than the loss of electricity in the wire conduction, and the optical fiber is used for long-distance information transmission.
Optical fiber can be divided into single mode fiber and multimode fiber according to the different mode of light signal. Multimode fiber is generally used for network connections in the same office building or in a relatively close area. Single-mode fiber delivers higher quality, longer transmission distances, and is often used to connect a wider network of offices or geographically.
If the optical fiber is used as the monitoring system, the transmission system should be added to the optical transmitter. Can solve video, audio, switch, intercom, data. The video signal is transmitted to the distal optical terminal through the video line through the video camera, and the optical transmitter is connected to the control center through the optical fiber. Normal connection mode: network monitoring camera - access to switch - optical fiber transceiver - fiber dielectric - optical transceiver - core switch - machine room NVR video recorder.