A set of video security system that is installed, debug-tested and constructed selects cameras, lenses, *** devices and video recorders that can produce or process high-quality images. However, without a good transmission system, the images seen on *** devices will not be satisfactory enough to reach the optimal state of the equipment.
According to the "bucket rule," the final image quality will depend on the worst part of the system. The worst part is often the data transmission system. The designers and installers of the system must choose the appropriate transmission mode, high-quality transmission cables and equipment according to the actual needs, and install them in accordance with professional standards, so as to achieve the desired transmission effect.
Generally, in a small range of monitoring systems, because the transmission distance is very close, the use of coaxial cable to directly transmit monitoring images on the image quality damage is not large, can meet the actual requirements. However, according to the analysis of the characteristics of coaxial cable itself, the attenuation of signal transmitted in coaxial cable is related to the transmission distance and the frequency of signal itself.
In general, the higher the signal frequency, the greater the attenuation. Video signal bandwidth is very large, up to 5.8mhz, and the color part of the image is modulated at the high end of the frequency, so that when the video signal is transmitted in the coaxial cable, not only the overall amplitude of the signal is attenuated, but also the attenuation amount of each frequency component is greatly different, especially the color part has the largest attenuation. Therefore, coaxial cable is only suitable for close transmission of image signals, when the transmission distance reaches about 220 meters, the image quality will be significantly reduced, especially the color becomes dim, there is a sense of distortion.
In engineering practice, coaxial amplifiers are used to extend the transmission distance. Coaxial amplifier amplifies video signal to a certain extent, and compensates for different frequency components through balanced adjustment, so as to minimize the distortion of video signal output at *** end.
However, coaxial amplifiers cannot be cascaded indefinitely. Generally, in a point-to-point system, coaxial amplifiers can only cascade two or three at most. Otherwise, video transmission quality cannot be guaranteed and it is difficult to adjust. Therefore, when using coaxial cable in the monitoring system, in order to ensure better image quality, the transmission distance range is generally limited to about 500 meters.
In addition, coaxial cable transmission of image signals in the monitoring system also has some disadvantages:
1) coaxial cable itself is greatly affected by climate change, and the image quality is affected to some extent;
2) coaxial cable is relatively thick, which is not convenient for wiring in intensive monitoring applications;
3) coaxial cable can only transmit video signal in general. If control data, audio and other signals need to be transmitted simultaneously in the system, additional wiring is required;
4) coaxial cable has limited anti-interference ability and cannot be used in strong interference environment;
5) coaxial amplifier also has the disadvantage of difficult adjustment.
The data cable
The use of data cable has a long history, telephone transmission is the use of twisted-pair, in many industrial control systems and interference in large places and long-distance transmission have used twisted-pair, we are widely used today in the LAN is also the use of twisted-pair. Twisted-pair cable is so widely used because it has many advantages such as strong anti-interference ability, long transmission distance, easy wiring and low price.