Video signal transmission generally uses direct modulation technology in the form of baseband frequency (about 8MHz bandwidth). The most common transmission medium is coaxial cable. 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 commonly used 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 adopt 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 image, resulting in image distortion; farther away fiber transmission, fiber transmission has small attenuation, frequency bandwidth, free from electromagnetic interference, A series of advantages, such as light weight and good confidentiality, are mainly used for national, provincial and municipal backbone communication networks, cable TV networks and high-speed broadband computer networks. In CCTV surveillance systems, fiber optic transmission has also become the preferred method for long-distance video and audio and control signal transmission.
The video signal can also be transmitted over twisted pair, which requires a twisted pair transmission device. Twisted pair transmission equipment is essential in some special applications. For example, when a large number of twisted pairs (called Category 3 or Category 5 in the standard) have been laid in the building according to the integrated wiring standard and the corresponding information interface (RJ45 or RJ11) is left in each relevant room, then new When the closed-circuit television monitoring equipment is added, there is no need to re-wire, and the audio and video signals and control signals can be transmitted through the twisted pair. The transmission of the video signal requires the use of a twisted pair transmission equipment. In addition, for applications where a twisted pair (or two-core sheathed wire) has been applied and the image of the front camera needs to be transmitted to the central control room equipment, a twisted pair transmission device is also required. The function of the twisted pair video transmission equipment is to convert the video signal suitable for unbalanced transmission (that is, suitable for 75Ω coaxial cable transmission) into a video signal suitable for balanced transmission (ie suitable for twisted pair transmission) at the front end; The opposite of the front end, the video signal transmitted over the twisted pair is reconverted to an unbalanced video signal. The twisted pair transmission device itself has video amplification and is therefore suitable for long distance signal transmission. For the different transmission methods above, the transmission components and transmission lines used are quite different.