According to the "barrel rule," the final image quality will depend on the worst link in the entire system; the worst link is often the transmission system. The system designers and installers must select the appropriate transmission method, high-quality transmission cables and equipment according to actual needs, and install them according to professional standards in order to achieve the desired transmission effect.
Video signal transmission generally adopts direct modulation technology and takes the form of baseband frequency (about 8MHz bandwidth). The most commonly used transmission medium is coaxial cable. Coaxial cable is specially designed to transmit video signals. Its frequency loss, image distortion, and image attenuation are relatively small, which can well complete the task of transmitting video signals. Commonly used special SYV75 ohm series coaxial cable, commonly used model is SYV75-5 (its non-relay transmission distance for video signal is generally 300-500m); when the distance is longer, SYV75-7, SYV75-9 or even SYV75-12 coaxial cable (In practical projects, the thick cable has an unrelayed transmission distance of up to 1km or more); there are also video amplifiers to increase the brightness, chrominance, and synchronization of the video, but the interference signal in the line is also Will be amplified, so the loop can not connect too many video amplifiers, otherwise it will appear saturated, leading to image distortion; farther away from the use of optical fiber transmission, optical fiber transmission has a small attenuation, frequency bandwidth, free from electromagnetic interference, A series of advantages, such as light weight and good confidentiality, are mainly used in the national and provincial-level backbone communication networks, cable television networks and high-speed broadband computer networks. In the closed-circuit television monitoring system, optical fiber transmission has also become the preferred method for long-distance video, audio and control signal transmission.
Video signals can also be transmitted in twisted pair, which requires twisted pair transmission equipment. In some special applications, twisted pair transmission equipment is essential. For example, when a large number of twisted pairs (three or five types of wires in the standard) have been laid in the building according to the comprehensive wiring standards, and corresponding information interfaces (RJ45 or RJ11) are left in the relevant rooms, the new Increased closed-circuit television monitoring equipment does not need to re-wiring, video and audio signals and control signals can be transmitted through the twisted pair, which transmission of video signals will use twisted pair transmission equipment. In addition, twisted-pair transmission equipment is also required for applications where twisted-pair cables (or two-core sheathed cables) have been laid and front-end camera images are transmitted to central control room equipment. 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) to the video signal suitable for balanced transmission (that is, suitable for twisted pair transmission) at the front end; at the receiving end In contrast to the front-end processing, the video signal transmitted through the twisted pair is converted back to an unbalanced video signal. The twisted pair transmission device itself has a video amplification effect and is therefore also suitable for long-distance signal transmission. For the above different transmission methods, the transmission components and the transmission lines used are quite different.
The communication cable is generally used in an imaging device equipped with an electric pan/tilt head and an electric lens, and a remote control decoder needs to be installed at the site during use. The communication transmission cable between the on-site decoder and the video matrix switching host in the control center generally uses a 2-core shielded communication cable (RVVP) or a category 3 twisted pair UTP, and each core has a cross-sectional area of ​​0.3 mm2 to 0.5 mm2. The basic principle for selecting a communication cable is that the longer the distance, the larger the wire diameter. For example: The basic communication distance that RS-485 communication stipulates is 1200m, but in actual project choose RVV2-1.5 sheath wire to expand the communication length to above 2000m. When the communication distance is too long, an RS-485 communication repeater is required.
The control cable usually refers to the multi-core cable used to control the pan/tilt and the electric variable lens. One end of the control cable is connected to the pan/tilt/electric lens control terminal of the controller or decoder, and the other end is directly connected to the pan/tilt/electric drive. On the corresponding terminal of the lens. Since the control cable provides DC or AC voltage, and the distance is generally short (sometimes less than 1m), there is basically no interference problem, so no shielded cable is required. Commonly used control cables mostly use 6- or 10-core cables, such as RVV6-0.2, RVV10-0.12, and so on. The 6-core cable is connected to the top, bottom, left, right, automatic and public terminals of the PTZ respectively. The 10-core cable includes the zoom, focus and aperture of the electric lens in addition to the six terminals of the PTZ. , Public 4 terminals. In the closed-circuit television monitoring system, the control cable from the decoder to the gimbal and the camera does not require special requirements due to the relatively short distance; however, the distance from the controller of the central control room to the gimbal and the electric lens is a few tens of meters. As many as several hundred meters, there are certain requirements for the control cable, that is, the wire diameter should be thick, such as the selection of RVV10-0.5, RVV10-0.75 and so on.
The audio monitoring cable generally uses a 4-core shielded communication cable (RVVP) or a category 3 twisted pair UTP with a cross-sectional area of ​​0.5 mm2 per core. In the non-interference environment, it can also be selected as unshielded twisted pair, such as commonly used in Category 5 twisted pair (4 pairs of 8-core); because the audio signal from the monitor head in the monitoring system is transmitted to the central control room is The use of point-to-point wiring, high voltage and low current transmission, so the use of non-shielded 2-core cable, such as RVV2-0.5.
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