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DIGITAL CCTV A Security Professional’s Guide phần 5

cho khoảng cách xấp xỉ lên đến 750 hoặc 1.000 feet, tùy thuộc vào nhà sản xuất mà bạn yêu cầu. Nó sẽ được cần thiết để lựa chọn đúng cáp đồng trục cho công việc ở bàn tay. 75 ohm trở kháng cáp là tiêu chuẩn được sử dụng trong các hệ thống CCTV | Guided Video Transmission 115 for distances of approximately up to 750 or 1 000 feet depending on which manufacturer you ask. It will be necessary to choose the correct coaxial cable for the job at hand. 75 ohm impedance cable is the standard used in CCTV systems. RG is the cable specification for use as a radio guide and the numerical value distinguishes the specifications of each individual cable. Even though each cable has its own number characteristics and size there is no difference in the way these different numbered cables work. Coaxial cable is used primarily for the transmission of analog video. The most common place it will be seen even with digital video systems is from the camera to the control equipment. Since many DVRs still require an analog video input they are still equipped with BNC connectors for the connection of coaxial cables. Some new DVRs will allow for inputs from analog and digital cameras so until analog cameras are replaced completely with digital output cameras coaxial cable will still be very common. Optical Fiber Optical fiber is a thin flexible material used to guide optical rays. Fiber optic transmission was designed to transmit digital signals in the form of pulses of light. A fiber optic cable contains one or more hair-thin strands of glass fiber each wrapped within a plastic tube and an external coating and capable of transmitting messages modulated onto light waves. See Figure 7-2. Cladding No light lost - cladding allows complete Internal reflection. Without cladding light gradually leaks out. Figure 7-2 Optical Fiber With Cladding 116 Digital CCTV Optical fiber is made up of a cylindrical cross-section with three concentric links Core the innermost section containing one or more very thin strands or fibers Cladding a plastic or glass coating with optical properties that surrounds each strand Jacket the outermost layer made of plastic and other materials that surrounds one or more claddings and protects them from environmental