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paging systems, mobile telephone, and emergency or public-safety communications systems to see if they are the cause. A procedure that works well for high- frequency interference is to fi rst check the video output from the camera. If there is no sign of interference, this indicates the interference is probably being induced into the cabling. With the cable still disconnected from the camera check to see if there are any signs of interference on the blank monitor screen. If so, that is a sure sign that the interference is induced into the cable, assuming the video locally at the camera shows signs of interference. T e eff ects of high-frequency interference can be reduced by the incorporation of fi ber optics and, to a lesser degree, UTP cable for your video transmission method. However, if the RF is penetrating into the camera body itself, it may be unfi xable. Some alternative approaches that may produce positive results include using a special housing lined with a RFI shielding material, and RFI mesh positioned on the face of the housing, but the most successful method may wind up being simply moving the camera to another location. If the interference appears to be induced into the cabling, a RF choke may be the answer. A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor that blocks higher-frequency AC in an electrical circuit while passing signals of much lower frequency and DC by having impedance largely determined by reactance, which is proportional to frequency. T is choke can be placed either around the suspected cable or in series with the cable as a pass-through. CASE NO. 5: CHECK THE CABLING Our fi nal troubleshooting challenge, as illustrated in Figure 6, involves a system that has been installed for Figure 5: Cabling Issue under test to a VOM. Use a BNC to banana plug adaptor if available. 3. Place the volt-ohm meter in the ohms position. 4. Measure the DC resistance of the cable (should not exceed 7-10 ohms). After completing the test and Not only is the video image distorted but also the time, date and camera title. Check the camera's video output; if it is normal, the logical problem will be in the cabling. many years. T e problem also occurs at all times of the day and night, and does not vary. T e transmission method is RG-6u coax, and it is running in an underground conduit. At fi rst glance, it appears to be an RF interference problem. However, note the date, time and titling on the monitor screen. Not only is the video image distorted but also the time, date and camera title. In most analog equipment, graphics is keyed by the vertical sync pulse that's generated by the camera's electronics. So our path for troubleshooting is going to focus on the video signal from the camera. T e fi rst check is going to be the actual video output from the camera; if it is normal, the logical problem area will be in the cabling. Since the cable is RG-6 and is in conduit, the next test should be the verifi cation of the quality of the cable. T is procedure, known as the cable compliance test, incorporates a simple volt-ohm meter. By measuring the DC resistance of the cable, one can determine if the cable is at fault. T e test procedure for cable is as follows (see Part I of this series for more on cable testing): 1. Short one end of the coax, center conductor to the shield using a BNC shorting stub. 2. Connect the other end of the cable reviewing the results, we fi nd that the cable has an open shield conductor. T e cause is easy to determine. Underground conduits are never without moisture and after a period of time the water fi nally penetrates the outer coating of the cable, and the corrosion process of the shield begins. T e shield turns from a metal conductor into nothing more than a pile of basically dust without conductivity. But how and why are we still seeing some video if there is no return path for the signal? T at is a good question. All equipment is referenced to some type of ground in the overall system design. T e ground for this system, poor as it may be, could be in the metal structure of a building, the actual earth itself, or some other associated equipment that is running back to the main console room. In any case, the only solution to this problem is to install new cabling. Robert (Bob) Wimmer is president of Video Security Consultants (cctvbob.com) and has more than 38 years of experience in CCTV. His consulting firm provides technical training, system design, technical support and system troubleshooting. Originator of the D.U.M.I.E.S. series (dumies.us.com), Wimmer was inducted into SSI's Industry Hall of Fame in 2006. OCTOBER 2012 / SECURITYSALES.COM / A7 ss1012dumies.indd A7 9/26/12 1:45 PM