The use of Unshielded Twisted Pair (UTP) in CCTV systems is increasing, as is an interest in identifying common pitfalls or causes of poor video quality after installation. An ideal UTP CCTV system consists of several simple components, each deserving careful consideration in order to have a hassle-free installation. The key to troubleshooting is recognizing those ingredients. Compare the details of a problem installation to an ideal one and observe the video to identify possible culprits. Use these logical steps to reduce the time spent on the troubleshooting process.
When It is Not About The UTP
* Camera is Adequately Powered: Verify the camera's input voltage is within manufacturer's tolerances under load conditions (including heaters, blowers, etc.) With the camera connected and operating, measure the input voltage at the camera's power input terminals. Typically, it should be higher than 21 Volts AC for a 24VAC camera, and 11.5 VDC for a 12VDC camera.
* Camera is Properly Adjusted: Set focus, iris and shutter speed using a portable monitor. It is worth noting that many LCD monitors have internal Automatic Gain Control (AGC), making brightness adjustments difficult. The use of a Video Level Meter will result in more accurate levels, as demanded by today's sensitive DVRs. Alternately, use an oscilloscope or an older glass “jug” monitor to verify the signal.
* Avoid Ground Loops and Transient Damage: A ground loop is a system-grounding problem that occurs when one device in a system is connected to an electrical ground different from the ground of the other devices. At low voltage, the ground difference is detectable at voltages as low as 200mV, and can be identified as a “Hum Bar.” At high voltage, it is identified as a lightning hit or other transient. Large currents can travel through the wire to the other ground, damaging equipment along the way.
Leaving the camera floating is a far superior approach than local grounding. This may require rubber pads or plastic screws, but it is well worth the time in lightning-prone areas. Parking lot cameras or those mounted on metal-skinned buildings are particularly vulnerable.
If a transient protection device is used, place it right at the camera. Connect the protector's ground wire to the camera's chassis (or shield of the BNC connector), rather than the local ground, to prevent transients back-flushing into the system. This “single-point grounding” practice is highly effective for both high-voltage transients and low-voltage ground loops. These practices should always be subject to regulatory safety requirements such as Electrical Code.
Ground loops can also occur between cameras connected to a multi-output power supply if one or more of the cameras has an additional path to ground. Most multi-output power supplies do not isolate outputs from one another. Instead, consider powering cameras and peripheral equipment using supplies with isolated outputs.
Getting Ready For Twisted Pair Video
* Use Correct UTP Transmitter and Receiver: Confirm distance by taking a resistance measurement of the wire, and use the transmitter and receiver specified for that distance. If power is required for the UTP device, ensure that the power supply in use conforms to the UTP transceiver manufacturer's specifications.
* Wire and Connections: Use unshielded twisted pair Category cable. Use conductors that are twisted as pairs. Do not double up pairs for video or use any untwisted wire. Do not use shielded wire. An exception is multi-pair wire (6+ pairs) with an overall shield, or wire that is specifically rated Cat5 or better. Test wire pair using a Wire Map Tester, and verify RJ45 connections. If wire is likely to come in contact with water (buried or in underground conduit), ensure that moisture-resistant cable is used. Water will damage unprotected cable.
* Head-end Considerations: Correctly adjust the UTP CCTV receiver as needed, per manufacturer's instructions. Verify the control room receiving equipment has one (and only one) 75-ohm termination, and that it is at the end of the transmission path. For best receive-end level adjustments, temporarily connect the video signal from your UTP receiver directly to a video level meter, oscilloscope or monitor that does not have AGC, bypassing the DVR or other receiving equipment .
If plagued by poor or no video when installing or servicing a UTP CCTV system, use observations to identify possible causes. It may be necessary to test each cable segment separately. For example, test the camera and the monitor together without the other equipment. Add in the UTP transmitter and receiver back-to-back. Testing each segment of a long cable run independently allows you to isolate the problem.
Video Symptoms and Solutions
Faint or blurry picture with little or no color: This symptom most often indicates loss of signal strength. Check for excessive wire distance, incorrect equalization of adjustable UTP transmitter or receiver, improper camera adjustment or output level, the use of shielded wire or water damage to wire. Testing for possible water in the line involves measuring the capacitance between the conductors that have been disconnected from other equipment. Cat2 or 3 wires should read 19pf per foot or wire; Cat5 is 16pf per foot. (Example: 1000ft of Cat5 should read 16nf or .016µf).
Extremely faint picture with only shadows of an image: There is likely a wiring problem such as an open conductor or short between conductors. Verify using an ohmmeter.
Over-saturated colors, high-contrast, grainy, bright, wavy or torn image: This could indicate incorrect equalization of the transmitter or receiver, improper termination or a ground loop problem. To measure for a ground loop using an AC voltmeter, test between each UTP conductor and earth ground. The reading should be less than 200mV.
Image out of sync: This could indicate incorrect equalization or a ground loop. But it is also indicative of reversed polarity if the image appears scrambled or as a negative image.
Faint stripes or bars gliding up or down in the image: This usually indicates crosstalk, which occurs when nearby signals are induced as noise onto the transmission path. It most frequently occurs when untwisted wire, mis-wiring or inferior baluns are in use. Not all UTP devices are created equal. Noise can also easily be induced onto coax in the path. Horizontal, rolling bars indicate ground loops.
Random lines, noisy image or faint shadows from the image of another camera: Crosstalk is the most likely culprit.
Ghosting — faint shadows of original image shifted to the right: This is an indication of an impedance mismatch, most often the result of a bridge tap — an extraneous length of dangling, un-terminated cable connected to the transmission line. Locate and remove bridge taps, and check for extra conductors at punch-down connections. Ghosting can also indicate improper termination.
No video, flickering image or signal LED: This could indicate a poor UTP or BNC connection. It could also be the result of an intermittent signal originating from the camera. Verify the camera is not experiencing a brownout condition due to insufficient power.
While wiring problems, signal level issues, ground loops and improper termination are the most commonly identified culprits when troubleshooting UTP CCTV Transmission Systems, product failure periodically occurs. If you suspect a transmitter or receiver has failed, replace the device with a known good device or contact the manufacturer for support and any recommended testing procedures. Armed with the correct test equipment and the facts about common UTP transmission issues, don't be surprised if your troubleshooting hours are significantly reduced, leaving you more time to design and install new systems.
Christine Baeta is technical services manager for Network Video Technologies Inc.
