UTP Video Transmission: Tips & Troubleshooting
If you are in the CCTV industry, it is likely you have noticed a substantial increase in the cost of coaxial cable. As you know, the two main substances used to manufacture coaxial cable, copper and petroleum, are becoming hot commodities.
The overall cost of copper has been on the rise for months and hit a record high of more than $4 a pound in early May (see page 88 of this issue for related information). As for the cost of petroleum, it would be difficult to find someone who has not noticed the consistent increase in cost at nearly every visit to a gasoline station. Because this situation appears to be long-term, many installing companies are looking for alternative cabling solutions.
Unshielded twisted pair (UTP) cabling has been a proven cabling solution for CCTV installations for more than 20 years. One of the biggest advantages of UTP cabling is the cost. Category 5/6 cabling has been consistently 45 to 50 percent less per thousand-foot spool than coaxial cable.
UTP cable also provides resistance to electromechanical and radio interference. Through active electronics, cable distances of up to 12,000 feet can be achieved. It also can help to create a forward-compatible cabling infrastructure that can support IP-based camera solutions as technologies advance.
As with any technology, possible miswiring and setup scenarios will result in an unfavorable installation. The following quick-list of obstacles to avoid, along with troubleshooting techniques to deploy when faced with these obstacles, should help alleviate any problems.
One of the greatest cost advantages to UTP technology is the ability to use existing in-house pairs or unused pairs of vacant UTP cables. If the existing UTP cable was used for phone or voice service in the past, it is recommended that you find and remove any T-taps before beginning your installation.
T-taps, also called bridge taps, are ancillary pairs of wires connected in parallel to the main pair being used. They are unused pairs and usually are found on telecommunication circuits connected at a punch-down block or pedestal. They are from previous extensions or lines that have been disconnected.
In audio circuits, they do not pose a problem, but in video circuits, they cause reflections that appear as “ghosts” in the display. It usually is possible to tell how many T-taps are present by the number of reflections in the display. (Their existence is often denied by telecom personnel!)
Fix Crossed Polarity
In order to utilize UTP cabling for a CCTV installation, it is necessary to install electronic devices to the CCTV equipment which, in turn, connect to the ends of the UTP cable. The majority of CCTV equipment today has a 75-ohm unbalanced BNC connector to attach coax to its input/output.
To use UTP cable to send a video signal, technicians must connect a balun-type device to the BNC of this device. Most baluns are designed with a BNC to a two-wire output. The two-wire output is labeled +/-. If the polarity is crossed, the image will appear to be scrambled. To correct this, one must simply change swap pairs on one end of the cable.
Isolate Ground Loops
Many UTP cable installations require running video between two buildings or in large buildings with lengthy cable runs. Unfortunately, both applications have a higher likelihood of potential ground loops.
A ground loop occurs when the source and destination of a video signal are at differing AC or DC earth potentials. Earth loop currents flow and cause longitudinal hum to be introduced into the video signal.
Video hum is low-frequency noise from the ground lines (usually 50- or 60-Hz mains frequency or its harmonics) which has influenced the video signal and is causing degradation of the displayed signal.
Video hum is usually observed as bars rolling vertically through the video image. Video hum also may cause video distortion or even tearing of the picture in severe cases.
Video hum may be a problem in any system where video sources and display devices are connected to different AC power sources with varying grounding potentials. Although ground loops are unpredictable and difficult to troubleshoot, they can be anticipated and planned for.
When looking at installations where ground loops may be an issue, the use of an active receiver that offers ground loop isolation is recommended. Although this does increase the cost of the receiver slightly, it will be far less than the cost of the labor to troubleshoot, diagnose and eliminate a ground loop.
Guard Against Attenuated Video Signals
Attenuation is the loss of signal. The longer the run, the more attenuation the cable will have. In other words, as the signal travels down a cable, part of it is absorbed by the cable, and the signal arrives at the end of the cable with less strength than when it started.
With a video signal, as the wire attenuates, the high frequencies are affected first. A video image may appear to be washed out, or in more severe cases, it may lack color altogether. Just as with coaxial cable, video amplification may be required on longer cable runs to compensate for losses that occur from wire attenuation.
Prepare for Network Connections
Most recently, many UTP installations have been designed around a network wiring scheme in anticipation of future IP technologies. A network wiring scheme involves the use of RJ45 jacks, patch panels and telecommunication connection points.
Network-type installations move away from the traditional two-wire balun design to a balun which has an RJ45 connector. While it is possible to field-terminate an RJ45 connector directly to the end of a Category 5/6 cable, it is recommended that you terminate the cable to a jack on one end and a patch panel on the opposite end.
The use of patch cables can be used to connect the baluns without concern for crossed polarity, misterminated or poorly terminated RJ45 jacks. Additionally, each cable run can be certified as a network connection.
How to Use 66 and 110 Punch-down Blocks
UTP cable can run to centralized locations where several cable runs can migrate to a multi-pair UTP cable through the use of a punch-down block. This helps to create a wiring scheme that resembles a star in shape and has proven to be more efficient, especially for moves, additions and changes, than the traditional home run approach previously used in coaxial installations.
The use of UTP cable for transmission of CCTV signals can help reduce cabling and labor costs, improve video performance and create a future-proof cabling infrastructure.
The majority of UTP transmission installations that have been completed with less-than-stellar results were not due to the electronic technology, but to installation problems. But with the tips set forth in this article, more proper cabling can be done that will reduce installation errors, as well troubleshooting time.
October 1, 2006 -- (Reprinted with permission from SDM magazine, Copyright 2006)