Working with CCTV systems typically offers a select number of application technology choices. You can take the old standard route of laying an array of coaxial cable and connecting standard analog cameras or, as we see more and more today, you also have the option of creating a network configuration with category cable, such as Cat-5, -5e or -6, and connecting IP cameras to the network.
Sounds pretty cut and dry, but is that all there is to it? Not exactly. This month, I am going to focus on a special little device that can play a large role in assuring trouble-free operation of video surveillance systems.
A Balancing Act
Let’s say you recently had a chance to bid on a CCTV system in adding cameras to an existing facility. You lost the bid, not by a little bit, but by a lot. A big part of the job’s cost was the labor and expense of running coax cable through the existing structure. The difference in the winning bid was that it proposed hardly laying any new cable at all, and none of it was coax. The competition noticed a considerable amount of existing unshielded twisted pair (UTP) cable in the building that could be used for the CCTV system. However, a special device would be needed to use it.
In another instance, a customer wanted to add some cameras to some older elevators. Again, the existing elevator wiring system did not include coax. It was very expensive to upgrade the elevator wiring control harness, but there was some spare twisted control cable. Again this special device was summoned to save the day.
In another scenario, a home automation contractor has a customer who wants to receive a competitive bid on upgrading a large house with some S-video and stereo audio locations. He has decided to run it over standard Cat-5, even though one location is more than 500 feet away from the distribution point. He will also rely on this special device.
What is this amazing device that can help in special applications like these, and allow the low-voltage professional increased technology versatility? It is called a BALUN, which is actually an acronym for BALanced-UNbalanced. While this device is actually a longtime cousin of the transformer, it has gained renewed popularity as the worlds of digital and analog technology increasingly become merged and integrated.
Key Concepts to Keep in Mind
In past “Tech Talk” articles we have discussed the special performance of running a variety of signals over UTP in which outside electrical noise is canceled due to the opposing cancellation effects of a pair of uniformly twisted wires. However, in order to interface these balanced signals with unbalanced electrical circuits such as a coax with a shield, the balun is needed (see diagram).
However, nothing is guaranteed technically perfect in the world of baluns and the following conditions and specifications should be understood by the technician:
Delay skew — Delay is the amount of time for a specific length of cable to deliver a signal. Delay skew is the difference in that delay time from one twisted pair in a cable to another. Cat-5 cable typically has a maximum delay skew of 45 nanoseconds (ns) for data to work. Ironically, the delay skew or timing of a RGB video signal is around 40ns. Cable manufacturers such as Belden also have special “Nanoskew” cable in which the four twisted pairs are identical, making for minimal delay skew. However, there can be a problem with crosstalk.
Crosstalk — This is the interference from one twisted pair in a cable bundle to another next to it. The challenge is that the better the delay skew is, or the closer the cable lengths match each other, the more crosstalk you will get. One way to minimize crosstalk is to reduce the number of video cable pairs in a cable bundle.
Another is to look at the common mode rejection ratio (CMMR) of the video balun. Category cable such as Cat-6 actually has dividers that will push the pair apart and help with crosstalk. Also, you can look at special “Media Twist” cables from companies like Belden as well.
Ground loops — Since we are dealing with circuits that may have different ground potentials at different locations in the video circuit, we can have the potential for ground currents or ground loops. This is often noticed by lines on the video image. You may need to use balun devices that will help in isolating the ground loop.
What Baluns Do Best
Some have asked if you can run camera power, or even more specific, 24VAC camera power, in the same UTP cable bundle as video. Typically you can, as long as it is on another twisted pair. However, don’t forget Ohm’s Law and the length of the cable as far as voltage drop.
So in reflecting on what we have already discussed; what are some of reasons I would want to use a balun?
However, baluns are not ideal for all applications. Following are some reasons you may not want to use a balun:
- Care should be taken when running UTP cable near florescent light fixtures. At least a 1-foot distance should be applied
- Avoid mixing computer data in the same cable as audio/visual signals
- Be careful when running category cable in a facility with large AC motors or other electrical equipment that can produce large amounts of RFI
- Excessive signal loss can occur when a signal is converted or cables jumped multiple times in the same run. Signal performance, however, can be improved with active balun devices
- Be careful when running cable more than 3 feet parallel to high voltage lines. Remember the right angle rule when crossing high voltage lines as well
Turning the Tide on RG-59 Coax
So what if you have a customer who wants to do just the opposite of what we have been talking about? Say they want to add some new IP CCTV cameras to a system that has standard 75-ohm RG-59 coax. Baluns to the rescue again!
Companies such as Energy Transformation Systems Inc. have low cost baluns that allow an existing run of coax to replace a two-pair UTP with improved near-end crosstalk (NEXT). The supplier’s EIP-XX Series of baluns are designed to exceed IEEE 802.3 10BASE-T NEXT specifications with signals being send 200 meters at up to 40Mbps.