UTP Presents an AV Technology Twist
One of the most interesting aspects to the convergence of AV and IT is the crossover of techniques and technologies between the two camps. The two cultures sometimes think differently in terms of new technologies, but techniques are another story.
ONE OF THE MOST INTERESTING ASPECTS TO THE CONVERGENCE of AV and IT is the crossover of techniques and technologies between the two camps. The two cultures sometimes think differently in terms of new technologies, but techniques are another story. Technicians from both camps take great pride in their installation work. For example, qualified technicians from both sides know how to create a truly well-dressed rack system. In our industry, we even have competitions to judge this skill set. Sharing techniques — such as using hook-and-loop straps instead of zip ties on data cables — will help us merge these worlds faster and more smoothly.THE SPECIAL CASE OF UTP
One technology to make the leap from IT to AV is Unshielded Twisted Pair (UTP) cable for the propagation of AV signals. Having mostly IT backgrounds, the people in my division quickly embraced this technology and are thoroughly familiar with TIA/EIA T568B termination techniques. We're used to the correct handling of this cable.
What we, as IT guys, quickly learned was that all UTP is not created equal. We had become used to Category cable — cable certified to carry data to very rigid specifications and defined by Cat-5, Cat-5e, and Cat-6 — and its reasons for different uses from the data side.
But while a lot of AV gear is able to accommodate Category cable, it's not the optimum cable for this equipment. Let me explain. UTP is defined as a class of cable that has eight 22 or 24 AWG wires that are bundled into four twisted pairs and encased in an unshielded package. These pairs of wires are twisted in order to minimize any noise that a long run of this cable would inherit — and therein lies the problem.
Category wire has different twists per a given length of each pair. This is necessary for data transmission, and the difference between various pairs actually increases as you move up the categories.
In AV, the cable is used to move a number of signals from point A to point B. With the right equipment at each end, you can move RGBHV/L&R Audio/Duplex Serial signals on a single UTP cable —and the distance between point A and point B can be as far as 2,500 feet. Think of the cable run for that same scenario using conventional AV cable. A mini-coax bundle cable, two audio cables, and a serial cable, either bundled together or separate cables, that's still a lot of copper.HOW IT WORKS
With UTP and simple (RGBHV only) transmit and receive devices, the red positive signal goes on one wire, the green on another, the blue on another, etc. The trick is the sharing of some wires. It gets a little more complicated when all kinds of signals are combined, but that's each vendor's magic.
Now let's investigate what that means to a cable. If you send red down one wire and green down another, for example, and the two wires are different lengths (due to the number of twists using more wire in one pair compared to another pair with fewer twists), the resulting signals will arrive at different times. When the signals don't arrive at the same time, this causes skew — the delay between one pixel arriving from one wire and another pixel arriving from another wire. This is expressed in nanoseconds and is usually negligible on small runs and lower resolutions.
For example, at XGA resolution, a pixel is pumped out about every 15 nanoseconds; the maximum skew for Category cable is 45 nanoseconds, according to its specification. Doing the math for long runs, it's obvious to see that pixels can't arrive at the same time on this cable. With long runs and displaying something like a spreadsheet, you would drive a person nuts. Colors would tend to smear or tear, and lines would actually disappear from a displayed spreadsheet's grid.DEALING WITH THE PHENOMENON
There's two ways to compensate for this phenomenon. First, use skew-free or at least low-skew UTP cable. This will eliminate the problem and ensure a perfect image at the other end with the least effort. While this cable will cost more per foot than the cheap Cat-5 we're all used to, the cost will outweigh the need for other equipment. The second way to address skew is to use equipment with either built-in or external skew compensation circuitry. This allows the installer to add delay to the various wires, thus making all the pixels appear to arrive at the same time.