By: Joseph D. Cornwall, CTS-D
Technology Evangelist — C2G
Technical lingo is a kind of shorthand that's used to express concepts common to that specific topic or area of study. Technical lingo is important because it provides a very precise or unique "shorthand" description of a device, effect or concept. Unfortunately, if you aren't comfortable and familiar with the lingo of a topic it can be a tall hurdle to communicate efficiently with folks who consider the jargon of their field to be "self-explanatory." In this series of articles we'll lift the veils of misunderstanding from the lingo of the A/V industry.
The dictionary offers many definitions of skew. Skew can refer to a bias. For example, the results of a poll can be "skewed" by a poor phrasing of the questions. Skew can refer to engineering—a bridge can be built with a skew arch, which allows it to cross and obstacle at an angle. Skew can even refer to financial statistics, where volatility skew can be used to describe a downward trend. In the A/V industry, we view skew in the context of time.
Category cables are composed of at least four twisted pairs of unshielded wires. The wires are twisted in pairs to assure that the electromagnetic field—which surrounds any metallic object experiencing a changing electrical charge—is evenly distributed in the space around the wires. Running a changing electric current through a metallic object makes an electromagnet—as you may recall from your sixth grade science class. Twisting the wires ensures that each conductor is equally exposed to any external magnetic fields that could induce unwanted noise, including the magnetic field from adjacent wires.
In a Category cable, each pair is twisted with a different pitch. The word "pitch" as used here refers to the twist rate, usually defined in twists per meter. When nearby pairs have equal twist rates, the same conductors of the different pairs may repeatedly lie next to each other. This causes crosstalk between the pairs. For this reason the twist rates must differ. Now imagine opening up a Category cable and untwisting each of the pairs. Because the twists-per-meter of each pair are different, it follows that the untwisted length of each pair will be different, too.
Electricity flows through wire at a finite speed, defined as the "propagation velocity" of the signal. If a signal is flowing at the same speed through different lengths of wire, then it's only logical to conclude that it will take a different amount of time for the signal to transverse each pair. This results in skew.
In analog video applications where a signal is sent across Category cables, the red, green and blue components of the signal will each travel along its own twisted pair. Because these pairs have varying lengths—and therefore varying time intervals for transport— the differing arrival times of each of the color components can result in subtle color defects and ghosting. The image components will not align correctly when recombined in the display device.
To combat skew a selection of UTP cables known as "low skew" or "no skew" have been created. Because these cables don't allow for the same variation in pitch, crosstalk is increased. This isn't a huge problem for analog RGB applications, but it is very detrimental to data carriage such as in a network application. Therefore "no skew" or "low skew" cables must be specified with caution as they may not be the best choice for data lines like HDBaseT or 1000BaseTX applications.
This white paper is for informational purposes only and is subject to change without notice. C2G makes no guarantees, either expressed or implied, concerning the accuracy, completeness or reliability of the information found in this document.