Adapting Audio/Video to Unshielded Twisted Pair: Understanding Skew

Quite often it is necessary to adapt audio and video signals from a computer, digital signage server, teleconferencing codec, or video matrix switch from their native coaxial connectivity to a system that will work over Cat5e or Cat6 unshielded twisted pairs. Whether the need is driven by the opportunity to leverage existing wiring, the need to find a slimmer and more flexible connection, or to move the signal over a longer span than is practical on coaxial cable, transporting video signals via UTP is a frequent challenge for the integration professional. It is important to understand all of the things that are happening to that delicate signal in the process of adapting it to an RJ45 environment.

To adapt a single-ended source to a balanced transmission system, two techniques are used. The signal can be adapted passively — by using a transformer, or actively — by using a specially designed amplifier/equalizer. Both systems match impedance (video impedance will be explored in a separate article) and provide the requisite balanced signal pair output. Only one solution provides full control over the transition to the UTP environment. Decisions are always influenced by cost, and passive solutions may be attractive because they are very inexpensive. However, when using a passive system you sacrifice the ability to adjust for skew.

What is skew? A UTP cable (Cat5 or Cat6) is composed of unshielded twisted pairs. Multiple pairs are used so signals can leverage the benefits of parallel communication in networking, and a balanced signal topology is used to maximize the system's signal–to–noise ratio. Each of the four pairs in a Cat 5/6 cable employs a specific number of twists per foot to minimize signal crosstalk between the pairs; the distance per twist is commonly referred to as pitch. A Cat5e cable will use 3.88 twists per inch for the green pair, 3.91 for the blue, 4.52 for the orange, and 4.93 for the brown pair. This means that if each of these pairs were untwisted, they would correspondingly increase in length, relative to their differing twists–per–inch specification. Skew describes the difference in length between the pairs in a single cable.

A high resolution video signal, such as the output of a computer or digital signage server, is composed of three primary color signals (red, green and blue) along with a sync or timing signal. When adapted to a UTP environment, these four signals are each assigned one of the twisted pairs. An analog video signal is sensitive to the arrival of each of these primary colors at the right time because it must precisely combine the red, green and blue raster to provide the full gamut of color and brightness we perceive as the image. As you can see, if each of these signal components has to travel through a different length of wire, they must therefore arrive at different times. A longer road means a longer drive when speeds are held the same!

If each of the color signals arrives delayed relative to the other, the signal will begin to distort. This can happen on surprisingly short runs. Cat5e may exhibit as much as a 10% difference in length between color pairs, and it only takes about 6 feet of actual difference to begin to cause a perceptible loss of quality. Even at relatively low resolutions, such as 1280x800, the color components can shift by a full pixel or more on just a 60 foot run! A passive balun (balanced–to–unbalanced transformer) has no ability to adjust for skew. Should you want to use a passive system for distances over 60 feet, or with higher quality video signals that may distort with the lower levels of skew exhibited at even shorter distances, a skew–free or low–skew UTP cable should be employed.

Active conversion to balanced connectivity uses matched active (powered) devices that offer time (skew) correction, making the use of standard Cat5 cables possible while eliminating or minimizing skew. Active conversion devices will also offer equalization and amplification to overcome signal loss over the run due to capacitive effects, ensuring full contrast and color saturation of the image. Depending on product design and signal demands, an active balun system might correct for skew developed over as much as 300 feet of UTP, or fifty times the range of a passive solution.

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.