Extron electronic TPX 88 manual Equalizing pair skew with Skew Equalizers, Installation, cont’d

Installation, cont’d

The Microtest OMNI SCANNER 2 performs comprehensive certification testing to the proposed CAT 6 standards. Other manufacturers also make testing equipment. The tests include advanced diagnostics for troubleshooting the cause and location of many cable and termination problems. For simple installation testing, the Microtest MICRO SCANNER PRO tests wire map and cable length, including individual cable pair length.

The manufacturing process for network (CAT 5) UTP cable leads to a condition called pair skew. Skew exists between pairs when the physical length of one wire pair is different from another. As the transmission cable length increases, the amount of skew increases. Skew affects the displayed image when the differential length between wire pairs exceeds 2 feet, causing the timing of the red, green, and blue video signals to appear out of alignment (horizontal registration errors). A white vertical line on a black field can appear as individual red, green, and blue lines that are close together; the signal transmitted on the shortest wire pair leads the other colors and appears to the left on the display.

Pair skew can be measured with test equipment. UTP cable test equipment measures and reports wire pair length. In certain circumstances, the report on the various pair lengths can be used in equalizing pair skew.

If test equipment is not available, skew can be identified by viewing a crosshatch test pattern with a critical eye to determine if either the red, green, or blue video image leads (appears to the left of) the other two video images.

For best results when using CAT 5 cable in A/V applications, pair skew needs to be equalized. Extron suggests several ways to equalized the pair skew as follows.

The design of Extron’s Skew-Free A/V UTP cable minimizes pair skew to the point that equalization is not required.

If there are different cable types and/or big differences in input cable length, skew compensation will not work.

Equalizing pair skew with Skew Equalizers

Try using the following methods to minimize or eliminate pair skew:

•If using MTP 15HD (RGB video) receivers, switch to SEQ versions of the receivers.

•Install separate SEQ 100 15HD or SEQ 100 BNC Skew Equalizers on the receivers’ video outputs and adjust the skew for the leading video image.

Equalizing pair skew with skew compensation cables

The nominal velocity of propagation (NVP — the speed at which the signal travels on the transmission line, measured as a percentage of the speed of light) of TP cable is very close to that of conventional coaxial cable. The similarity in NVP means that, when all transmitter cables systems use the same cable type and are approximately equal in length, by adding an additional length of coax, equal to the length of pair skew, placed on the receiver’s output, you can equalize the effects of pair skew (figure 2-4).

If UTP cable test measurement cannot be done, pair skew can still be equalized, assuming that all of the transmitter cable systems are equal, by viewing a crosshatch test pattern with a critical eye. Examine the test pattern for loss of horizontal registration and, through a process of trial and error, equalize any pair skew with coax extensions on the red, green, and/or blue outputs.

Extron skew compensation coax cables are available in lengths of 2 through 20 feet, see Appendix A for part numbers.

2-6 TPX 88 Matrix Switchers • Installation