AV Technology - January 2009 - (Page 27)

Advances in cable, connector, and driver designs make it possible to send DVI and HDMI signals over longer distances. ow that sending RGB analog video long distance over Cat 5 is well accepted, it seems only fitting that DVI and HDMI are next. Accomplishing such a feat seems almost magical considering the challenge of simply buying a good quality dedicated DVI or HDMI cable. A fundamental understanding of the challenges and guidelines of DVI/HDMI transmission is necessary in order to take full advantage of long distance extender systems. N WHY THE DIFFERENCE IN CABLES? The original DVI and HDMI cables satisfied a limited-distance interface. For DVI, the interface is usually from the computer graphics card to the monitor. For HDMI, the interface is typically from a DVD player or AV receiver to a home TV. In each case, no more than about 6 feet is necessary, and certainly no more than three times that distance for typical uses. Within this realm, cable construction employs wire gauges smaller than the 24 gauge wire in the typical UTP cable. Twisted-pair wire size in those dedicated cables ranges from about 28 gauge to 32 gauge. This step-down in wire size yields substantial additional cable loss and is manageable only for short distances. Why use it? Small wire size makes for small diameter, flexible, easy-to-handle cables. Some DVI and HDMI cables support distances to 75 feet or so, but the physical diameter is significantly larger. These designs often use 22 gauge wire and lower-loss dielectric construction. Larger wire size translates to larger overall diameter and stiffer, heavier cable. With proper active equalization circuits (cable drivers or equalizers) extension to 200 feet is possible. It’s important to remember that DVI cables historically extended only about 5 meters. Release of the HDMI specification ushered in the notion of 75-foot transmission distances as if the standard or the electrical interface somehow improved. In truth, nothing changed. Both specifications include the same electrical requirements for signal transmission. So why is 75 feet promoted in HDMI? After much complaining about short DVI cable distance capability, cable manufacturing improved to the point that this distance is possible under the same specification. A SKEWED NOTION Knowing this DVI/HDMI cable history, the notion of transporting DVI and HDMI over low-cost category cabling may seem technically www.avtechnologyonline.com troubling, but it is possible. Successfully sending DVI and HDMI signals over Category 5e, 6, or 7 cable is possible with special cable drivers for two reasons: [1] Those cable driver/receiver circuits are designed for 100 meters distance; and [2] Wire gauge for UTP starts at 26 AWG, with most being 24 gauge, and the better cables as large as 22 gauge. UTP cable works with network systems because it employs differentially driven wire pairs along with varied twist rates that minimize data crosstalk. Integrated circuit manufacturers invest heavily into IC designs capable of the varied demands of network cabling and its increasing transmission speeds. But while the varied twist rates for pairs within a cable assembly minimizes crosstalk, it also creates timing skew. ICs driving DVI/HDMI transition-minimized differential signaling (TMDS) data over UTP cable must compensate for the timing differences between pairs, as well as attenuation variation. Migration of transmission speeds for networks has necessitated flexibility in cable transmitter/receiver IC combinations. AV applications can take advantage of this technology to exploit DVI and HDMI over inexpensive twisted-pair cable. CHALLENGING CABLE SPECS Using available network cable driver technology, 24 gauge UTP cable can transport DVI and HDMI over modest distances of about 100 to 150 feet. Why not farther? It’s not that cable drivers cannot overcome the attenuation of longer cables, but that crosstalk between pairs impedes the receiver’s ability to recover data. Even with the best 23 gauge UTP cable, crosstalk effects ultimately take over. In addition, most receiver blocks available with built-in equalization are limited to about +40 dB, a gain of 100. As receiver gain increases, crosstalk effects magnify. With increasing cable length, crosstalk rises and gain decreases. As attenuation and crosstalk approach intersection, receiver circuitry can no longer recover clock or data due to crosstalk interference; thus, data recovery becomes increasingly difficult and eventually fails. As the margin narrows to less than 10 dB, transmission failure becomes imminent. While shielded cable (STP) might help, the cable is only part of the problem. A significant contributor to the crosstalk issue is the RJ45 connector. Parasitic capacitance between connector pins provides significant cross-coupling that exacerbates the problem. january 2009 | AV TECHNOLOGY | 27 http://www.avtechnologyonline.com

Table of Contents Feed for the Digital Edition of AV Technology - January 2009

AV Technology - January 2009 Contents Precedent Corporate: Hunkering Down Government: The Great Unifier Education: Technological Self-Sufficiency Integrating Communications into Your Business Process DVI/HDMI Signals over Twisted-Pair Cable AV Helps Explain the Unexplainable AV Enhancements at the Coliseum Product Forum Tech Horizons Product Spotlight New Products Ad Index AV MO

AV Technology - January 2009

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