AV Technology - January 2009 - (Page 31)

mum channel clock frequency is 165 MHz. Transmission at 1.65 Gbps means that we need to be concerned about the fundamental at one-half the data rate, or 825 MHz. This makes transmission more complex because the video/audio signaling bit rate is ten times the clock rate. Although the clock frequency does not carry video information, it’s important in this discussion to provide simplified insight into the basic challenge that long UTP and STP network-style cabling presents. The actual attenuation and crosstalk performance is governed by the actual loss effects at one-half the video data symbol rate, which is 825 MHz for standard DVI/HDMI and 1.7 Gbps for HDMI Category 2 performance. These frequencies are well beyond the standard data specifications for network cable. Therefore, transporting DVI and HDMI over long twisted-pair cables requires some “command of the art,” so to speak. Pushing performance requires one to pay careful attention to the matching of network cabling to specific drive ICs along with careful signal processing and board layout in order to handle these extreme data rates needed for DVI and HDMI. Figure 1 shows that, for the 165 MHz clock, the Cat 5e cable will have attenuation and crosstalk approaching intersection with a very small margin. The 825 MHz associated with the bit rate is way beyond the usable range based on the attenuation and crosstalk curves. Certainly, the clock signal just falls inside the usable range. What gives? How does it work at all? By the time video/audio data completes the journey through 100 feet or more, the UTP cable’s low-pass filtering effect reduces the data symbols to mere sine waves. This is where the receiver’s gain and equalization range become important. Physics works in our favor since the rise and fall time on the data signal diminishes, so does a proportionate level of crosstalk. Remember that we need at least a 5 dB margin. With careful receiver design and board layout, 100 feet is just attainable for DVI and HDMI. Cat 6 cabling provides us additional margin due to its 250 MHz specification, or, more distance with the same margin. At 165 MHz, Cat 6 margin is about twice that of Cat 5e cable. The Cat 7 STP cable exhibits very ample margin at 165 MHz. In fact, considering HDMI 1.3, an HDMI Category 2 cable design supporting the 340 MHz clock and 3.4 Gbps data channel still has almost 30 dB margin at 100 feet. In addition, we can extrapolate past the 600 MHz limit in the chart to realize that Cat 7 has possibilities with the one-half signaling rate, or 1.7 Gbps. Now, crosstalk is no longer the main issue, but attenuation and available receiver gain become the limiting factors. Consider for a moment the slope of the Cat 7 attenuation and crosstalk curves in figure 1. Due to the wide margin between attenu- http://www.bestcableforless.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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