AV Technology - October 2008 - (Page 28)

STAR G R O U N D I N G WARS A signal reference grid is a network of copper or aluminum wires installed below a raised floor in a data center. ment share a path, the impedance of that path causes the noise to couple between them. This mechanism is called “common-impedance coupling” (which is what makes unbalanced signal interfaces so prone to noise problems). In most systems, the single point, called the MGB (master ground bar) by some and “Mecca” by others, is the zero voltage reference ground for an entire system. Generally, this is the bus bar in the main power panel where neutral, safety ground, and earth ground are bonded as required by NEC. While the voltage at this connection point may rise above zero volts with respect to earth ground under power fault conditions, the entire system will also rise to the same voltage. This minimizes transient currents between pieces of equipment due to lightning or power surges. For obvious reasons, it would be great to have Mecca as close as possible to the audio system equipment. Only a transformer configured as a separately derived system per NEC can establish such a new N-G bond to improve system noise. “Technical power” using isolated-ground outlets is often part of a single-point grounding scheme, where the intent is to keep unknown currents that may be flowing in a metallic conduit from flowing in the system safety ground wiring. Star grounding is very well suited for audio systems since 60 Hz and its harmonics are in the audio signal frequency range, and audio dynamic range requirements are often extremely high. Star grounding is favored (at least in the U.S.) for audio and most AV systems. “Mesh” or “multi-point” grounding interconnects everything with a network of ground wires that attempt to approximate an equi-potential ground plane at both low (60 Hz) and high (radio) frequencies. Mesh grounding has been traditionally favored for higher-frequency systems such as IT or telecommunication systems worldwide. Although it generally can’t reduce power frequency voltage differences as well as star grounding, such low frequencies are “out of band” and of little concern in highfrequency systems. However, mesh grounding is also being promoted by many Europeans as the “preferred solution” for audio systems. At least one highly respected acoustical consultant clearly sees the potential for conflict. According to Kenneth Fause, principal consultant with AV consulting firm Auerbach Pollock Friedlander, “The major difference between IT/telephone and AV is dynamic range. A dynamic range of 20 to 40 dB may be adequate in the IT context. A dynamic range of 120 to 140 dB may be desired in some practical AV applications. The Ethernet physical layer is differential, and the signaling issue is detection of change of state. This is intended to be fairly robust on a noisy channel. Worst case, Ethernet backs off and tries again. A signal reference grid at gigahertz frequency must be on the order of 25mm [a grid of 1-inch squares]. Not too practical at an AV system scale. A “signal reference grid” (SRG — see figure) is a very popular version of www.avtechnologyonline.com 28 | AV TECHNOLOGY | october 2008 DIAGRAM COURTESY JOHN WILEY & SONS http://www.avtechnologyonline.com

Table of Contents Feed for the Digital Edition of AV Technology - October 2008

AV Technology - October 2008 Contents Precedent Corporate: Residential AV Does Not Equal Commercial AV Government: Funding Problems? Education: DIY ISP.EDU, Part Two The Cost of Bandwidth Star Grounding Wars Understanding IoIP How to Equip a Smart Classroom Two Rooms With A View From Primitive to Professional Product Review:�Visionary Solutions AVN 420 Encoder Product Spotlight: Document Cameras New Products Ad Index AV MO: Using Infrared Microphones

AV Technology - October 2008

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