AV Technology - October 2008 - (Page 27)

caused (or made far worse) by small ground voltage differences between the individual pieces of equipment that make up the system. “Line-level” interconnections (as opposed to “mic-level” or “speakerlevel” interconnections) are always the most troublesome in audio systems. Because microphones and loudspeakers are rarely grounded independently of their connected cables, their interconnections rarely become part of a “ground loop.” But line-level interconnects are usually between two pieces of equipment that are each independently grounded, either directly by their power cord, by rack mounting, or by a myriad of paths to other (grounded) equipment. These interconnections beg to complete ground loops. Ground loops occur because no two points in a grounding system are ever at exactly the same voltage. The most significant source of these voltage differences is magnetic induction (essentially a parasitic transformer) in the premises’ power wiring. This induction creates a small voltage over the length of every safety ground wire in the premises’ wiring. Since safety ground is connected to signal reference ground inside virtually every piece of audio or video equipment, the ground voltage difference causes a small power-line current to flow in signal interconnect cables. Depending on the type of interface (i.e., balanced or unbalanced) and the quality of the equipment design (i.e., CMRR of balanced inputs and freedom from “pin 1 problems”), the resulting noise problems can vary from severe to non-existent. TWO BASIC SCHEMES It would be wonderful if all system ground connections were equi-potential (equal voltage) regardless of ground current flow; however, in the real world, this must remain an engineer’s fantasy. In the real world, ground systems consist of wires. Wires have not only resistance, but also inductance, which makes their impedance non-zero at power frequencies and rise steadily with increasing frequency. Therefore, there will likely be a voltage difference between any two physically separated points in such a grounding system. According to Henry Ott, author of Noise Reduction Techniques in Electronic Systems, the type of grounding system you use should depend on the specific system application. “The proper ground system is determined by the type of circuitry, the frequency of operation, the size of the system (self-contained or distributed), and other constraints, such as safety,” writes Ott. “No one ground system is appropriate for all applications.” Over the years, two popular strategies developed to reduce system-wide ground voltage differences. According to Ott, “Normally, at frequencies below 1 MHz, a single-point ground system is preferable; above 10 MHz, a multi-point ground system is best.” “Star” or “single-point” grounding takes the approach of separating individual ground currents by forcing them to flow in different paths (wires) wherever possible. If ground currents from two pieces of equip-

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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