From Issue #2, Page #68
-Contributed by TechLiving

by Daniel Sweeney

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Squelch electrical noise and get better audio and video

The new home entertainment system is all wired up. You're ready to watch The Matrix Reloaded, but something's not quite right.

There's an annoying hum in the background coming from several speakers. Worse yet, the picture on the projection screen is a tad unsteady, and a series of faint bars appear seemingly behind the image.

In most cases, noise and picture artifacts have their origins in the house's electrical system, or more precisely, in the way that the various A/V components interact with the AC electrical system. Let's look at the simplest concepts and the most basic proven procedures for noise suppression.

Noise Problems
The alternating current transmitted to American homes by utilities is nominally 120 volts and 60 cycles per second. The two parallel blade-shaped prongs on an AC power cord represent hot and neutral, while the third pin represents ground, which rests at the same zero voltage potential as neutral. Electrical noise normally enters an A/V system through the ground paths.

Hot and neutral both connect with the power supply of each component and supply the alternating current to it. Ground, on the other hand, connects to the metal chassis of the component and serves as a path to drain away any voltages that occur on the chassis. It's a safety measure, just like the ground return wire extending down from a lightning rod.

Now in the case of the RCA connector cables used in most A/V components, the outer sheath is grounded on the chassis and thus is part of the same circuit as the third pin ground. In other words, each RCA has a direct electrical connection to the third-pin socket on the wall.

Normally, some residual 60-cycle AC winds up on the chassis, generally at levels of at most a couple of volts. Since AC tends to follow the path of least resistance, anything on the chassis is generally going to follow the very-low-resistance, heavy-gauge third-pin wire in the AC cord and from there

ultimately to the grounding point for the house wiring. In some cases, however, an RCA shield represents a lower resistance, or perhaps the third-pin wire on another power cable does, in which case you get a "ground loop" where electrical currents circulate through the ground paths and into the signal circuits via the RCA shield, causing hum.

Ground loops are the most common source of electrical noise, but noise can occasionally enter electrical power supplies through the hot lead, or it can result from the magnetic fields thrown off by power cords, which, incidentally, all AC power cords generate. Higher-frequency electrical noise can also be radiated from A/V connectors in the form of radio waves. In this case, the noise ultimately emanates from high-frequency circuits within the components, such as digital decoders.

Finding the Problem
Noise can manifest itself only in the last components in the signal chain, the video display and the loudspeaker, but it usually enters at the beginning of the chain.

In addressing noise problems, the first order of business is to locate the noise, which one does by progressively removing components, starting at the signal source. If, for instance, the noise stops when you've disconnected the DVD player, you've managed to localize it. When disconnecting equipment, be sure to turn everything off first, then turn the system back on to listen for noise.

Once you've identified where the noise is entering the service, you must find its actual source, which is largely a matter of trial and error. First examine the position of power cords in relationship to the RCA connectors. If power cords cross RCAs, they should always do so at right angles and should never run parallel in close proximity. Make sure the two cables maintain the right-angle relationship.

If that doesn't work, clean RCA terminations thoroughly with electrical contact cleaner. Noise voltages develop across an electrical resistance, and reducing resistance in the ground path may solve the problem.

If noise persists, try a different RCA cable and strive to avoid the use of cables of over two meters in length. Better to reposition equipment and use shorter cables if the cable run is excessive. If this still doesn't cure the noise, then you must resort to more radical and expensive palliatives.

If the two components being connected have provisions for balanced three-pin XLR cables, utilize those in preference to RCAs. XLRs are immune from induced hum from power cables and from radiated noise, and their ground circuits cannot be modulated by noise. You can buy the expensive kind from an audio specialist for hundreds of dollars, or purchase XLR microphone cables from a large electronics retailer like Fry's for about $15. A middle ground would be Monster Cable Pro's Z200i-x Reference ($99.95 per meter pair).

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