The ultimate goal of sound and image reproduction is realism — the kind that creates suspension of disbelief in the listener or viewer. And nothing breaks the spell like background buzz during a quiet musical passage or a suspense-filled moment in a movie. No matter how good the reproduction technology is, interference can really spoil the experience.
While professionally balanced interfaces largely preclude noise problems, most audiophile and home theater systems consist mostly, if not entirely, of audio and video equipment with unbalanced inputs and outputs — a danger zone for signals. In all but the smallest systems, which use very short cables, noise problems are likely to exist. Nevertheless, there are ways to combat this problem and minimize its effects on your customer's audiovisual systems, which, in turn, improves their listening and viewing pleasure.
What's making all that noise?. Any signal accumulates noise as it flows through the equipment chain; and once it's contaminated, no process can remove the noise without degrading the original signal. Since the dynamic range of an entire system can be no better than its weakest link, noise must be avoided everywhere along the signal path. In most systems, the worst problem is not signal processing in the equipment itself but so-called pickup, or noise coupling, in the interconnect cables.
This noise is most often a mixture of 60 Hz harmonics and other high-frequency noises that normally exist on AC power lines. Since it couples to the audio or video signal path via grounded cables, this noise is usually referred to as “ground noise.” This should not be confused with “random” noise, which manifests itself as a hiss in an audio system or granular movement (snow) in a video image. A predictable amount of random noise is inherent in all electronic devices and must be expected. Ground noise produces artifacts such as hum, buzz, clicks or pops in audio systems. In video systems, it generally produces horizontal bars (light or dark) or bands of specks that slowly move upward in the image.
Common-impedance coupling. There's a common misconception among many people that this noise is something airborne, picked up by cables, which can, therefore, be cured with more cable shielding. See Sidebar on page 24. However, in real-world AC-powered systems, small leakage or ground noise currents will always flow in any wire connecting two devices. The tiny voltage drop this creates is actually what causes 99% of consumer system noise.
As shown in Fig. 1 above, when two pieces of equipment are connected via an unbalanced interface, the noise current flows in the shield conductor of the cable. Because the shield has impedance, a small noise voltage drop appears across the length of the cable, according to Ohm's law. Since the cable shield is also part of the signal circuit, the noise voltage will be directly added to the signal at the receive end of the cable, which is the sum of all the voltages in the loop from point A to point C. Because the shield impedance is part of two circuits (noise current and signal current), this mechanism is called common-impedance coupling.
Suppose you have two devices connected by an RCA audio cable. Both devices have non-grounding type power cords, and their power-line capacitances cause a 300µA, 60 Hz noise current to flow between them through a typical 25-foot unbalanced cable. The cable has a foil shield and a 26-gauge drain wire, which makes its resistance (or 60 Hz impedance) about 1 ohm. Using Ohm's law, we calculate the resulting voltage drop to be 300µV. This noise level is only 60 dB below the nominal consumer signal reference level of 300mV. The high-frequency noise level may be even worse and more audible as buzz. Compare this to the 95 dB dynamic range of an ordinary audio CD and you can see how a musical recording wouldn't fade into “silence”, but rather fade into a noise signal.
The magnitude of the noise current can be much higher if both devices are served by grounded type power cords. Referring to Fig. 1 again, consider that a significant voltage difference normally exists between outlet ground connections. Cumulative leakage currents and inductive coupling in the premises AC wiring system cause this voltage difference. It's not uncommon to have voltage differences of 100mV between the ground pins of two different receptacles.
And this voltage difference can get much higher between two outlets on different branch circuits. For two grounded devices, this voltage will be impressed across the length of the signal cable shield and be directly added to the signal. For 10mV of noise voltage, the noise would be only 30 dB below reference level.
In a standard video interface, reference level is 1V (peak-to-peak), including sync. The active range from reference black to reference white spans about 600mV (peak-to-peak). Since our previous noise voltage of 10mV (rms) equals about 30mV (peak-to-peak), our signal-to-60 Hz hum ratio would be 26 dB. This would cause a visible hum bar in a video display. Even higher voltage differences can result if one of the devices is connected to an outside ground point such as a separate earth ground or a CATV drop.
Pinpointing the source of the noise. Weeding out power-line noises such as hum and buzz from an audio or video system can be frustrating and time-consuming. The following method is safe, effective, easy to understand, and requires no test equipment other than your ears.
Gather data. The success of any troubleshooting process has a lot to do with how you think about the problem and what data you gather. Ask a lot of questions such as: Did the system ever work right in the first place? What are the symptoms that tell you it's not working now? When did it start working poorly or stop working altogether? What other symptoms showed up just before, just after, or at the same time as the failure? Gather as many clues as possible before you try to solve a problem. And write everything down. Imperfect recall wastes a lot of time.
Tinker a bit. Use the equipment's own controls, with some logic, to provide additional clues. For example, if the noise is unaffected by the setting of a volume control or selector, then it must be entering the signal path after that control. If the noise can be eliminated by turning the volume down or selecting another input, then noise must be entering the signal path before that control.
Use a visual aid. Sketch a block diagram of the system. Show all interconnecting cables and their approximate lengths. Mark any balanced inputs or outputs. Generally, stereo pairs can be indicated with a single line. Note any equipment grounded via a 3-prong AC plug, and any other grounds such as cable TV or DSS dishes.
Ground dummy tests. The term “dummy” refers to special adapters that don't pass a signal. Dummies allow the system to test itself and pinpoint the exact entry point of noise or interference. By temporarily placing the dummy at strategic locations in the interface, precise information about the nature of the problem is revealed. To review the requirements of a simple four-step “dummy” test procedure, go to www.jensentransformers.com/an/ts_guide.pdf. The tests can specifically identify common-impedance coupling in the cable, magnetic or electric field coupling to the cable, or common-impedance coupling inside equipment.
Dummies can be made from standard parts and wired as shown in Fig. 2 on page 23. Since a dummy doesn't pass a signal, it should be clearly marked so it doesn't accidentally permanently find its way into a system.
How do I get rid of this noise? When a system contains two or more ground connections, whether through power cords to safety ground or other grounded signal cables such as the CATV connection shown in Fig. 3 at left, a ground loop is formed. This ground loop provides a complete circuit path for ground noise current.
Because a voltage difference, which is often substantial, exists between the ground for CATV and the safety ground at the sub-woofer, a noise current will flow in the shield of all signal cables that are part of the loop. Common-impedance coupling then adds noise to the signal in these cables. In general, the noise added is directly proportional to the cable's length.
This sample system would probably exhibit a loud hum regardless of the input selected or the setting of the volume control because of ground noise current flow in the 20-foot cable. The hum might be slightly louder if the TV input was selected and the volume was turned up because the same ground noise current also flows in the 3-foot cable. Because the ground loop is a series circuit, opening the circuit at any point can interrupt current flow. You might be tempted to open the loop with a 3-to-2-prong adapter at the sub-woofer AC plug, but that creates an electrocution and fire hazard for which you would be legally liable.
There are two basic ways to reduce common-impedance coupling in unbalanced interfaces. You can either reduce the coupling impedance or reduce the circulating ground current.
Reduce coupling impedance. The cable's grounded conductor (typically the shield) is the most common impedance. You reduce it by following these steps:
Keep cables as short as possible. Longer cables increase the coupling impedance. Even short cables can produce severe coupling if ground currents are high. Never coil excess cable length.
Use cables with heavy, braided copper shields. Cables with shields of foil and thin-gauge drain wires increase coupling impedance.
Maintain good connections. Contact resistance is part of the common impedance. Connectors left undisturbed for long periods can develop high contact resistance. Hum, or other noise that changes when the connector is wiggled, indicates a poor contact. Use a good commercial contact fluid and/or gold-plated connectors.
Reduce the circulating ground current. Some tips for reducing circulating ground current include:
Don't add unnecessary ground connections. With rare exception, additional grounds increase ground noise currents. Of course, don't disconnect required safety grounds to reduce noise current either.
Use ground isolators at problem interfaces. Commercial isolators are available for audio, video and CATV signal paths as well as for digital interfaces.
Fig. 4 shows how a ground isolator breaks a ground loop. Since no current can flow between the insulated transformer windings, noise current can no longer add noise to the signal by flowing in the shield. A ground isolator cannot remove hum and buzz if it's placed randomly in the system: It must be installed at the interface where the noise is coupling. This is easily determined by the testing outlined earlier. High-performance ground isolators not only provide true audiophile signal quality, but also use internal shields to suppress ultrasonic and RF interference.
An audio isolator is a safe solution for the ground loop shown in Fig. 3. The isolator could be installed in the audio signal path either between the TV and the preamp or between the preamp and the sub-woofer. High-performance isolators should always be installed at the receive end of an interconnect cable.
Another safe solution is to break the ground loop by installing a CATV isolator at the antenna input of the TV. If the TV in Fig. 3 were driving a video projector equipped with a grounded type AC plug, the ground loop might cause hum bars in the display, especially if the video cable is long. Because the signal is baseband video (composite, component, or S-video), different types of isolators are required.
The only property of cable that significantly affects noise coupling is shield resistance. Expensive and exotic cables have no significant effect on hum and buzz problems. So rather than agonize over which cable makes the most improvement, prevent this unwanted coupling effect by installing a ground isolator.
Whitlock is president of Jensen Transformers, Inc., Chatsworth, Calif.
Sidebar: Coping With Fields
Magnetic or electric fields can sometimes induce noise in cables. Electric fields are generated around wiring or devices operating at high AC voltages. Their coupling is prevented by a conductive outer shield, which completely surrounds and covers the inner signal conductor in cables. Braided shields provide 80% to 95% coverage, which is adequate for all but extreme cases. Electric fields are rarely a problem in AV systems.
Magnetic fields are likewise generated around wiring or devices operating at high AC currents. Building wiring, power transformers, electric motors, and CRT displays are a few sources of strong AC magnetic fields. Ordinary cable shielding, whether copper braid or aluminum foil, has virtually no effect on audio-frequency magnetic fields. Increasing distance between signal cables and offending fields is the best cure for electric and magnetic field problems.