DEALING WITH LONG LINES

Dec 1, 2001 12:00 PM, BILL WHITLOCK

IN MY OCTOBER 1999 “CLEAN SIGNALS” COLUMN, I talked about long lines. In terms of electrical wavelength, a long audio cable (20 kHz bandwidth) was defined as one over about 4000 feet (1200 m), and a long video cable (5 MHz bandwidth) as one over about 15 feet (4.5 m).

Such long lines require termination to avoid signal degradation due to transmission line effects. Cable capacitance and line-driver output impedance form a low-pass filter that can significantly degrade high-frequency response in audio cables over about 100 feet (30 m) long. This is especially true in consumer equipment where output impedance is often quite high.

I also mentioned a more subtle current-limiting problem that can occur when low-impedance, professional, balanced outputs drive long lines. It distorts or clips high-level, high-frequency program material such as cymbal crashes or vocal sibilance. However, none of these problems cause noise or interference. But when it comes to noise and interference, problems are much more likely with long lines, and the severity will depend on whether the interface is balanced or unbalanced.

Significant ground voltage differences between driver and receiver are almost certain with long signal lines. These ground voltage differences are an entirely normal feature of safe, well-designed and code-compliant electrical power distribution systems. Balanced signal interfaces are inherently immune to these normal ground voltage differences, while unbalanced signal lines are especially susceptible.

THE POWER LINE CONNECTION

To get some idea how the power system couples noise into signal interconnections, take a look at Figure 1. It shows how the wiring between just two pieces of equipment in a system forms a physically large loop. Although the green safety ground wiring is intended to carry only fault currents from defective equipment, under normal conditions it carries small leakage currents from every safety-grounded device, including our system gear, which is powered by the branch circuit. Leakage currents of several milliamperes per device are not uncommon, and accumulated current in the safety ground can reach 100 mA or more.

Because safety ground wiring has resistance like all real-world conductors, leakage current causes voltage drops, generally double-digit millivolts, between outlets on a branch circuit. Voltages between outlet grounds on different branch circuits, especially if they're wired to opposite phases of the utility power feed, can be considerably higher — often 100 mV or more. The voltage can reach several volts between two outlets in different buildings! This voltage is effectively impressed across the length of the signal interconnect cable. When it couples into the signal path, it is heard as a hum or seen as a slowly upward-creeping disturbance in video displays.

It's important to realize that similar leakage currents are also generated by equipment without ground connections on their AC power plugs. Although UL regulations limit leakage from equipment with no safety ground to 0.75 mA, this amount is still significant. This is the current that causes the tingle sometimes felt when touching equipment or appliances with 2-prong AC plugs.

In systems using this kind of equipment, the leakage current simply flows from one device to another through the signal interconnect cable, causing 99% of hum and buzz problems in consumer audio systems. For example, consider that the shield conductor of a typical 20-foot RCA cable may have a resistance of 1 ohm. If a leakage current of only 0.3 mA flows in it, a 300 microvolt noise voltage will exist between its ends. Because leakage currents are capacitively coupled (effectively a high-pass filter), they are most often heard as a buzz when coupled into the signal path.

UNBALANCED INTERCONNECTS OPEN THE DOOR TO NOISE

The grounded conductor in an unbalanced signal interconnection, usually the shield, carries both signal and the aforementioned noisy leakage current. At the receive end of the cable, the signal from the driven end and the voltage drop caused by leakage current flow in the shield are simply added together. This is called common-impedance coupling. In the example above, the noise will be only 60 dB below a 300 millivolt consumer-reference-level signal. If our unbalanced interface were between safety-grounded devices, or devices connected to safety-grounded devices through other signal cables, the noise situation could obviously be much worse.

For this reason, unbalanced interfaces in long lines should be avoided whenever possible. If equipment with unbalanced inputs or outputs must be used, then prevent leakage current from flowing in the cable shield — and prevent noise. Breaking the shield connection in one place is sufficient. There is no need to convert the signal to balanced at the driven end and back to unbalanced at the receive end. A single transformer anywhere in the signal path will break the direct electrical connections and magnetically couple the signal across the barrier, stopping leakage current flow through the cable.

Although widely available output-type audio transformers will reduce leakage current flow, a transformer whose design is a bit more sophisticated will always produce superior results. An audio input-type transformer incorporates a Faraday or electrostatic shield between its primary and secondary windings to prevent capacitive coupling of noise. When used at the receive end of an unbalanced audio cable, it can reduce noise coupling from leakage currents by 60 dB or more! Likewise, video hum bars and other artifacts can be effectively eliminated with special wide-band video isolation transformers that can be placed anywhere on a video cable run.

THEORETICAL PERFECTION

In balanced interconnections, power-line leakage currents flow in a third conductor or a shield conductor, if present. Since this grounded conductor is not part of the signal path, common-impedance coupling is avoided altogether. The voltage drop caused by leakage current in the shield, or any ground voltage differences between driver and receiver, appear on both signal lines. Such voltages are common-mode and are, in theory, completely rejected by the receiver, which ideally responds only to differential or signal voltages.

The degree to which this ideal is approached is measured as common-mode rejection ratio, or CMRR. Even a slight impedance imbalance in the lines allows slightly unequal noise or interference to be developed on the two lines: Any difference will be indistinguishable from signal when seen by the receiver. Because the sub-system consists of a line driver, the line itself and a line receiver, impedance imbalances in any of them can cause conversion, the process whereby a portion of common-mode becomes signal.

Even with balanced interfaces, long lines also increase the likelihood of inductive noise pickup from magnetic fields near the cable. Twisting of the signal conductors, which ensures equal pickup and subsequent common-mode rejection in the receiver, is the first line of defense. But remember that wherever twisting is opened up, the result is a magnetic pickup loop. It's easy to forget that this happens at terminal strips, punch-down blocks, and inside XLR connectors. Keep these points, as well as signal cables in general, as far as possible from the strong magnetic fields radiated by power transformers, motors, TV or computer CRTs, and electrical wiring. Avoid long runs where signal cables run parallel to power cables. Keep them far apart and allow them to cross only at right angles whenever possible.

Speaker lines are an interesting example of an audio interface that inadvertently qualifies as balanced. Since the output impedance of a typical power amplifier is only a few hundredths of an ohm, the impedance (to ground) of the two signal conductors are essentially equal — satisfying the fundamental requirement for balance. As I constantly emphasize, signal symmetry has absolutely nothing to do with noise rejection in a balanced interface. Since the speaker at the receive end of the line has no power line or ground connection of any kind, it is floating and qualifies as a differential receiver with high CMRR. As a result, hum and buzz problems in speaker interfaces are virtually unheard of.

However, if a misguided installer grounded one of the connections at the speaker, noise problems could certainly arise. This brings me to some general advice: Don't add unnecessary grounds! Grounds that are required for safety and code compliance are, in the overwhelming majority of systems, the only ones necessary for noise-free system operation. Additional grounds will usually create new paths for leakage currents and new opportunities for noise coupling into signal paths. Some so-called experts claim that normal ground voltage differences can be reduced to zero using massive amounts of heavy copper wire or bus bar. Such heroic efforts are usually a waste of time and money.

Bill Whitlock is president of Jensen Transformers. He may be reached via e-mail at whitlock@jensen-transformers.com.

References:

B. Whitlock, Hum & Buzz in Unbalanced Interconnect Systems, Application Note AN-004, Jensen Transformers Inc., available as .pdf at www.jensen-transformers.com.

B. Whitlock, “Grounding and Interfacing”, Handbook for Sound Engineers, 3rd Edition, Glen Ballou Editor, Focal Press.