OTHER TECH

When home theater enthusiasts talk about their audio systems, they usually concentrate on components such as the disc player, receiver (or preamp-processor and power amps), and speakers. But there's another component that deserves just as much attention: the cables that connect the other devices together. Without cables, those other components would be nothing more than expensive boat anchors.

The effect of various components on audio signals is relatively well-known; specs like frequency response, total harmonic distortion, and dynamic range indicate how a signal changes as it passes through a piece of gear. However, the effect of audio cables is not so clearly understood. How do their design, materials, and construction influence the audio signals they carry from one component to another?

This question is at the heart of an ongoing and heated debate throughout the audio community. For some people, exotic designs and materials make a clearly audible difference in the quality of the audio signal, albeit at a steep price. For others, these exotic cables are a waste of money; going beyond certain minimum electrical and mechanical requirements does not improve the quality of the signal to their ears. Unfortunately, objective measurements are scarce, so the debate rages on.

Electrical Factors
Everyone agrees there are several electrical factors that clearly affect the aural performance of audio cables. For example, all conductors (except superconductors) exhibit electrical resistance, which is called impedance in the presence of an alternating current such as an audio signal. Resistance to DC current is measured in ohms per foot; the longer the cable, the higher the DC resistance. However, the size of the conductor must also be considered; the larger the diameter of the wire, the lower the DC resistance. AC impedance is also measured in ohms, but it is independent of length.

DC resistance is important only in speaker wire. According to Marc Dimmitt, former technical support manager for Clark Wire and Cable, "If you run a small-gauge wire 100 feet from a power amp to a subwoofer, you're just going to burn up the wire. Most of that energy is going to dissipate as heat; it's not going to make it to the voice coil and move the speaker cone back and forth. This tends to compress the dynamic range of the sound as well."

AC impedance is important mostly for electrical digital audio cables. (Of course, none of this discussion is relevant for optical digital audio cables.) Unlike analog audio, digital audio signals are sent at frequencies in the megahertz range. This requires a cable that exhibits a very specific AC impedance: S/PDIF requires an impedance of 75Ω:, while AES/EBU requires 110Ω.

This is particularly important in the pro-audio world. As Dimmit explains, "A lot of people try to make AES/EBU transfers from one digital machine to another with regular microphone cable and wonder why they get errors and glitches. Mic cables are not specified at any particular impedance; it may be all over the map. With high-frequency digital audio, the impedance must be carefully matched. If something is out of tune, you get stray bits, dropouts, and glitches."

One problem to overcome in digital cables is return loss. According to Bruce Jackson, former vice president at Apogee Electronics, "If the cable and connectors are mismatched in terms of impedance, the digital pulses bounce back and forth, interacting with each other, which causes them to distort. When you have it all correctly impedance-matched, you don't get reflections."

Another important electrical factor in analog audio cables is capacitance, which arises when two conductors are separated by a small distance. A high capacitance tends to resist abrupt changes in voltage, so minimizing capacitance is most important in cables connected to high-impedance inputs, such as most line-level inputs, which bring about more fluctuating voltage than current in the cable.

As Dimmitt explains, "All manufacturers spec their cable at some number of picofarads per foot, conductor-to-conductor and conductor-to-shield." Michael Laiacona, president of Whirlwind Cable, explains the audible result. "The lower the capacitance, the better the frequency response of the cable. If the capacitance is too high, you start hearing a high-frequency loss."

Perhaps the most difficult factor to optimize in audio cables is inductance, which arises when current flows through a conductor; the resulting electromagnetic field interacts with the current in that conductor and any nearby conductors. Unlike capacitance, inductance resists abrupt changes in current. For low-impedance, high-current sources, such as power amplifier outputs, minimizing the cable inductance is of primary importance. This is often accomplished by separating the conductors as much as possible.

Exotic Conductors
One of the most hotly contested issues is the conducting material itself. Most of us are familiar with copper; it's electrical conducting properties have been exploited for many years. But how do different copper formulations affect the audio signal?

Bill Low, founder of AudioQuest, explains the different grades of copper. "Tough-pitch copper, which is also known as high-purity or electrolytic copper, is the standard grade. Oxygen free copper, or OFC, is a step up from that, and there is a significant advantage to it. The only way to prove that is to listen; you can hear the difference. You can also see the difference under a microscope. A strand of copper consists of many separate grains. The current has to jump between them and suffers as a result. Impurities in the copper, including oxygen, coalesce at the grain boundaries, which is why reducing the oxygen content makes such a big difference.