Cable Reviews

Why cable again?

Well, the obvious reason is that it has been a while since my last foray into Cableland (July 1988). Many new products have been introduced in the interim, so it appeared appropriate to once again open Pandora's Box. Those of you who still remember my speaker cable article of 2½ years ago will recollect the considerable controversy that evolved from that project.

Some of the response was quite predictable, though the venom with which it was laced was not. The manufacturers of those outrageously priced "garden-hose"–type cables that I failed to rave about were more than just perturbed. After all, if you had a goldmine with market momentum primed by pseudoscientific technobabble and another magazine's endorsement, the last thing you'd want to see is even a murmur of critical dissent. One of these good fellas went so far as to threaten me with a personal lawsuit should my survey result in the loss in sales of "even one cable."

Others objected to my findings because I wasn't rigorously "scientific" in my methodology. I was guilty of using the time-honored audiophile method of forming a subjective opinion on the basis of careful comparative listening tests; essentially the same approach as I would have used for testing a loudspeaker or an amplifier.

There's a popular myth about scientific measurements that regards such data as enjoying an objective and independent existence in the outside world. The problem with this view is that the data must be perceived by an observer. The observer structures and interprets the data in accordance with his cognitive or theoretical framework. A scientist's preconceived notions, or theoretical view he is out to prove, will provide cues as to which data are essential or on how to pattern the data in order to support the theory (footnote 1). Thus, the data are imprinted with an unconscious subjective bias. James Clerk Maxwell, the father of classical electromagnetism, is said to have remarked once that "There are two theories of light, the corpuscle theory and the wave theory; we used to believe in the corpuscle theory; now we believe in the wave theory because all those who believed in the corpuscle theory have died."

The point, of course, is that the significance of particular data patterns depends very much on who is perceiving these as cues for confirming particular theoretical assumptions and inferences. It's amazing how many times scientists have reported positive findings on the basis of "data" that were actually buried below the noise floor of the experimental apparatus. Eddington's measurement of the gravitational bending of light by the sun during the solar eclipse of 1919 was hailed as confirmation of Einstein's theory. Einstein's theory of general relativity has, of course, been verified numerous times since then. But it was later discovered that Eddington's results were fortuitous. The experimental errors associated with his photographic plates were such that he could just as easily have obtained a negative result.

Scientists are basically deterministic in outlook and are conditioned to search for causality. The simplistic reduction of such an attitude leads to the following dictum: If it exists, it can be measured. The corollary of which is that if something cannot be measured, it does not exist. Thus, one can understand the logic behind the assertion that all cables that measure identically should sound alike. This might be true if we could assure ourselves that the measurement set was all-inclusive and sufficiently refined or sensitive to establish a particular pair of cables as identical twins. But how can you know a priori all of the factors which impact sonic performance? And at what level do these factors make an audible difference? To argue simply, as opponents of exotic cable have done, that impedance variation is all that matters because nothing else appears to matter, reflects a lack of imagination. It is to such skeptics that this article is dedicated.

Science is about the search for a hidden reality. To say that all of the important design considerations for cables and amplifiers can be condensed into a simple recipe is to say that these aspects of audio are closed and require no further investigation. This mirrors the view of many physicists in the latter part of the 19th century. Many budding physicists were advised to pursue another discipline because pretty soon there would be nothing left to discover about reality.

Fortunately, for the time being, high-end audio remains largely an art. A high-end product should evolve on the basis of extensive listening tests. The same circuit can be made to sound differently with different boards, layout, or part selection, and in all these cases the differences would be impossible to discern, measurement-wise, at the current state of the art. Take the soundfield produced by a conventional two-channel audio system. Aspects of this soundfield could be measured at various levels of accuracy, but none of these measurements can reliably answer the question of "how close to 'live' will that soundfield be perceived?" The soundfield at the head is processed and interpreted by the ear/brain and results in the perception of a soundstage projected outside of the head. Just how realistic that soundstage is must be decided by the audiophile on the basis of subjective listening tests. High-end audio is about the conviction with which the "illusion of live music" can be reproduced.

There's another, and even more important reason, for treading these waters again. The Lindsay-Geyer cable, or L-G for short, represents a conceptual breakthrough in cable design, one that could revolutionize the industry. When a product single-handedly redefines the state of the art, as this one does, it becomes a cause célèbre and a story that must be told.

Making waves
Traditionally, audio cable has been conceptualized as a conductor facilitating current flow between various components. From this standpoint, a cable's cardinal design aspect is minimal impedance to current flow over the bandwidth of interest, hence negligible resistive losses. The ideal cable, from such a perspective, would be one with optimum power transfer. Clearly, such a cable is mandatory for a low-impedance circuit, such as that between a power amp and a loudspeaker. A modern power amp's output impedance is a fraction of an ohm. A loudspeaker's impedance magnitude is typically in the range of 3 to 8 ohms. This is a low-impedance circuit characterized by low voltages and high currents. Peak currents in such a circuit may exceed 40 amps; because ohmic losses go as the square of the current, the cable's impedance becomes a major factor. In this case, the speaker cable's impedance had better be minimal, or else the cable will soak up power from the speaker to a degree dependent on the speaker's impedance curve. The frequency at which the speaker's impedance dips to a minimum is where the percentage of power dissipated by the cable will be greatest.

However, the situation is drastically different for an interconnect. An interconnect typically operates in a high-impedance circuit where the source and load impedances are several hundred to several thousand ohms. Currents are in the milliamp range at most, and ohmic losses are therefore not a major concern. Here, it is much more instructive to investigate the propagation of the voltage signal rather than the current distribution.

We normally focus on the current distribution in a wire. There is a "skin effect" in that the current is progressively squeezed toward the periphery of the wire with increasing frequency. But we can gain more insight into the physics involved by analyzing the electromagnetic (EM) signal propagating down the wire. Such a description is complementary to and dependent on the current distribution in the wire, but sheds more light on how information is propagated along the wire. The following discussion is based on David Lindsay's white paper on highly magnetic wire, L-G's patent application, and information readily found in any graduate-level text on electrodynamics. My favorite text is Holt's (that's Charles A. Holt's—not JGH's) Introduction to Electromagnetic Fields and Waves (Wiley & Sons, 1963).

With a perfect conductor, say a superconductor, the signal propagates on its surface. However, until room-temperature superconductors become a reality, we'll have to make do with copper and silver. In an ordinary or "imperfect" conductor the signal sinks into the wire as an inverse function of frequency (the skin effect). The magnitude of the signal decreases exponentially with depth of penetration because of ohmic losses. At each frequency, a skin depth may be calculated at which the attenuation is exactly 1/e, or 36.8%. The signal is also retarded in phase as it sinks into the wire because of the finite velocity of propagation inside the wire. At one skin depth, the phase angle lags the phase angle at the surface by one radian, or 57.3°.

The problem, as Lindsay points out, is that if the wire is thin enough—less than several skin depths—some of the signal can sink entirely through the center of the wire and come out the other side. The time delay of the re-emergent signal can be significant. We all know that EM signals propagate at the speed of light—but that's true only in a vacuum. In copper at 1kHz, the signal speed is a relatively pedestrian 13 meters per second. At that speed, a signal will sink through a 1mm wire in about 70 microseconds. A 70µs delay should clearly be audible, assuming the magnitude of the delayed signal is significant—which is the case here. The skin depth at 1kHz is 2.1mm in copper, so the magnitude of the re-emergent signal for a 1mm wire is only down about 4dB.

Next, let's consider what happens to a transient waveform propagating down this 1mm copper wire. Because the waveform is composed of many harmonics and because the "sinking speed" is a function of frequency, a transient that sinks through the wire will be smeared out in time. The typical interconnect then propagates the original signal plus a smeared-out copy of that signal. It is possible for the smeared copy to sink through the wire again and generate another smeared copy of itself. But for every traversal of the wire, the magnitude is reduced by 8.7dB for each skin depth traversed. So only the initial few traversals are likely to be significant.

What can be done to improve the situation? One possibility is to make wire which represents a fractional skin depth over the entire audio bandwidth in order to make time dispersion negligible. Such a wire would have to be thinner than a human hair, and as a result it would be extremely fragile and impractical. But even if a thin wire could practically be made, multiple traversals of the wire would simply build up the delay to the threshold of audibility.

All right—if thin wires won't do the trick, how about fat ones? The idea here is that a signal "dies" out as it sinks through the wire at the rate of 8.7dB for each skin depth it sinks through. Therefore, make the cable fat enough so that the re-emergent signal is negligible in amplitude. If we define "fat" as five skin depths, that will provide 43dB of attenuation. At a frequency of 100Hz in copper, the skin depth is 6.6mm. Thus, to make a copper wire appear fat at 100Hz will require a diameter of 33mm or about 1.3". Again, that's rather impractical. Incidentally, a ribbon construction would not work, as the signal would simply sink through the thin part of the ribbon.

Another strategy is possible, one representing the essence and insight of the L-G cable: Make the wire appear "fat" over the audio bandwidth by reducing the skin depth itself. In other words, maximize the skin effect over the audio band in order to minimize dispersion.

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Footnote 1: As a physicist at Los Alamos, DO knows of that of which he speaks.—John Atkinson