Knowing Audio: A Mess of Cables
We’ve now spent several months in this series traveling the world of audio, and along the way we’ve looked at the different parts of a Hi-Fi system, from speaker to source. It’s been an enjoyable ride full of technical details and examination of Hi-Fi myths in equal measure, but now it’s time to descend into one of the simplest but most controversial areas of audio reproduction. Every audio component, whether digital or analog, needs to be connected to the system it’s part of, and that’s the job of audio cables, sometimes called interconnects. They’re probably the component most susceptible to tenuous claims about their performance, with audiophiles willing to spend huge sums on cables that claim to deliver that extra bit of listening performance. Is there something in it, or are they all the same bits of wire, with the more expensive ones being a rip off? It’s time to take a look.
What Makes an Almost Good Cable
In a typical home audio system with both digital and analog signals, you can expect to find two types of cables, electrical interconnects that can carry either analog or digital signals, and optical cables for digital signals. We’re here to talk about power cables here because they’re the ones used for analog signals, so let’s start with a little transmission line theory.
Perhaps one of the first electrical circuits you ever built had a battery and a flashlight bulb connected to a length of two-conductor cable. When you touched the wire to the battery terminals the bulb lit up and when you released it the light went out. It was a DC circuit with two states, off and on, and that’s all there was to it. But if you were to plug a storage oscilloscope into the wire when you plugged in the bulb, you might notice something interesting. Instead of jumping from off to on in an instantaneous transition, the voltage would actually curve up in microseconds. The DC circuit suddenly doesn’t look as perfectly bi-state as we thought, so what’s going on?
The voltage bends upwards because the wires and the bulb are not perfect. They have a small amount of resistance, inductance and capacitance, called parasitic, and it is the interaction between these that causes the voltage to rise over a short period of time rather than immediately. It is almost immediate, so it’s fine for a flashlight, but as soon as similar wires are used to carry a signal, that spurious RCL circuit will start affecting it. Early telegraph and telephone engineers faced this problem because their wires spanned hundreds of miles and therefore had large R, C, and L values that gave the effect of a low-pass filter. Their attempts to understand the phenomenon gave rise to what we now call transmission line theory, with which anyone who has worked with RF should be intimately familiar.
That said, an audio interconnect is a transmission line in which parasitic R, C and L values should be taken into account, I’m now going to reverse that completely and say that within reason the performance of the transmission line of interconnection as we would understand for radio circuits, it doesn’t matter much at audio frequencies. The reason comes down to the short length of an audio interconnect, which at something on the order of a few feet (or a meter) has spurious values that are so small that they make little difference as a pass filter. -low. When this is compared to the wavelength at audio frequencies – 300 km at 1 kHz – this is insignificant.
Going back to our flashlight bulb, the current in these battery wires was DC, always flowing in the same direction. If we imagine them as thick single-stranded copper wires, we can further imagine the current inside them as if it were a flow of water in an idealized plumbing system, with a flow evenly distributed over its cross section. We know that electric current creates magnetic fields, so the wires powering our light bulb will be surrounded by a static field as long as direct current is flowing.
With an alternating current such as an audio signal, the magnetic field is different. As the current changes, the field changes, and since changes in magnetic fields induce currents in nearby conductors, they will induce additional currents in the wire. These do not conveniently flow as linear currents along the length of the conductor, but as so-called circular eddy currents within it. Because part of the circular current flows forward and part backward, toward the center of the conductor, eddy currents cancel the direct current.
This gives rise to what is known as the skin effect, in which alternating currents flow primarily outward from a conductor, and going back to the previous paragraph, this can produce the result of a significant increase in this parasitic resistance at AC audio frequencies. For an audio interconnect, this can detract from its quality, so it is common for audio cables to increase their surface area as much as possible by having many small strands of wire instead of a single larger one. In case that’s not enough, premium cables ensure the lowest surface resistance of the wire strands by plating copper with silver or gold.
Busting some cable myths
So we’ve established that a good audio cable should have minimal parasitic resistance, inductance, and capacitance. Due to its relatively short length, its performance as a transmission line in the RF sense is largely unimportant, and skin effect can be reduced by using stranded cable. But there are other things to consider when buying a decent cable, and these are perhaps the most interesting as we enter the world of audiophile woo here. If you look at cables in an audiophile catalog you will see terms such as ‘oxygen free’ and ‘directional’, what do they mean?
Oxygen-free copper is a very high quality form of refined copper. It has very slightly better conductivity than regular copper due to the removal of impurities, and so audiophiles claim it offers significantly better quality. The reality is that the length of an audio interconnect is so small that the slightly better conductivity is not significant in its performance. Applications that require longer cables on the order of hundreds of meters could see an advantage, so we would expect to find this in scientific instrumentation for large projects such as CERN, but for short audio interconnects it it’s just a marketing tool.
If you buy a decent interconnect, it will probably use oxygen-free copper, but its performance will come from using a large cross-section of thin and possibly silver-plated wires and not super-pure copper. Directional cables are another matter, you will find many audio cables with small arrows on them indicating the direction the current should flow. A web search will reveal a variety of explanations for this which generally rely on the parasitic action of the diode between the individual grains in the copper mass, and some of these even suggest that the directionality will increase with use. . It’s another great marketing tool for gullible audiophiles, but unlike oxygen-free copper conductivity, it has no basis in truth. Audio cables or any other cables are simply not directional, they work equally well whichever way they are plugged in. Sorry audiophiles, you got screwed.
Any silly cable can count as one
So far, we’ve only looked at analog audio cables in this room, but of course, those aren’t the only cables sold to audiophiles. For example, you can buy “special” IEC mains cables at exorbitant prices, or audiophile-grade digital cables for Ethernet, USB, TOSlink or HDMI.
A mains cable is just a mains cable as long as it has conductors sized for the appropriate current. Digital cables are almost so simple.
With digital cable myths, there is an element of truth, but it’s not one that should cost you hundreds of dollars. Digital cables are different from analog audio cables in that the bit rate is at a much higher frequency than the bit-encoded signal. Thus, their transmission line performance becomes an important issue, and sometimes it may show up in the choice of cable.
Find the cheapest HDMI cable under $5 on the market and chances are it will work with a 1080p signal but not a 4K signal because its transmission line bandwidth isn’t up to par. height of the additional requirements of 4K bitstreams. But before that $1,000 HDMI cable rolls off the shelf, try a $10 cable to replace the $2 one, and you might be pleasantly surprised.
Even the cheapest HDMI cable can carry several gigabits per second and chokes your digital audio throughput in the megabits. And as long as the ones and zeros remain intact all the way to the other end of the cable, it makes no sense to spend more money – there is no better one or zero.
Some audiophiles may read this article and get mad because clearly I don’t know what I’m talking about when it comes to directionality or oxygen-free copper, and especially with AC cables or 1000 ethernet cables $. I will make them this offer: there is a pint of old addict in an Oxford pub for the first person to prove me wrong. But the standard of evidence is pretty high, I won’t accept any of that mumbo-jumbo “Gold-plated oxygen-free USB cable lends a rich chocolate tone to the wider soundstage.” Instead, I’ll do side-by-side testing with a high-end professional audio analyzer. Let’s see what Audio Precision has to say, shall we? I hate to turn down a sale to the very excellent Hook Norton Brewery, but something tells me I won’t be buying that pint anytime soon.
We’ll be back with another one in this series, and having explored the components of a home audio system in detail, now is the time to look at it another way. How to measure audio performance?