Know Audio: A Mess Of Cables

We’ve now spent several months in this series journeying through the world of audio, and along the way we’ve looked at the various parts of a Hi-Fi system from the speaker backwards to the source. It's been an enjoyable ride full of technical detail and examining Hi-Fi myths in equal measure, but now it's time to descend into one of the simplest yet most controversial areas of audio reproduction. Every audio component, whether digital or analogue, must be connected into whatever system it is part of, and this is the job of audio cables, sometimes referred to as interconnects. They are probably the single component most susceptible to tenuous claims about their performance, with audiophiles prepared to spend vast sums on cables claimed to deliver that extra bit of listening performance. Is there something in it, or are they all the same bits of wire with the expensive ones being a scam? Time to take a look.

In a typical domestic audio system with digital and analogue signals you might expect to find two types of cable, electrical interconnects that could carry either analogue or digital signals, and optical ones for digital signals. We’re here to talk about the electrical cables here as they’re the ones used for analogue signals, so lets start with a little transmission line theory.

Perhaps one of the first electrical circuits you ever constructed had a battery and a flashlight bulb connected with a length of two-core flex. When you touched the wire to the battery terminals the bulb lit up, and when you released it the light was extinguished. It was a DC circuit with two states, off and on, and that's all there was to it. But if you were to hook up a storage oscilloscope to the wire as you hooked up the bulb you might notice something interesting. Instead of jumping from off to on in an instantaneous transition, in fact the voltage would curve upwards over a few microseconds. The DC circuit suddenly doesn't look as perfectly bi-state as first thought, so what's going on?

The voltage curves upwards because the wires and bulb are not perfect. They have a small amount of resistance, inductance, and capacitance, referred to as parasitics, and it's the interaction between these that causes the voltage to rise over a short time rather than immediately. It's nearly immediate so it's fine for a flashlight, but as soon as similar wires are used to carry a signal this parasitic RCL circuit will start to affect it. Early telegraph and telephone engineers faced this problem as their wires stretched hundreds of miles and thus had significant 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 refer to as transmission line theory, with which anybody who's worked with RF should be intimately familiar.

Having said that, an audio interconnect is a transmission line in which consideration should be given to parasitic R, C, and L values, I am now going to turn that entirely on its head and say that within reason the transmission line performace of the interconnect as we’d understand it for radio circuits doesn't matter much at audio frequencies. The reason comes down to the short length of an audio interconnect, which at something in the order of a couple of feet (or a meter) has parasitic values that are so tiny as to make little difference as a low pass filter. When this is compared to the wavelength at audio frequencies — 300 km at 1 kHz — it is insignificant.

Going back to our flashlight bulb, the current in those wires from the battery was DC, always flowing in the same direction. If we imagine them as single strand thick copper wires, we can further imagine the current within them as though it was water flow in an idealised plumbing system, with the flow evenly distributed across its cross-section. We know that electrical current creates magnetic fields, so the wires powering our bulb will be surrounded by a static field as long as the DC current flows.

With an AC current such as an audio signal, the magnetic field is different. As the current changes so does the field, and since changing magnetic fields induce currents in nearby conductors it will induce extra currents in the wire. These don't flow conveniently as linear currents along the conductor's length, but as circular so-called eddy currents within it. Because part of the circular current flows forward and part backwards, towards the centre of the conductor the eddy currents cancel out the forward current.

This gives rise to the so-called skin effect, in which AC currents flow predominantly towards the outside of a conductor, and harking back to the earlier paragraph this can produce the result of increasing significantly that parasitic resistance at AC audio frequencies. For an audio interconnect this can adversely affect its quality, so it's usual 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, higher quality cables ensure the lowest resistance on the surface of the wire strands by silver- or gold-plating the copper.

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 irrelevant, and the skin effect can be reduced by using a multi-stranded cable. But there are some other things to consider when buying a decent cable, and they are perhaps the most interesting because here we enter the world of audiophile woo. If you look at cables in an audiophile catalogue you’ll see terms such as "Oxygen-free", and "directional", what do they mean?

Oxygen-free copper is a very high-grade form of refined copper. It has a very slightly better conductivity than regular copper because of the removal of impurities, and thus audiophiles claim that it delivers noticeably better quality. The reality is that the length of an audio interconnect is so small that the marginally better conductivity is not significant in its performance. Applications that require longer cables in the order of hundreds of metres could see a benefit so we’d expect to find it in scientific instrumentation for large projects such as CERN, but for short audio interconnects it's simply a marketing tool.

If you buy a decent interconnect it’ll probably use oxygen-free copper, but its performance will come from using a large cross-section of fine and maybe silver-plated wires and not from the extra-pure copper. Directional cables are another matter, you will find many audio cables with little arrows on them indicating the direction in which the current should flow. A web search will reveal a variety of explanations for this that usually settle upon the parasitic diode action between individual grains in the mass of copper, and some of them even suggest that directionality will grow with use. It makes yet another great marketing tool for gullible audiophiles, but unlike the conductivity of oxygen-free copper it has no basis in truth. Audio cables or indeed any other cables simply are not directional, they work just as well whichever way round they are plugged in. Sorry audiophiles, you’ve been had.

So far we’ve only looked at analogue audio cables in this piece, but of course they aren't the only cables sold to audiophiles. You can buy "special" IEC mains cables at outrageous prices for example, or audiophile quality digital cables for Ethernet, USB, TOSlink, or HDMI.

A mains cable is just a mains cable as long as it has conductors rated for the appropriate current. Digital cables are almost as straightforward.

Along with digital cable myths is one element of truth, but it's not one that should cost you hundreds of dollars. Digital cables are unlike analogue audio cables in that the bitrate comes at a much higher frequency than that of the signal encoded in the bits. Thus their transmission line performance becomes a significant issue, and occasionally this can show up in a choice of cable.

Find the cheapest sub-$5 HDMI cable on the market and the chances are it’ll work with a 1080p signal but not a 4K one, this is because its transmission line bandwidth isn't up to the extra demands of 4K bitstreams. But before that $1,000 HDMI cable comes off the shelf, try a $10 one to replace the $2 one, and you might be pleasantly surprised.

Even the cheapest HDMI cable can carry multiple gigabits per second, and laughs at your digital audio bitrate way down in the megabits. And as long as the ones and zeros make it intact to the other end of the cable, there's no sense in spending more money — there is no such thing as a better sounding one or zero.

There may be some audiophiles reading this piece and becoming irate, because clearly I don't know what I’m talking about when it comes to directionality or oxygen-free copper, and especially with $1,000 mains leads or Ethernet cables. To them I’ll make this offer: there's a pint of Old Hooky in an Oxford pub for the first person to prove me wrong. But the standard of proof is quite high, I’ll accept none of that "The oxygen-free gold-plated USB cable gives a rich chocolatey tone to the broader soundstage" mumbo-jumbo. Instead I’ll take side-by-side tests with a high-end professional audio analyser. Let's see what the Audio Precision says about it, shall we? I hate to deny the most excellent Hook Norton Brewery a sale, but something tells me I won't be buying that pint any time soon.

We’ll be back with another in this series, and having comprehensively explored the components of a domestic audio system it's now time to look at it in another way. How can we measure audio performance?