I was struck by something today. It probably would never have come to my attention if the
item that struck me were properly shelved and faced, but that is something I'll have to
fire someone for later. What is notable is that the item which struck me in the head was a
pair of $90 3 foot RCA cables. I decided to read what made them worth $90, and found
myself laughing aloud at the "features" which made these cables
"superior". Knowing full well these features were complete bullshit, I was
suddenly struck with the euphoric sense of realization that placed me half way between
Einstein's Theory of Relativity, and P.T. Barnum's sucker-every-minute: If you build it,
they will come, and they won't even know why.
It is very
true that audio interconnect technology improves on the sound quality possible in cars.
Many features are essential for good sound, while others are hooey, complete and utter
hooey. In this paper, I will discuss the common features of many manufacturers products,
and common myths that accompany some of them.
The Skin Effect
One of my favorites, and one I fell victim to early on is the Skin
Effect. Quite simply, the Skin Effect is the tendency of current to move to the outside of
a conductor at high frequencies. This means that there is less total conductor area, and
thus more resistance and higher signal loss. This is a real issue, mind you. However, the
skin effect occurs at much higher frequencies than your typical rational audiophile will
be concerned with, and it is almost nonexistent at even extended audio frequencies. The
skin effect is more or less a borrowed problem from high speed digital networks, and high
frequency communication equipment.
Gold is the Best Conductor
This is categorically untrue. Anyone with a physics background
will tell you that the lowest conducting alloy at room temperature is Silver (Ag).
However, Gold (Au) is more suited for automotive applications. Gold is a very inert metal,
which means it will not readily corrode. Any tarnish that does occur is easily cleaned
with a soft cotton cloth. Gold is also soft and highly malleable, so when interconnects
are plated in gold, it yields more contact area than when not (albeit, on a very
microscopic scale). But finally, and most importantly, gold is pretty. Gold is also
expensive, and when you plate things from speaker wire to distribution blocks with gold,
you can charge more.
Multi Path Interconnects
This was one of the many things that inspired laughter from those
$90 rca cables. In their theory, conductors of different length are wound into a cable.
It's common knowledge that given sound frequencies travel faster than others, and other
sound frequencies carry farther. By using conductors of differing lengths, these given
frequencies should arrive at the other end of the cable at the same time. That is complete
and utter bullshit. Here's how:
- Sound is comprised of changes in pressure in a gas. We hear those
sounds because our ears convert those changes in pressure into nerve impulses. However,
audio signals carried on an interconnect consist of fluctuations in electrical flow. They
do not suffer the same problems as sound travelling through a gas.
- Voltage changes at near the speed of light, or at least damn near
it. When you change the voltage on one side of the cable, it almost instantaneously
changes at the other end.
- Current flows extremely slowly. It may take hours for an electron
at one end of an interconnect to bump it's way to the other end. A length of a few feet or
centimeters will make no difference.
- To ensure that the frequencies travel the specified path, you would
have to build a crossover into the interconnect. There was not one. Otherwise, electricity
takes the path of least resistance, which would be the shortest sweetest conductor.
To anyone, the basic construction of an audio interconnect cable,
and the construction of a capacitor are virtually the same. In a capacitor, you have two
metal plates separated by a dielectric (insulator). In an interconnect cable, you have two
wires, separated by insulation.
The determining factors in a capacitors capacity is the area of
the two plates, and the distance between them. The more area, the more capacitance. The
closer they are without actually touching, the more capacitance.
Capacitors in car audio, among other things, are used for
filtering. A capacitor, depending on it's value, will resist low frequencies, while
passing high ones. A very large capacitor will pass all frequencies, because all
frequencies will appear to be high. A very small capacitor on the other hand, will block
audio frequencies. Many cable makers tout the very low capacitance of their cables, but in
truth, the capacitance in even the cheapest of RCA is on the order of picofarads, and
would not have any effect on the audio signal.
Insulation is important. The automotive environment is a very
hostile one for electronics. Materials should be able to stand up to the extremes of heat
and cold, as well as vibration. For the most part, any good quality pvc or plastic will
work. However, there are a few situations when you will need to consider the insulation.
Some cable manufacturers use special gelatins in their insulation, which reseal any
punctures or breaks. However, these tend to also be easier to damage. There is also the
normal PVC jacketing common to cables, which is common, durable, and cheap. However, some
manufacturers are experimenting (on your pocket, no less) with silicone, oxide impregnated
plastics, and good old fashioned wool. Remember....we are not dealing with high voltages
here, no exceptional currents, so concentrate on insulations which offer thermal
protection, and long life.
In spite of it's simplicity, terminators on an interconnect have
to be very precise. RCA's, which are the most common audio interconnect, have a very
specific dimension that they have to conform to. While there is some room for error, a
grossly mis-sized connector may not fit, or worse, could damage your equipment on
insertion. Many manufacturers produce split center pins and split outer rings. This
actually works two fold. If it doesn't precisely match tolerances, the springy nature of
the pin/ring will accommodate the tolerance. Also, the tension will make the interconnect
hang on harder.
Many design configurations are used for the actual cable itself.
The most common is a coaxial design. In this design, there is the center conductor, which
is insulated. Then, the outer conductor is wound around that (usually woven, which adds
strength). This is again, insulated. Finally, good cable makers add a foil shielding (this
is called mu metal) to reject high frequency interference, and of course, another
jacketing of insulation. Coaxial cables are favored for their high bandwidth; The ablility
to carry very high frequencies. However, these are often up into the radio frequency
range, and do not necessarily enhance transmission of audio frequencies. This type of
design does have more strength, which is a plus if you're pulling the cables under
Another common design is using twisted pair. In twisted pair, the
two conductors for each signal are simply twisted. By twisting them, they are at a
constant 45 degree angle from each other. This reduces the likelihood that the magnetic
field around one wire will induct onto the other. It's also pretty. A lot of installers
will twist wires together to keep them from turning into a rats nest. Grizz Archer hosts
seminars on car audio from time to time, and one thing he likes to ask is:
"Why is it that I can pick up the phone and call half way
around the world to Japan, and get a crystal clear connection, but most installers can't
run a cable ten feet without getting noise? It's because phone companies use twisted pair
wires for their phone lines...."
One thing to avoid is shielded wires, and special "Anti
Noise" interconnects. The shielding consists of a large foil wrapping, which is
connected to the shield of the RCA's. This is a borrowed technology from radio and high
frequency transmission, where induced RF noise is captured by the shield and shunted out
of the signal path. However, in your car, that large metal foil provides the center
conductor of your RCA with more surface area, increasing the likelihood of ground loops.
Anti noise interconnects, on the other hand, use an inductor at one end. This inductor
will tend to squelch high frequencies, and in some designs, will actually cause a ground
loop if the cable is put in backwards. Any RCA with some type of "Anti Noise"
technology, is bullshit. It's just a bandaid to a bigger problem, and it doesn't matter
whose name gets slapped on it.