| 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.
Capacitance
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
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.
Connector Construction
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.
Cable
Configuration
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 carpets.
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. |
