Cables and Connectors: They're More Important Than You Think

Walk into any Radio Shack and wander through the sections for audio and video cables and connectors. Thousands of choices confront you. What do you need? What should you buy?

How should you buy it. Why should you care.

After all, a cable's a cable, right? Won't any bit of wire with the right ends do?

The answer to that question is yes, no and it depends. In this article we'll explore these answers. But first, we must determine what a cable really is, and what it really does.

According to The Histoy of Electric Wires and Cables by R.M. Black, the humble electrical cable boasts irnpressive origins-in duding the wire draving systems of the ancient Egyptians (machines used to make wire from metals for jewelry making) and the rope weaving techniques of even earlier civilizations. Experiments with wet silk threads and wires to conduct electricity date back to 1730, even before Ben Franklin and his kite.

The real demand for wires and Cables to conduct electric power began in 1871, with the invention of the first dependable dynamo.

The telegraph and telephone swelled demand for efficient methods of transmitting electric current and signals over long distances. In the case of one early telegraph system, wires for each letter of the alphabet were bound together into a single manageable Cable. Paper-coated pitch wrapped each individual wire, insulating it from the others. The whole batch was then wrapped in paper. Some even suggested laying these batches in tubes coated in resin and placing them underground to keep curious young boys away from them.

Today's cables are slightly less arcane. In audio-visual, cables tend to be one or more wires wrapped in insulation and terminated by two metal connectors. Often, shielding as well as insulation wraps the wire(s) of the cable.

You really only need to know about three kinds of cables and a handful of connectors. To find out about them, we've consulted a panel of experts: Oran J. Sands, video engineer and journalist extraordinaire, who currently writes on video topics for Amazing computing and Video Systems Magazine, Carl Cornell, Senior design engineer for Whirlwind Music; Michael Wulf Axelrod, computer engineer and animator; and Ethan Funk, director of engineering for WITR Radio in Rochester, New York.

In any conductor of electricity or electric signals, there are a few different factors that come into play. Note: we'll talk about these in general terms, avoiding most of the numbers and using only two or three formulas. Our apologies in advance to engineers and other formula fans.

Keep in mind that no one really knows why this all works anyway.

"We have all these models and formulas and explanations for this stuff," says Axelrod. "When you get right down to it, they're little better than guesses at why and how this stuff works."

In the case of cables, factors like resistance, capacitance and impedance all play a role. There's also a great many more electrical sources, magnetic sources and broadcast radio frequencies today than there were in the days of paper and pitch insulation. All of these factors can affect a cable's transmission. As a result, shielding is another important variable in audio and video cabling.

Electrons, and therefore electric charges, move through conductors. If they didn't, the world today would be a very different place. A flow of electrons (a charge) flows from a voltage or signal source (like a battery) through two conductive surfaces (that is, materials that allow electrons to pass through them).

Hook a wire to two ends of a battery and you've got flow. This movement of electrons has many characteristics. The two of interest at the moment are voltage and current. Voltage is like the "pressure" created by the flow of electrons. Current is the rate of the resulting flow.

Resistance is the quality of a material that restricts the free flow of electrons. As a conductor resists the flow of current through it, a small percentage of the energy dissipates.

Metal wires are good conductors and resist little ofthe current that runs through them, though there is some loss. They also have a large surface to volume ratio. This is crucial because, oddly enough, electrons travel over the surface of the wire and not through the middle.

Capacitance is the tendency of a material to store energy and block constant or low-frequency currents. Capacitors filter and store energy, among other things. Different conductors have different resistive and/or capacitive properties. A shielded cable has similar properties to a capacitor, as both consist of two conductors separated by some type of insulator.

Capacitance is a factor in cable design, because it reacts with the impedance in a length of wire to strip away high frequencies. In video, that means a loss of detail; in audio, a loss of clarity. In cables, the lower the capacitance, the better.