Wiring Fundamentals: Connections, Fittings and Cabling

Audio and video systems consist of many different components.

There are video displays and video sources. There are audio devices, including amps, preamps, tuners, receivers, compact disc players and more.

You get the picture.

All of these devices -- from inexpensive to top of the line -- have connections that need to be made for the system to work. No matter how quickly the industry moves, we'll still need connections to make the components talk to each other.

There are a number of connections in a system. First, most consumer audio equipment works with an unbalanced type of signal and connection. Professional equipment, including audio mixers, amps, processors and the like, use balanced signals and connectors. The difference between the two is that a pro signal consists of a positive signal, a negative signal and a ground.

By sending a signal in this fashion, the chances of noise infiltrating the signal is far smaller. A typical professional connection will have three conductors. (With a microphone cable, chances are the jack on the end will be an XLR or Neutrik connection.) Consumer equipment, working with an unbalanced type of signal, requires two conductors -- for signal and ground.

When it comes to consumer connections, most audio connections, and many video connections, use an RCA style of fitting. This fitting uses the center conductor for the signal and the sleeve for the ground connection. Analog two-channel audio signals are color-coded, with red for right and white for left channel information.

RCA connections aren't just for audio. You can find an RCA connection on a digital audio port, which is called a coax digital connection. That will be an orange connection, by the way.

You'll find RCA fittings on baseband, or composite, video connections. Those connections are almost universally yellow in color. RCA fittings will also appear on component video connections.

Those fittings will be red, green and blue in color, but will also be labeled Y, Pb and Pr. As a video signal is broken down into more specialized categories, the inherent signal quality potential increases.

A component connection, therefore, will offer better quality than a composite connection.

Why Wires Matter

Different types of wires are used for different types of signals. Coaxial cable refers to a cable with a center conductor surrounded by a plastic-like material called a dielectric. The dielectric is then covered with some sort of shielding -- often, a combination of braiding and foil.

Coax cable is frequently used for video types of connections. When terminating coaxial cables, the objective is to maintain impedance.

Video cables are generally 75-ohm impedance. When working with video signals, such as composite or component, it's a good idea to work with a coax cable, which has a solid copper center conductor as well as a copper braid. The alternative type uses copper over steel for the center conductor. Since the signals we are discussing are voltage-based and copper is a better conductor than steel, all copper gets the nod.

You can attach fittings to coaxial cable in a variety of ways. The cable is generally prepared the same way; a stripper is used to cut through the outer jacket, shield and dielectric materials, leaving about 1/2 inch of bared conductor.

The stripper will also remove about half an inch of the overall outer jacket and shielding. Once the cable is in this condition, connections can be crimped or compressed into place.

Crimping is a type of connection that was widely used for antenna and cable television signals. The typical antenna post uses an F-style of fitting. An F-connector is inserted onto the cable so that the dielectric reaches a specified area inside the fitting.

Then a heavy pliers-like device applies pressure on six sides of the connection simultaneously, crimping the connection in place.

Getting Fit

With an F style of fitting, installers can screw on a connection that converts the fitting to an RCA style of plug. This type of connection is acceptable for lower resolution types of signal, but really should be avoided with component video signals. F to RCA adapters frequently can have problems maintaining a true 75-ohm connection and can cause service calls in the future.

There are a variety of manufacturers that produce compression styles of fittings. The concept is basically the same across the board -- a compression style fitting applies pressure all the way around the cable.

This tends to make for a stronger connection, and maintains the desired impedance more successfully. Compression fittings are available in F style, but also for RCA and a third style of video connection, a BNC fitting.

BNC fittings (which Bedrock Learning's Helen Heneveld tells me stands for "Bayonet Neill Concelman") have posts that the fitting slides over and, when a quarter turn of twist is applied, locks the connection on quite well.

You'll find BNC fittings most frequently in commercial video applications, such as security cameras, monitors, DVRS and also on video processors and scalers. Again, adapters can be used to convert an F-style of fitting to a BNC fitting, but the preference is to terminate it correctly the first time.

Typical higher quality patch cords will use a topology that incorporates two conductors with an overall shield and drain per channel. When assembling the fittings, attach the signal and ground on one end of the cable. On the other end, attach the signal and ground and drain wires.

The drain wire will attach to the same connection point as the ground. Assembling cables in this manner dramatically reduces the chance for noise being induced on the line and being injected into the audio signal path. This technique is known as "lifting the drain."

Regardless of what type of connection you are encountering, making the best termination will save you time and money in the long run.

It's worth your while to get a couple of bags of fittings that will work with the wire you have. Practice making better connections.

Regardless of the connection you're using, please label your wire so that, in the future, others can tell what specific wire is being used.

Fred Harding is in sales and technical support at Capitol Sales (http://www.capitolsales.com), a full service distributor of electronic installation hardware.