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.
A Cable History
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.
The Long and Short of It
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, Voltage and Current
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.
There’s a lot of electrical and magnetic interference in the air these days. Other power sources, appliances, close proximity to radio or TV stations-all these and more can create noise picked up by your cable unless properly shielded.
A shield is another conductive material, normally a metal foil or a wire mesh, that encircles and protects the integrity of your cable. It absorbs outside signals and/or reflects them back to the outside world. It also keeps your signal within your cable. The relationship between the conductor and shield within your cable can increase the Cable’s capacitance. Once again, careful design is everything.
Audio and video signals are alternating current signals. They’re similar to waves, like sine waves. For a wave to pass cleanly across a cable from one component to another, the impedance-or signal opposition-must be equal all the way across. The cable must be rated to carry signal at a certain impedance.
If the impedances do not match between signal source, cable and receiving component, the waves get out of phase and collide with each other. This actually reflects some of the power of the signal back to the source, causing noise and signal loss.
The more reflection, the less clean your signal, ultimately leading to loss of picture or color data. The video signal is a high frequency one, very susceptible to problems with impedance.
The video industry settled on a standard of 75 ohm impedance for video equipment. (Why did they choose this? Not one of our experts could say.)
“That 75 ohm impedance is most important in video,” says Sands. “Most coaxial cable for ham or CB radio is 50, 52 or sometimes 100 ohms. In a coaxial cable there’s a certain amount of resistance and capacitance per foot which impedes the signal. Using other than 75 ohm impedance cables will create problems like signal frequency loss and other distortions of the signal.”
Pro cables are sold with the 75 ohm rating stamped on them. In the consumer industry, they’re not marked; it’s not a one-size-fits-all standard. You can’t determine your cable’s impedance without an expensive meter. So how do you know what you’re getting?
“Ninety-five percent of the cables sold for video are 75 ohm cable,” says Cornell. “In fact, I wouldn’t be surprised to learn that all consumer audio and video RCA cables these days are 75 ohm impedance. The impedance is much less critical in consumer audio cabling, and it’s probably easier for the consumer manufacturers to make one cable and label it two ways.”
The Coaxial Cable
You’ve no doubt heard the term coaxial-coax for short-thrown about. Fancy word, simple concept: coax simply refers to any cable with a central wire surrounded by an insulator, a shield and a plastic outer jacket. if you’ve seen almost any kind of AV cable other than cheap speaker wire, you’ve seen coaxial cable.
“Today, 95 percent of the video and audio cable is coax,” says Cornell.
That holds true even for Y/C cabling, the cables used for S-VHS and Hi8 equipment. Y/C cables actually have two small coaxial cables inside one jacket. Both are 75 ohm impedance.
Cables for Audio
Impedance plays a factor in audio cables, too. Pro audio cabling is usually designed for low impedance, balanced equipment. The audio cables included in most consumer camcorder accessory packs is for high impedance, unbalanced audio gear.
“Most pros use low impedance audio line, 600 ohm or less, and also use balanced lines,” says Sands. “Balanced line is a three wire system. The signal on a balanced line is carried on two wires. The third is a ground and a shield. Balanced line can literally be run thousands of feet with little loss or noise pickup.”
Regular unbalanced line is simply a shielded conductor; balanced cabling uses two conductors and a shield. Balanced audio equipment uses both conductors in a special arrangement to cancel out noise.
The main difference between the two, aside from their connectors, is that only the latter has the ability to transmit your signal long distances without picking up external noise.
“Unbalanced line only has the two wires for the signal and should rarely be run more than 20 or 30 feet,” says Sands. “I see an awful lot of people. like wedding videographers, running 50 feet or more of unbalanced audio cable and getting noise.”
Depending on who you talk to, BNC stands for Bayonet Nut Connector, Bayonet Naval Connector or British Nut Connector. Whatever it stands for, a BNC connection is the standard for pro video equipment.
It’s that funny looking, twist-on connector that you see on a lot of professional grade video equipment. The BNC connector is found on composite video cables.
“It’s one of the few connectors designed for constant impedance,” says Sands.
This means that unlike other connectors, the BNC connector, as well as the cable, is rated at the right impedance for video.
“It’s also a coaxial connection, so it maintains the shielding all the way through,” adds Sands.
Its design allows the insulation and shielding of the coaxial cable to carry through the connector itself. This prevents breaks in shielding and assures a cleaner signal.
If you’ve got cable TV, you know what an F connector is. It’s the screw threaded connector at the end of the coaxial cable that never threads on correctly when you’re reaching behind the TV to hook it up. Like the BNC connector, this maintains the coaxial cable’s shielding, making a very fine connection.
Cables with F connectors are usually for RF signals, i.e., the picture and Sound data together in one signal. This is a signal combo useful for viewing on a TV, but not ideal for dubbing and editing. When you copy or edit tapes, avoid using the poor-quality RF signal. Stick with composite or Y/C cabling.
Used in hi-band video systems (Hi8 and S-VHS), these connectors are round, multi-wire connectors. In video, the four wire system carries luminance and chroma signals separately, as well as their associated grounds.
“These are named after the firm that invented them,” says Sands. “They vere invented for audio equipment and, as a friend of mine who works for RCA likes to say, ‘There’s a reason we invented them for audio.’ These are fine for connecting a VHS deck to a TV or monitor for viewing, or as emergency back up in a pinch. But don’t use them for serious video applications.”
RCA connectors are common on consumer video cables because they are cheap. They don’t match impedance and they don’t carry shielding through the way that BNC connectors do. So they’re more susceptible to noise and signal loss.
RCA connectors are pretty much the standard for unbalanced consumer audio. Good quality RCA connectors with high quality, shielded, unbalanced line can be a good choice for prosumer audio setups that don’t use balanced line.
These are the three pin pro mike and audio connectors used with balanced lines.
“They’re the best,” says Sands. “You can run a truck over them and they keep right on going.”
1/4-inch Phone Connectors
So called because they were the original telephone operator’s switchboard connector, these connectors come in both stereo and monaural versions. You see them on higher end headphones, as well as some pro video equipment to carry audio. They’re the standard for audio recording and mixing.
Mini and Micro Connectors
These types are smaller versions of the 1/4-inch phone connectors used on radios, camcorders, CD players and such. Most camcorders and VCRs have minijacks for connecting headphones and external microphones. Stereo microjacks-even smaller yet-often carry control-1/4 signals in 8mm-family camcorders.
You can find an adaptor to alter almost any connector. The question is: when should you do it and when shouldn’t you?
Only use adaptors in emergency or temporary situations. Any time you use an adaptor you risk a bad connection and the resultant signal degradation, or even a full scale break in the connection.
Is this because all adaptors are inherently shoddy? No, but they’re not permanent connections. Most don’t carry the cable’s shielding, making them more likely to pick up noise. They can slip when subjected to tension, resulting in a reduced connection.
Overall, the important thing about adaptors is that they add another variable into the equation, another item to troubleshoot.
That said, sometimes you have to fudge. In emergencies, the safest course is to use the right type of cable and the best quality cable first; then match the adaptors.
Always remember that “line” inputs/ outputs and “mike” inputs/outputs on audio equipment are of such different impedances that one will not work with the other. You can’t run a cable between a “line out” from a camcorder or tape deck and plug it into a “mike in” on another recorder-no matter what type of simple adaptor you stick on the end.
There are special matching transformers available that will take you from a “line” input/output to a “mike” input/output. Like any other adaptor, they’re better than nothing in an emergency or temporary situation; but they’re not a long-term solution. The long-term solution: buy equipment that interconnects with the right signal level and impedance.
Mix and Match
A cable doesn’t have to have the same connectors at both ends. Many desktop video mavens with pro gear, like the Video Toaster, will connect their Hi8 camcorder’s RCA video out jack to their DTV system’s BNC video in jack. In an ideal world, they’d use a specifically macic video cable with an RCA connector on one end and a BNC connector on the other.
Generally the RCA to BNC cable is the one mixed video cable you are most likely to use in the prosumer arena. In the audio arena, those people using pro grade audio mixers may find themselves in need of XLR to RCA cables or XLR to phone.
Brand Names You Can Trust
Once you decide what type of cable you need, there are still some choices to make…multi-strand or single strand connectors, foil or mesh shielding, crimped or molded ends. Most cables are manufactured to such high standards today, that the main reason for choice comes down to application.
“Cables with single strand conductors and foil shielding are pretty rigid,” says Cornell. “If you’re buying Cable for a situation that doesn’t require flexibility, go with these. Shielding is rated in percentage of coverage and these are the highest rated, 1OO percent. If you require flexibility, multi-strand and mesh are the way to go. These cables once had greater problems with resistance to noise, but good ones are now also up to about 95 percent coverage.”
Molded cables will cost a little more, but offer some strain relief to the connections. Strain relief refers to the ability of the cable to withstand tension and fatigue from bending or pulling of the cable. Have you noticed how the headphones to your portable music machine tend to short and break right by the base of the mini plug? Strain relief will help prevent that type of breakage. Some cables come with rubber protectors or springs at the base of the connector to provide strain relief.
The bad news: once a molded cable has broken, you can’t repair it.
Crimped ends on cables have metal rings and cable insertion points squeezed tightly around the ends of the cable. They can suffer from strain and come loose or even disconnect from the end of the cable.
The good news: you can easily repair crimped ends.
As for so-called special cables, you might want to avoid gold connector cables for video and audio.
“You don’t find gold ends in the pro video world,” says Sands. “Sometimes silver ones, but mostly they’re nickel. There may be a certain advantage to using gold connectors on lousy cable, but it’s better just to buy good, standard cables. The gold is really more of a marketing ploy.”
Yes, No and It Depends
Should you buy expensive cables? Should you worry about the impedance of your video cables? It depends.
Videomaker readers are a mixed group. If you, like many readers, are a mid-level hobbyist who’s spent a sum total of $3,000-$4,000 on video equipment, you’re probably editing tapes for your own pleasure. If so, expensive cables are probably unnecessary. The signal loss and color timing problems won’t be significant.
If, on the other hand, you’re a hobbyist who believes in “the right tool for the right job,” you may want to spend the extra money, secure in the knowledge that you’re doing your best to make your tapes “run smooth under the hood.”
If you’ve set up a $10,000-plus desktop video studio, and are doing work for duplication or broadcast, the extra investment may be worth it.
The benefits of better cables will be worth the minimal additional extra expense.
No matter what your goal as a videomaker, remember these points:
1) Use video cables for video and audio cables for audio.
2) For unbalanced audio, keep cable runs as short as possible. If you can, run balanced.
3) Buy the right length for the right application. Longer cables mean more exposure to hostile forces, and more potential for failure.
4) Buy flexible cables for portable situations and more rigid ones for permanent installs.
5) Avoid using connector adaptors/converters. In emergencies, try to use a combination of cable and adaptors that will still provide you with high quality signal.
Follow these simple outlines and your productions will boast crisp, colorful pictures and clear, clean audio.
Videomaker contributing editor Stephen Jacobs is a free-lance video producer (Video Sound).