Shielded Audio
AUDIO – CONTENT VS. QUALITY
In 1997, musician James Taylor, and his recording engineer Frank Filipetti, won a Grammy Award for best engineered album "Hourglass". The big surprise was that this album was recorded in James Taylor's living room. The explosion through the recording industry could be heard around the world. Now anyone could produce a ‘world class' recording virtually anywhere.
Of course, the CD produced in the band's garage is nothing new. It only proves that "content is king". But perhaps we should add a corollary to that: "Content is king, and quality makes it sell." In these days of MP3 players and low-bit rate downloads, it is amazing what low quality levels are acceptable to many listeners. But, even if we assume that 20% of the audience will listen regardless of the quality, that leaves a lot of audience who might also listen if the quality of the material, and the quality of its delivery, improved.
So quality still matters. It's just that getting to that top level is now a lot easier (and a lot cheaper) than it used to be. Just note that condenser microphones, that a decade ago were rarely available below US$1,000 are now routinely available for US$199 or even less.
Are they the same? No, of course not. The question really is, can that $199 model be good enough for your project? Will it last on the road with abuse it might get? How well does it work outdoors as well as indoors? And a thousand similar questions. You might well end up back at the $1000 model just because you can't afford to have a mic fail during your next recording or stage performance.
And this also applies to the cable between that microphone and the preamp or mixer. And don't be confused: microphone cables, or any other cable, is just as technologically advanced as the device it is plugged into. And, like links in a chain, your signal quality is limited only by that weak link. And if that weak link is cable then, shame on you, because that's the cheapest link to fix. You just have to know what to choose.
For analog microphones, there are really three basic choices: ruggedness, low noise, and performance. We'll look at the first two of these factors and determine just what that cable would look like. The third, performance, will be the subject of a following column.
RUGGEDNESS
The most rugged microphone cables currently are made with rubber. These are very rare cables. You might think the cables you are currently using are made with rubber. It is very likely you would be wrong. There are formulations of PVC that look and feel like rubber, but are nowhere near as rugged. If you want to know for sure, just check with the manufacturer. You can also do a simple test. Get three feet of rubber-jacketed cable (such as Belden 8412) and tape three feet on a floor. Next to it tape three feet of any other cable you wish. Now find an office chair with metal wheels and go over these cables, back and forth, for a few minutes. If you know someone considerably heavier than you, have them go back and forth as well.
After this "torture test" un-tape the cables and look at them. Will you be surprised! The rubber cable is almost indestructible, while the PVC or other plastics, can get shredded quite easily.
LOW-NOISE
This can mean different things to different people. There is ‘shot noise' also called ‘impact noise' that is a common problem with guitar cables. However, that kind of noise is much rarer with professional balanced-line twisted-pair microphone cables. And the reason it is rare, is that the impedance of most professional microphones is very low, around 150 ohms, compared to guitars which are very high, up to 50,000 ohms.
More often, with mic cables, "noise" refers to EMI (electromagnetic interference) or RFI (radio frequency interference) which can easily work its way into the cable and interfere with the audio, or worse, work its way into the preamp, mixer, or console using the mic cable as a really nice "antenna".
The way to reduce this effect is first, to run the twisted pair as a balanced line. This is what all professional audio devices already do. An explanation of balanced lines, with common-mode noise rejection, would take an entire column, and more. Goggle those terms out if you want to learn more. And one of the things you will find is that the ideal twisted pair has two wire very close to each other. (Wires close together make the noise on each wire as close to identical as possible.) This is one reason why we twist the two wires in a twisted pair, to keep them close together.
However, when you bend and flex this mic cable, the two wires in the twisted pair tend to untwist slightly. This makes them vulnerable to EMI and RFI, or to leak out the audio signals running on them to cables around them. There are two solutions to this problem.
The first solution is a very new solution and it is to stick the two wires together when they are being twisted. There is only one cable made like this, and that is Belden 1353A. It is, in fact, a single pair "data cable", a single pair Category 5e computer cable. Now a computer cable installer would say this is crazy to have a "one-pair Category 5e data cable", but now you understand it perfectly. If you can stick the two wires together, then no matter how you bend and flex the cable, they will stay right next to each other, and give you maximum noise rejection. In fact, the noise rejection is so good that this pair isn't even shielded. Just stick the two wires into pins 2 and 3 inside the XLR microphone connector, and it will work just fine…with one exception. And that exception is microphones that are powered through the mic cable, "phantom powered" microphones. They need that shield to help deliver the power to run the microphone. Obviously, this unshielded cable won't work there. But for all the self-powered microphones, dynamic, ribbon, and other designs, this cables would be an excellent choice.
The second way to reduce EMI and RFI is to have a special construction with not a single twisted pair, but four wires, something commonly called "starquad". If you open up this cable, you will find that the four wires are in one spiral (not two twisted pairs). To use this cable, you must combine the two wires opposite each other. This then turns four wires into the two wires for your connector (pins 2 and 3 again). These starquad cables also have a shield that connects to pin 1.
How starquad gives you ultra-low-noise performance would take another entire column, but suffice it to say that the performance of this design, especially at EMI – low frequencies – is dramatic. It is especially good at 50 Hz power noise. So if you must hang microphones from a lighting grid, for instance, starquad would assure minimum 50 Hz pickup in each mic line. Just be sure that the mics themselves are resistant to 50 Hz interference.
About the Author
Steve Lampen has been with Belden for 19 years and currently is Multimedia Technology Manager and Product Line Manager for Entertainment Products. He has an FCC Lifetime General License (formerly an FCC First Phone License) and is a BICSI Registered Communications Distribution Designer. His latest book "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill.
Visit our web site www.beldenapac.com or email us info@beldenapac.com
I can hear my laptop think!
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