In the comments section of the last post one of our readers mentioned star quad cable for use in areas where RF interference is a particular concern. Sometimes just called "quad" cable it was brought to us by the phone companies, like so many of the things we use in audio today. [Hat tip to Audio Analyzer]
The concept is actually pretty simple. It utilized the twisted pair technique to help cancel out noise in the signal conductors that gets past the shielding. The wires inside a normal mic cable form a twisted pair, so that a spike of noise getting on one conductor is also getting on the other one but that conductor is carrying the same signal in the opposite polarity. When the cable finally gets to the balanced input of a piece of gear, the signals are combined and since their amplitudes match are passed on, but the noise is not. It's a bit of a tough concept so don't feel bad if you don't get it, just go look it up.
With quad cable, each side of the balanced line uses a pair of conductors. It takes advantage of the phenomenon twice over which reduces noise even further. Here's a link to a data sheet that extols the virtues even further.[Hat tip to Brandon] I'd have to do some more research before I could quote an exact number, but it looks like noise reduction on the order of six to nine decibels is to be expected.
This technology is quite effective. Ethernet cable uses this method with great success. In fact the only difference between grades of cable is how tight the twist is. Even speaker and power cables utilize a twist inside the jacket. Although in those cases it has more to do with manufacturing methods than noise reduction. On a speaker line, a millivolt spike of noise is practically nothing, on a mic line it could be as loud as the signal.
So what's the importance? Well, in a mastering studio it might make a slight bit of difference. That's an area where absolutely fanatical measures are taken to provide a pristine listening experience. Beyond that it's not as common to see it in the lower levels of live sound. Where it becomes more important is at larger concert or sporting events. When you have miles of cable and hundreds of channels of wireless gear, including higher powered two-way radios, that's a lot of opportunity for noise to get in. Quad is one answer, going digital is fast becoming another. But even on the smaller gigs there are places where it could be a big help.
I remember one small music venue that was just a few blocks from a major broadcast tower. There was a not so subtle hiss in everything. One of the effects units was practically unusable due to the amount of RF getting into it. Short of tearing the club apart and installing a Faraday cage (look it up) quad cable would have been a worth while investment and probably would have greatly improved the sound of the system.