Star-quad cable

Star quad cable is a type of balanced audio cable that specializes in reducing electromagnetic interference. Quad cable is better at rejecting external noise than an equivalent single pair cable. Star-quad cable is a four-conductor cable that has a special quadrupole geometry which provides magnetic immunity when used in a balanced line. Four conductors are used to carry the two legs of the balanced line. Star-quad cable typically provides a 10 dB to 30 dB reduction in magnetically-induced interference compared to standard two‑core mic cables. The up side is better rejection of very close‑source electromagnetically induced interference — the kind of interference that might come from laying a mic cable alongside noisy mains cable. Starquad cables are very good at rejecting very close sources of EM interference — such as when laying mic cables in a cable duct with lighting cables — but are essentially no better than standard balanced cables when the interference source is more than a few feet away.

There are three types of interference that can affect signals in cable: electrostatic, radio frequency, and magnetic interference. Of these, the first two can be minimized effectively using shielding. An overall shield only helps prevent RFI (radio frequency interference). It won’t do a thing for lower frequency noise, or crosstalk, which is where a well twisted pair or quad will prove its worth. Unshielded CAT 5 cable is actually used to carry audio in many broadcast facilities.

The third type, however, requires a different approach as magnetic fields can pass quite easily straight through shielding. The star-quad configuration was invented for use in telephone cables back in the ’30s, and was adopted in the TV world in the ’60s and ’70s. It is widely used today, particularly where there is a likelihood of having to lay mic cables alongside strong sources of interference.

The magnetic immunity of star quad cable is a function of the accuracy of the star-quad geometry, the accuracy of the impedance balancing, and the common-mode rejection ratio of the balanced receiver. All four conductors must be an equal distance from a common point (usually the center of a cable). The four conductors are arranged in a four-pointed star (forming a square). Opposite points of the star are connected together at each end of the cable to form each leg of the balanced circuit. All points of the star must lie at equal distances from the center of the star. In order to maximize noise rejection, Star Quad must be properly wired to the XLR-3 connector (or terminal block).

To a magnetic field, both legs of the balanced circuit appear to be in the exact center of the star. This means that both legs of the balanced circuit will receive exactly the same interference from the magnetic field and a common-mode interference signal will be produced. This common-mode interference signal will be rejected by the balanced receiver. The 4-conductor Star Quad arrangement, cancels electromagnetically induced noise from SCR dimmer packs, fluorescent lighting ballasts and AC power transformers.

So, is it worth paying more for star quad cable? It depends on your usage and how important interference reduction is for you. If the chance of external noise is low, quad cable may very well be “over kill”. But many people prefer to play it safe and use quad cable anyway, just in case they run into a situation where that noise could be a problem. With the portable situations that most of us deal with, external noise is a variable that we have no control over, and quad cable becomes an “insurance” policy. Star quad cables are clearly beneficial for certain needs, but they come at a cost. They are a popular choice in professional audio settings where interference can be an issue, but for simpler settings, they may be a “nice to have” but unnecessary luxury.

The main disadvantage of star quad compared to normal shielded twisted pair microphone cable is higher price and the increased capacitance. Obviously, with four wires in the cable, the core‑core and core‑screen capacitance will be much higher than in a more conventional two‑core cable, and that’s the down side because it can result in a greater HF loss in very long cable runs — although it really won’t be a problem over 10 metres, or even 50. For microphone or line level signals, capacitance of the cable is by far the dominant parameter affecting audio performance. High capacitance will limit frequency response at the high end (treble). The downside is that quad cable has roughly twice the capacitance of equivalent single pair cable. That means you can only run half the distance before the top end starts to roll off, but that distance will still be several hundred feet, even under worse case conditions. The extra capacitance can become an issue in very long cable runs or if you are trying to force AES3 digital signals down the cable!

Sources:
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