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The Reference Room
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Wired for Sound: Steve Lampen
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| A Star Glimmers in the Night |
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by Steve Lampen,
5.25.2005
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Steve Lampen is Technology Specialist, Multimedia Products for Belden Electronics Division. Steve holds an FCC Lifetime General License (formerly a First Class FCC License) and is an SBE Certified Radio Broadcast Engineer. On the data side he is a BICSI Registered Communication Distribution Designer. His latest book, "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill.
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We've talked about how balanced lines reject noise because the two wires are as close together as possible. Besides bonding pairs, as in some UTP computer/data cables, there is another technique that actually puts the two wires in the same place.
Yes, I know, boys and girls, that certainly sounds impossible. But it's done all the time, and it works well in rejecting noise. It's called the "starquad" design and is most commonly used in microphone cables, although there are multiquad snake cables available.
Fig. 1 shows a generic starquad construction. It consists of four conductors spiraled together. This is the only construction that will have a noise-reducing effect. Two twisted pairs, or four random conductors, will not have this effect.
If you've not used starquad, you might wonder how to wire up a cable. It's really simple. Fig. 2 shows how you combine the conductors from four into two. (You already know that "standard" mic cable has two conductors with a shield around them.)
Specifically, in an XLR, the shield would go to Pin 1, the two wires (from the combining step described above) go into Pin 2 and Pin 3. The secret is to combine the wires opposite each other into the two conductors.
In Fig. 1, for instance, you would combine the two light conductors into one and the two dark conductors into one. Most starquad cables are color-coded so you know which to combine.
Think about the four spiraled conductors. You will see that this approach puts the two (combined) conductors in the same place. Fig. 3 attempts to show this.
Sound vs. light
Because both conductors are in the same place, any noise that hits these wires will be close to identical by the time it reaches a transformer or active balanced circuit. The ability to cancel out noise is dramatic. This approach is effective especially at low frequencies, say 50 Hz or 60 Hz power. So if you have to run mic lines near power or lighting cables, starquad would be a good choice.
Some makers of starquad cables add a stripe or other marker onto one conductor of each color so that you can use it as a four-conductor shielded cable. Of course, once you use it as a four-conductor cable, it has no noise reduction properties.
Because most starquad cables are limp and flexible mic cables, they can make great remote control cables. There are even some miniature starquad cables made of special alloy conductors for added strength. This type often is used as headphone cable thanks to its ruggedness. Of course, there's no noise rejection needed in this use, and none provided.
The noise rejection is really thanks to the fact that this is a balanced line cable. And the amount of noise rejection - called "common mode noise" because it is common or the same on both conductors - is only as good as that of the devices at each end. If those devices have good CMRR - common mode rejection ratio - noise will be rejected. In fact, this is true in any balanced line circuit.
My friend Bill Whitlock of Jensen Transformers has a great definition of a balanced line. It is "a two-conductor circuit in which both conductors and all circuits connected to them have the same impedance in respect to ground and to all other conductors."
So you can see that every part, from source to destination, can have an effect on the "balance" of a balanced line and its ability to reject noise.
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