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view all articles in:
The Reference Room
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Wired for Sound: Steve Lampen
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| A Perfect Balanced Line |
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| by Steve Lampen, 2.02.2005 |
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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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Here starts my 60th column for Radio World. I have been writing about wire and cable on and off for eight years now. Occasionally, a reader who stops to chat at a trade show or SBE, AES or SMPTE meeting will ask how I could write about one subject for so long and without running out of material.
Believe me, it's easy.
Wire and cable are no less "high tech" than any other part of broadcasting; and improvements, even "breakthoughs," occur with alarming regularity. I predict I will never run out of things to talk about.
This time, we're going to look closer at balanced lines.
Fig. 1 shows a balanced line, with transformers (or active balancing circuits, which mimic transformers) at each end. Where the signal is differential (black arrows), electromagnetic noise hits the two wires and travels to each end, where it cancels itself out.
What makes a perfect balanced line? There are three essentials:
1) The two wires must be the same length. If they aren't, the noise will not arrive at the same time. This means the two signals will not cancel out exactly. A timing or phase difference will exist between them. In the data world of Category 5e or 6, they use a term for length inconsistency, "resistance unbalance." Maybe we should start testing and referring to this for audio cable pairs.
2) The two wires must be the same size. If the wires are different in size, the intensity of the noise signals will be different and will not cancel out completely. This also shows up in resistance unbalance.
3) The two wires must be in the same place. The example shown here is probably the worst twisted pair because the wires are far apart. When they are spread, any noise will hit them at different times. The two noise signals in the wires don't arrive at the same time at the end of the cable. They are out of phase with each other and do not cancel out completely. Whatever is left of the noise will travel through the transformer to the next device. This shows up in the data world as changes in capacitance or "capacitance unbalance."
Now or later
As any engineer knows, it is much harder to remove noise once it is part of a signal. It is much easier to remove it before it becomes part of that signal.
This is true especially in analog audio circuits, which cannot recognize the difference between noise and the audio. In digital bit streams, noise looks different than the bits and often can be filtered out.
But it's still a lot cheaper to put in good balanced line cable attached to good transformers (or active balanced circuits) and get rid of the noise before it becomes part of the signal.
The problem is that you take a "perfect" balanced line and, by attaching it to something that is not well-balanced, destroy that circuit's ability to reject noise. So, really, you have to look at all passive components, cable, connectors, patch cords, patch panels and the source and destination devices as well.
The actual definition of a balanced line - with thanks to Bill Whitlock at Jensen Transformers - is a circuit where each wire and all passive components attached to it have the same impedance in reference to ground. That's quite a mouthful! But because "impedance" includes resistance, capacitance, inductance and the reactance of C and L produced when frequencies run through them, it covers everything that could be attached.
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