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Help me understand EMI in microphone cables!

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Help me understand EMI in microphone cables!

Postby Dead Sound » Thu Feb 07, 2013 7:20 am

I'm left without a full understanding after reading this SOS article, What’s special about star-quad cable?.

Even if it's just a regular twisted pair, I can't shake the notion that each full twist ensures the leads are equidistant from the EMI source. So why does one lead induce more interference versus the other lead as explained in the article? Can someone please offer some clarity?
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Re: Help me understand EMI in microphone cables!

Postby Hugh Robjohns » Thu Feb 07, 2013 9:52 am

If the interference source is a long way from the cable, then the interference is fairly consistent in level along a long length of the cable, and the twist of the two wires inside will ensure that both wires are exposed to an equal amount of interference.

However, this theory falls down if the interference source is very close to the cable (ie, right alongside it) because by the time the twist has come around the now closer wire is actually further away from the interference source and so picks up less interference anyway. So a lazy twist isn't effective in cancelling out interference for very close sources. A tighter, shorter twist would be better, but its effectiveness is still restricted, depending on the proximity of the interference source.

Imagine a two core cable, laid alongside a point source of electromagnetic radiation, with the two cores twisting relatively slowly along the length of the cable. The EM interference is strongest where the cable is closest to the interference source and if the two wires happen to be in a part of their twist where they are equidistant from that close point source of interference, they will both receive exactly the same level of interference. All will be well because we have a true common-mode signal.

However, if the state of the twist is such that one wire is closer, it will pick up slightly more interference than in the previous example. Meanwhile the other wire must be more distant than previously and so will pick up slightly less interference.

These unbalanced levels of interference in the two wires defeat the differential receiver because it is not a common-mode signal any more, and hence the intereference will be heard. Not good!

In a starquad cable there are four core wires, connected in opposite pairs, and the twist length is much shorter than in a normal cable. The tighter twist length helps to ensure interference cancellation over short cable lengths, which helps, but the use of four wires connected in opposite pairs is the really clever bit.

Again, let's assume that one pair of connected wires happen to be exactly equidistant from the interference source (the red and blue wires in diagram 3 in that article), while the other two (green and white) are closer and further, respectively.

The hope is that the sum of the interference signals that break into the two wires that are equidistant (red and blue) in the middle will be the same as sum of the interference in the closer (green and slightly more) and further (white and slightly less) wires.

And generally, that is the case.

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Re: Help me understand EMI in microphone cables!

Postby Dead Sound » Thu Feb 07, 2013 11:49 am

Thanks very much!

What I hadn't accounted for is the distance needed to complete each twist down the length of the cable. Also, the interference is fanned out from the source, not moving toward the cable like a wall.

So there is a fine point where the interference will certainly be stronger inside one wire compared to the neighboring wire even though it's a small space.
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Re: Help me understand EMI in microphone cables!

Postby Hugh Robjohns » Thu Feb 07, 2013 11:53 am



H
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Re: Help me understand EMI in microphone cables!

Postby Jim Lockhart » Fri Feb 08, 2013 5:46 am

Hugh's response gives part of the story, but there's more. Faraday's law of induction tells us that a voltage will be induced in a wire loop when the magnetic flux through the loop changes. (This is how a dynamic microphone or an electric guitar pickup gets its signal.) A twisted-pair cable attempts to reject signal pickup from magnetic sources such as power (mains) cables and lighting dimmers by forming the two signal wires into a tightly-spaced series of very small loops with varying orientations. A loop with its plane perpendicular to the lefthand direction could be followed by one with a plane perpendicular to the upward direction, then one perpendicular to the righthand direction, and a fourth perpendicular to the downward direction. Signal from a reasonably distant source picked up by the lefthand loop would be roughly cancelled by signal from the righthand loop; same for the upward and downward loops. Star-Quad greatly improves on this magnetically-induced signal rejection by always having a "reverse-oriented" loop at the same location as each "forward-oriented" loop. (A "humbucking" electric guitar pickup uses a similar approach.)
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