XLR cables, commonly known as microphone cables, are designed to carry a so-called balanced AC electrical signal from point A (generally a microphone) to point B (generally and audio mixing desk). They have a relatively simple task but a big problem in accomplishing it: Electric current is prone to disturbances from pretty much any magnetic interference from fields in the space around it. This means just about every light fixture, cell phones, pacemakers, hell even the sun.

There is a point to this. Bear with me.


A sound is a vibration, carried through a medium (the air, usually) as a sine wave. A microphone is nothing but a converter, transforming this change in pressure into a change in electrical current. The end result is also a sine wave, travelling through the copper inside the cable. God, this stuff is so harmoniously beautiful, it brings tears to my eyes.Sine

<- Kind of like this. Not exactly, but close enough.
The amplitude and frequency (width and height of the arches) depend on the tone and volume of the sound.

SinedistDisturbances from halfway down the cable can be visualized by peaks in the waveform, like this ->

The very cool thing about this is that it actually looks this way in real life. If you measure the voltage on the cable hundreds of times per second, it actually goes up and down and when there is interference, you can see this in the measurements.
But how would you, as an engineer, make sure that this disturbance doesn’t end up as audible distortion?

That’s where the ‘balanced’ part comes in.

XLR cable doesn’t have 2 connectors, but 3. This allows for it to support not 1, but 2 signals simultaneously. And what a microphone or any device with a balanced output does, is take the original signal and split it. It then takes the copy that it made, and flips it upside down. Thus, the two signals running down look like this:


Cute, no?

So these two signals run down the cable simultaneously, and pick up the same interference. So when it hits and distorts both sine waves, it changes the sine identically:


I swear, drawing these is MS Paint requires a steady hand.

The audio desk picks up both signals, flips one back around so that they are copies again. But here’s the kicker: The disturbance that got picked up along the way, gets flipped around, too. Right before the two signals are added back together, they look like this:



You can see that the two waves around now identical again, and compliment each other when added together. But not the disturbance: The peaks don’t match. In fact, they are each other’s exact opposite. And what do you get when you add exact negatives and a positives?

That’s right, zero. I see you paid attention in school.
And the interference is gone. A problem is solved with such gracious simplicity, that it borders on magic.

What the hell is my point, you ask? Aside from the jaw-dropping beauty displayed in systems as efficient as this, there’s also a metaphor to be found. This and many other methods used in engineering, are a perfect example of a creation made in the image of its maker.

No, we are not sine waves, but we do have messages we carry over. Constantly, we are communicating, even when we are alone. And we like our message to arrive the same way we intended it when it was sent.

What we perceive as interference, is just random information floating by. About the state of the cosmos, the invisible effects of a dimmer on a lighting circuit, and so on. They are part of our surroundings just as much as the sound we are trying to amplify, but we aren’t interested in them at this point in time, if ever. So we filter them out the same way we would love to filter out any misinterpretation when we try to get a message across.

The thing with humans is that we can never erase misunderstandings completely. In fact, we manage to function in a world filled to the brink with endless white noise, constantly bombarding us with information true and false. How we manage to even isolate and understand a single sentence spoken into our ear, is a small miracle. But it is never exact.

More than most others, I try to filter these things down to the bare essence. I rarely say a thing I don’t mean, and sarcasm is often lost on me. If it were up to me, all information was shared in a curt but complete fashion, a militaristic style of communication that would last until we go back to ordinary banter.

But it isn’t, and I am often called rude or short for the way I talk to people. I still haven’t figured out a way to flip signals well enough for the message to come across in its purest form, but at least I have my job.

You should have seen me. A band onstage that had its own audio desk, sending its signal to mine, from where I sent back a mix of the voices of the actors onstage. Two complete PA systems interlinked like lovers on a sofa, as I ran back and forth to tweak knobs and shove faders. Where no solutions existed, I created them, abusing my FX Send port into an aux output for the monitors downstairs. It was a thing of beauty, invisibly pulsing with every word spoken into any microphone, with every note of the piano.

Some people can do this with the friends around them; Laughing, joking and understanding things that don’t make sense. Seeing the information through the noise.
Me, not so much. But where others see a handicap, I found a talent. And try as they might, with all their social skills and linguistic insight, they couldn’t hook up a microphone to save their lives. It’s a good thing we have each other.


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