Every sound/recording engineer is familiar with the terms unbalanced and balanced interfaces… more commonly referred to as “unbalanced or balanced lines.”
Yes, everybody knows “unbalanced line” can be very susceptible to noise (for very long runs), and that “balanced lines” are more preferred because they’re quieter, i.e. not as susceptible to noise.
But if you probe deeper, and ask them to explain exactly what the difference is between an “unbalanced” and “balanced line” is, you often get the common answer that:
“A balanced line has 2 wires (plus the ground), a HOT and COLD line, and the signal that is present on the 2 wires are 180-degree out of phase/polarity with one another.”
[BUZZER!!!!!] — I’m sorry to break your bubble, but that is not the correct answer.
But so often, you see the above answer (or some variation of it) on so many audio forums. And this incorrect answer seems to get propagated and spread more and more.
So what is the difference between “Unbalanced” and “Balanced” lines?
Well, the operative keyword here, and what is missing in the incorrect answer above is IMPEDANCE. To be more exact, the IMPEDANCE with respect to ground.
In an “Unbalanced line”… wait, let’s call it the more proper term “Interface” because a piece of cable is nothing special by itself and it takes a whole system (the output driver, the cable, and the input receiving end) to define if the system is “unbalanced” or “balanced.”
Okay, in an “Unbalanced Interface”, you have 2 wires. The ground wire has zero impedance (because it’s grounded), and other signal-carrying wire has a higher impedance compared to the ground wire.
In a “Balanced Interface”, you have 3 wires. You have the ground wire (same as the unbalanced interface), but the other 2 wires have EQUAL IMPEDANCES with respect to ground wire.
Now, there are some strict conditions for this scenario to be considered truly “Balanced.” Mainly, the driving equipment, the “line” or cable, and the receiving equipment” all must have equal impedances to ground. If one of these 3 items violate that condition, then it’s not “Balanced” anymore, and your connection becomes “Unbalanced.”
What do I mean by that? For example, using a plain TS cable instead of a TRS/XLR cable between the output and receiving end of 2 equipment, will invalidate your “Balanced” system. It will become “Unbalanced.” Why? You don’t have equal impedances anymore.
Another example, let’s say you have a Balanced output equipment, you’re using a TRS/XLR cable, but connecting to an RCA jack, or TS jack on the receiving equipment. The end result here is an “Unbalanced” system. Not “Balanced.” Why? You don’t have equal impedances anymore.
It’s all about the IMPEDANCE! — that’s the true nature of “Balanced Interfaces”, and everything from the driving equipment, the cable/line, and receiving equipment are important pieces of the puzzle working together.
Okay, now that we’ve cleared that common misconception about Balanced and Unbalanced, some of you may still be scratching your head. Some of you may be asking “What about those waveforms that are opposite/out-of-phase/out-of-polarity from one another?”
Good question. But first, let’s correct another common misnomer — “out of phase”. This is the wrong term to use. When we talk about the word “Phase”, it means with respect to time (t). A signal may be lagging or leading, i.e. out-of-phase with respect to time (t).
The 2 output signals here are not out-of-phase, with respect to time. However, they are “complementary” or “symmetrical” with one another. Electrical Engineers call this “Differential Signalling.”
In this screenshot, we see approx. 2Volts peak-to-peak on one line of the balanced output. The second line also shows the same 2Volts peak-to-peak signal, but complementary and out-of-polarity with the other line. The purple line is how the Balanced Receiving circuit will process these differential signals. [Vout1 – Vout2] and the resulting output is 2xVout.
I hear some of you screaming “Common-what?!”
Relax. Let’s explain it in more detail…. so we have our 2 lines having equal IMPEDANCES (with respect to ground), now if an external noise source, or noise generator (for example: Electromagnetic Interference, or Radio Frequency Interference) affected our Balanced system, the noise pickup by our cable or equipment will be equal in magnitude in our 2 lines. Why? Because of equal IMPEDANCES.
Think of Impedances as Resistance, and the Noise Interference as Current. The voltage generated (i.e. noise level) by Ohm’s law will be V = I x R. Since both our R is the same value (remember, equal impedances), and the same noise affected both our 2 wires, then the extraneous noise voltage developed on our 2 lines will be of equal magnitude or levels.
Some of you may be grumbling “Great! Now we have TWICE the amount of noise on our 2 lines!”
Ahhhh… but this is where the true beauty of a “Balanced” system lies.
Remember that Balanced Receiver I mentioned above? Well, Balanced Receivers have a nifty behavior where it REJECTS signals/voltages it sees that are identical/equal-in-magnitude present on both lines. The equal/identical voltages present in both lines is called the Common-Mode voltage. And Balanced Receivers treat Common-Mode voltages as a sworn enemy!
That stupid noise that is present on our 2 lines? Rejected/Gone/Erased/Eliminated/Deleted/Trashed-and-Recycle-Bin-Emptied! What’s left after our Balanced Receiver process the 2 signals is just our plain, clean signal.
And this is how Balanced systems can be so immune to noise interference, even in harsh electrical environments… it’s because of the ability of the Balanced Receiver to reject Common-Mode noise voltages. But everything must be “Balanced” (from driving equipment, cable, receiving equipment), otherwise Common-Mode noise rejection suffers.
Now, of course in the real world nothing is perfect and mis-matching tolerances prevent us from achieving a perfect 100% noise rejection, but hey, we can come very close to 100% noise rejection performance.
Now that’s out of the way, I hope you now understand what truly is a Balanced Interface and how it helps us keep our audio systems quiet.
Now, what about that Differential Signalling, 2 signals symmetrical and all that jazz…. we’ll save that topic for another day in the future.
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