What is a balanced input / output?

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What does it mean when an input or output is balanced?

Willbill

Posted 2010-12-07T22:09:37.353

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Answers

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This topic is commonly misunderstood, and Joel's answer isn't quite correct.

Transmitting "both the signal and its opposite along two wires" is called differential signaling. This is used to minimize emissions from a cable into other circuitry, because the equal and opposite electric and magnetic fields cancel out at a distance from the wire.

A "balanced line", on the other hand, means that the sender and receiver both have the same impedances connected to each of two wires (the source impedance for both lines is the same and the load impedance for both lines is the same). This way, any interference into the cable produces the same voltage on each line, and can therefore be canceled by a differential amplifier in the receiver.

Usually these are both used at the same time, but they don't have to be. The so-called "impedance balanced" outputs on some mixers are an example of a circuit that is balanced but not differential.

Here's a reference:

SIGNAL SYMMETRY HAS ABSOLUTELY NOTHING TO DO WITH NOISE REJECTION — IMPEDANCE IS WHAT MATTERS!

endolith

Posted 2010-12-07T22:09:37.353

Reputation: 1 864

1+1 I didn't know this distinction existed. I still think that Joel's answer is good in a practical sense though – None – 2010-12-16T14:40:42.753

1+1 great explanation. I always wondered why analog signal cables used opposite voltages because differential doesn't necessarily require opposite voltages to work (ie, you could send the same voltage down both lines). The twisted pair and magnetic cancellation makes perfect sense now. Thanks. – None – 2011-02-11T00:03:22.853

Actually I think the electric field cancellation is more important here, since very low currents are generally flowing in signal cables. – endolith – 2011-02-11T14:43:27.357

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It's really the cable that's balanced.

Imagine a long cable transmitting a weak signal (for example, a very low voltage signal from a microphone). As the signal travels down the cable, ambient noise might distort the signal.

In a balanced cable, instead of just transmitting the signal by itself, you transmit both the signal and its opposite along two wires, usually twisted together.

If there is any interference, the interference will apply equally to both wires.

That allows it to be cancelled out at the receiving end automatically.

Imagine that the signal is +5 and the noise is +1.

An unbalanced cable would transmit +5, but the noise would cause a +6 to arrive.

A balanced cable would transmit -5 and +5; the noise would cause a -4 and +6 to arrive. Now the receiving apparatus can figure out that the original signal must have been +5.

In practice, you'll recognize balanced cables because they use three conductors instead of two, for example, a TRS 1/4" jack:

alt text

Or an XLR jack:

alt text

Joel Spolsky

Posted 2010-12-07T22:09:37.353

Reputation: 654

Nice explanation, Joel. I especially like the D&D style signal to noise descriptions. :-) – None – 2010-12-08T13:44:10.527

Just to extend this, I'm curious if stereo cables are considered "balanced", since by my understanding both signals traversing such a cable are of the +5 variety (i.e. not negative). Or does this notion of a three-conductor balanced cable apply only to cables carrying a single audio channel? – None – 2010-12-14T05:29:13.587

that's a good question for someone smarter than me... ask it as a new question! – None – 2010-12-14T11:55:26.847

Voyagerfan5761 didn't create a new question, so here's the answer: No, a stereo cable is not balanced, because the left and right channels (a) have different signals and (b) neither is inverted. – Liudvikas Bukys – 2010-12-15T16:22:50.027

Geek gestalt: In the data world, use of balanced lines is called differential signalling, as exemplified by RS-485 serial lines. (RS-232 would be the unbalanced equivalent.) – Liudvikas Bukys – 2010-12-15T16:24:28.730

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Balanced lines and differential signaling are two different things. They're often used at the same time, but not always, and they have different purposes. http://en.wikipedia.org/wiki/Balanced_line#Balanced_and_differential

– endolith – 2010-12-15T19:55:14.703

@Voyagerfan5761 basically no they are not balanced. The positive/negative aspect has nothing to do with being balanced, it's all about the voltage referenced to ground. Any voltage that is seen on the ground shouldn't be there so the input removes whatever is equal across signal and ground. Joel has the right idea, but is somewhat wrong. – None – 2010-12-15T23:12:43.080

That's what I figured (@Liudvikas). Thanks to everyone who commented here; some really interesting reading. And maybe I would have made a new question, if I'd still been online. :) Perhaps I should anyway, so the answer will be in the archive when the site (hopefully) launches. – None – 2010-12-16T00:29:34.563

2@endolith gets extra geek credibility for being more precise than any of the rest of us. – Liudvikas Bukys – 2010-12-28T18:19:56.527

The image of a TRS jack should probably be replaced with a Mono one, to show that it's not balanced. Right? – None – 2011-03-06T12:02:03.507

3no... TRS means Tip, Ring, Sleeve = three conductors. It is mono. This is confusing because old stereo hi-fi gear used the same TRS connector for unbalanced stereo headphones. – None – 2011-03-06T19:18:37.300

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It's an interconnect method that lets you transmit signals over very long lengths of wire without having large amounts of extraneous noise injected in to the signal. The signal is duplicated on to two wires and the input impedance at the received is the carefully matched for both signals. This insures that noise injected during the journey is done so in equal amounts on both signals so it can be detected and rejected by the receiver. This is important for relatively weak signals (say from a microphone) where even minute amounts of RF noise picked up on the line are, relative to the signal level, fairly large.

This wikipedia entry describes the different signaling methods in balanced transmission design, in detail.

Ian C.

Posted 2010-12-07T22:09:37.353

Reputation: 3 984