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  1. #1

    balanced audio: inverted signal required or not?

    I am trying to wrap my head around balanced audio (again), but am not clear on whether two inverted signals are core to its operation. My reading of the following url is that this is true:

    http://www.mediacollege.com/audio/ba...-balanced.html

    ... however at the following page, the text appears to me to be saying that this is not a requirement. Any clarification would be appreciated.

    Bob

    http://en.wikipedia.org/wiki/Balanced_audio

    "Signals are often transmitted over balanced connections using the differential mode, meaning the wires carry signals of opposite polarity to each other (for instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an inverted version of the same signal).

    Despite popular belief, this is not necessary for noise rejection. As long as the impedances are balanced, noise will couple equally into the two wires (and be rejected by a differential amplifier), regardless of the signal that is present on them."

  2. #2
    Senior Member SC358's Avatar
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    http://en.wikipedia.org/wiki/Balanced_audio

    "Signals are often transmitted over balanced connections using the differential mode, meaning the wires carry signals of opposite polarity to each other (for instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an inverted version of the same signal).

    Despite popular belief, this is not necessary for noise rejection. As long as the impedances are balanced, noise will couple equally into the two wires (and be rejected by a differential amplifier), regardless of the signal that is present on them."
    Hello bobmc and welcome to Media College,
    I can understand your confusion. Even I had a hard time trying to figure out the writer's meaning and intentions. It is my belief that the writer is incorrect of their conclusion. Someone may have even thwarted the writing intentionally since it's Wikipedia and is totally open to be edited by anyone.

    The one thing that may not be made clear in Wikipedia and I would have to say needs to be corrected here in Media College is:
    Two identical signals that are 180 degrees out of phase from one another has to be created. If by chance, interference is induced, they will be in phase to each other on both lines.
    When it reaches the mixer, only one wire (+ or hot) is used as the reference and is untouched. It is the second wire (- or cold) that has it's signal inverted a second time. The intended audio on the -/cold is now in phase and is summed with the reference which pass through. On the same wire, the induced interference is now 180 degrees out of phase as well and is cancelled when it is summed with the reference.

    Hope I got this one right!!! Just kidding....
    it's basically algebraic.
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  3. #3
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    Two identical signals that are 180 degrees out of phase from one another has to be created.
    not exactly.... if you're simulating it with a true sine wave, then yes, but in reality they are just the same waveform, but one is at an opposite potential [negative instead of positive, if you will].


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    When it reaches the mixer, only one wire (+ or hot) is used as the reference and is untouched. It is the second wire (- or cold) that has it's signal inverted a second time. The intended audio on the -/cold is now in phase and is summed with the reference which pass through.
    And any interference is out of phase and cancels.... (note that this isn't exactly how it works in most, but it is mathematically equivalent)
    Eric Adler (tonsofpcs)
    http://www.videoproductionsupport.com/ Chat at: http://tinyurl.com/vpschat
    Follow me on twitter: @videosupport @eric_adler

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    Senior Member SC358's Avatar
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    but one is at an opposite potential [negative instead of positive, if you will].
    Yes-this is true because it is negative to ground. I stand corrected.

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    (note that this isn't exactly how it works in most, but it is mathematically equivalent)
    Would you mind elaborating that a bit more? Mathematically and in application of bread board experiments, it worked that way (at least to my recollection). I'm just interested in the part that, "isn't exactly". I've never had a discussion since school and was just merely accepted by me.
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  5. #5
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    Would you mind elaborating that a bit more? Mathematically and in application of bread board experiments, it worked that way (at least to my recollection). I'm just interested in the part that, "isn't exactly". I've never had a discussion since school and was just merely accepted by me.
    I'm not saying that you're wrong there, what you placed works, and some boards definitely use it, but many boards use different differential methods (according to block diagrams and schematics that I have seen) to get the same result.
    Eric Adler (tonsofpcs)
    http://www.videoproductionsupport.com/ Chat at: http://tinyurl.com/vpschat
    Follow me on twitter: @videosupport @eric_adler

  6. #6
    I found an article by Jay Rose that fills in a missing piece in my understanding of how balanced audio works. I don't know for a fact that this method is correct, but based on other reading, it appears to match the description of at least one method of achieving balanced audio. As was pointed out here, there is (at least) one other method.

    "Noises from nearby video or computer cables are picked up on each conductor. But remember, a balanced audio input cares only about the voltage difference between the two wires. Interference is radiated equally into each wire. Since the interference is equal on each, there's no voltage difference from it! The balanced input can't even see that the noise is there."

    Then Jay goes on to show a math chart that depicts how the the difference of one signal combined with another version of itself with inverted polarity would be double the original signal (+1 minus -1 = +2).

    <http://www.dplay.com/dv/balance/balance.html#balance>

    Bob

    (nice to be here, btw)

  7. #7
    Senior Member SC358's Avatar
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    but many boards use different differential methods (according to block diagrams and schematics that I have seen) to get the same result.
    You're right again Eric - different manufactures did design different circuits with different components to achieve the same result. That should have been my disclaimer at the end of it . I did ignore all the rest and went for the simplest explanation.

    Btw bobmc, Jay does provide a good explanation of Balanced lines - thanks for the info as well
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  8. #8
    Administrator Dave's Avatar
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    Ahh, I see this old topic has come back to haunt me.

    When I was very young I learnt about balanced audio from a respected sound engineer. He used a standard "180 degress out of phase" explanation backed up with a standard sine wave graph. It all made sense to me and I didn't give it much more thought. Eventually after many years I ended up teaching other people the same thing.

    Unfortunately my explanation was flawed. Like many people (including the respected engineer who taught me) I misinterpreted the whole 180-out-of-phase vs inverted signal concept. It wasn't until someone pointed out that I'd got it wrong that I stopped to think about it. It then seemed very obvious and I was very embarrassed.

    I think some of the confusion comes from the fact that the sine wave commonly used to illustrate this concept looks the same if it is shifted 180 degrees as it does when it's inverted.

    Anyway, I haven't looked at the page we have here at media college for a long time but I do recall having a discussion about it with a teacher from Europe somewhere. My last recollection of that page is him saying he approved of some changes I'd made and he would be using it for his students. Now the very knowledgeable SC358 tells me it's still wrong so I'd better take notice!

    Right now I need to go and do some urgent jobs but I hope to look at this some time in the next couple of days. In the meantime if any of you guys want to propose an exact description of the situation, perhaps by modifying the text on this page, I'd be happy to work with it.

    In any case thanks for this good discussion. You'd think that balanced audio would be one the the simplest things about audio theory to get right
    Dave Owen
    MediaCollege.com

  9. #9
    Senior Member SC358's Avatar
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    Now the very knowledgeable SC358 tells me it's still wrong so I'd better take notice!
    Oh I wouldn't say you're wrong - it' s just incomplete information which can be confusing. I've done that - said things and even wrote stuff that was not explained well so someone else would point out what's missing.

    To bad we just can't get together and have a few drinks in a round table discussion while writing this out!!!

    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  10. #10

    "out of phase"

    When I do seminars on this, I insist on using the words "opposite polarity" - not "out of phase." Because, when you say "out of phase," that implies questions about what frequency you're dealing with, and is the phase different at different frequencies, and so on. And none of that applies. We are simply talking about opposite polarity: when one wire goes positive by 1 volt, the other goes negative by 1 volt. Sine wave, square wave, anything.

    But here again, there is a misconception. The very expression "balanced line" is misleading as it only refers to the situation where there are actually equal but opposite voltages on the two lines. To be sure, that is usually the case - but it needn't be for the principle to work.

    A far better expression would be "differential line." Because the core of the issue is that the signal which a differential input "measures" is the voltage difference between the two wires. If, at a given instant, wire "a" is +1/2 volt and wire "b" is -1/2 volt, the differential input will see 1 volt. If wire "a" is 1 volt and wire "b" is zero, the diff input will see 1 volt. If wire "a" is zero and wire "b" is -1 volt, the diff input will see 1 volt. In theory (though not often in practice), if wire "a" is +10,001 volts and wire "b" is +10,000 volts, the diff input will see 1 volt. The difference is all that matters.

    Whenever you say the voltage on a wire is, for example, "1 volt," that implies a question: 1 volt relative to what? An input always "measures" a voltage against some reference. For an unbalanced line, it's relative to local ground. For a balanced line, it's 1 volt relative to the other wire. Thus each wire carries its reference - the other wire - with it.

    This combats interference in two ways. The one normally cited is that, if the two wires are close together - better yet, twisted around each other - it will be impossible for an interference source to affect one wire differently from the other. Any crud that appears on one wire will appear equally on the other, so from a crud standpoint, there is no voltage difference between the wires and thus the differential input can't see it.

    The other has to do with ground loops and ground references. In an unbalanced system, a given piece of equipment puts out "x" volts relative to its own ground. If that is connected to the input of another piece of equipment, it will "measure" the signal against its own ground. If the two grounds are not precisely the same (or if a voltage is induced in the ground wire between the two) the latter piece of equipment will "see" the ground voltage discrepancy as part of the signal. A balanced line eliminates this by making sure a differential input measures a signal's voltage at the sending end, thus bypassing the ground differential problem.

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