The above Ad will no longer appear after you Sign Up for Free!

Audio QOTD: Balanced Lines

Discussion in 'Question of the Day' started by Dillon, Oct 13, 2008.

  1. Dillon

    Dillon Active Member

    Messages:
    111
    Likes Received:
    14
    Location:
    New York
    Here's one of my favorite questions to ask when interviewing an audio tech:

    Balanced mic lines use three conductors to carry a signal. We all know that an audio signal can be carried with just two lines. Describe to me how balanced mic lines work.

    It's a simple question that not many audio guys have ever really stopped to think about. You'll be surprised at how difficult it can be to describe what you may think to be obvious.
     
    Last edited: Oct 13, 2008
  2. zuixro

    zuixro Active Member

    Messages:
    369
    Likes Received:
    13
    Location:
    Spartanburg, SC
    The way I understand it, is that one line carries the correct waveform, another carries the same waveform but inverted, and the last conductor carries a ground, which (should) always be the average of the two other conductors.
     
    Last edited: Oct 13, 2008
  3. David Ashton

    David Ashton Active Member

    Messages:
    829
    Likes Received:
    89
    Occupation:
    truck driver
    Location:
    perth W Australia
    I explain it like this, you have a pair of closely twisted wires so when a pulse of electrical interference passes through the pair it induces a + pulse in one conductor and an identical negative pulse in the other, which cancels it out.This is because the currents are flowing in opposite directions so that it adds to one and subtracts from the other
    The screen around the cable helps to reduce the interference by draining it to earth, which it does to a degree but the tight twisting of the pairs is the best factor.I found this out in a previous life as a high power measurement testing technician.
     
    Last edited: Oct 16, 2008
  4. waynehoskins

    waynehoskins Active Member

    Messages:
    782
    Likes Received:
    49
    Location:
    Arlington, TX
    The shield serves chiefly to keep RF out (and a braided shield does a far better job of this than a foil shield). Most all the rest of the noise-immunity is the result of the balanced signal on the twisted pair.

    It's a good question. I like it.
     
  5. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

    Messages:
    4,445
    Likes Received:
    2,845
    Location:
    Las Vegas, NV, USA
    How can this be? A braided shield has holes in it, whereas a foil shield "locks in freshness"?

    My understanding is the differences in shielding are mechanical, not electrical.
     
  6. philhaney

    philhaney CBMod CB Mods

    Messages:
    721
    Likes Received:
    40
    Location:
    Laguna Beach, CA
    As far as braided vs. foil are concerned it depends on frequency. To the appropriate frequency a chain-link fence looks like a solid wall, whereas we can see right through it.

    Now to the question at hand:

    A balanced mic line has three conductors. One crries the original audio, the second carries the audio 180-degrees out of phase with the original signal, and the third wire is the common. In a dynamic microphone, like the SM-58, this is done by using a transformer with a center-tap on the secondary side, the center-tap wire being the common or ground. Now, when outside noise gets introduced into the cable it will be in phase with itself on both audio conductors. At the other end of the line (i.e. mixing board), the audio on the second conductor is inverted 180-degrees, placing it back in phase with the original signal, and the two are combined. This causes any noise on this line to be 180-degrees out of phase with the same noise on the original audio line, and the noise cancels itself out. ;)
     
  7. Dillon

    Dillon Active Member

    Messages:
    111
    Likes Received:
    14
    Location:
    New York
    Allthingstheatre -- the idea of interference creating a positive charge in one conductor and a equal negative charge in the other defies the laws of physics. It's all or nothing to both. That's where the whole "flip one signal twice" idea comes in.

    Phil nailed it on the head. The second signal is taken 180° out of phase (inverted) inside the mic then sent down the line along with the positive, untouched signal. Any interference will be added equally to both lines and then canceled out when the signals reach the console.

    As far as shielding is concerned... I've never noticed a difference between foil and braided grounds. The braided stuff tends to hold up better under stress and tends to show up moreso in microphone cables with a nice heavy outer insulation. Foil shields are cheaper and tend to show up more in install-grade cable that won't experience the constant coiling/uncoiling/moving-related stresses that microphone cable sees. As far as I'm concerned, either works fine for keeping out the EMI demons.

    Good job, folks. Are people interested in more sound-based QOTD's? I tend to see a whole lot of lighting questions and very few for my audio friends.
     
    Last edited: Oct 13, 2008
  8. waynehoskins

    waynehoskins Active Member

    Messages:
    782
    Likes Received:
    49
    Location:
    Arlington, TX
    It has to do with the braided shield being electrically a bunch of little inductors in parallel. My dad had initially the same thought (it has holes!), but when he was doing some RFI research a couple of decades ago (in making a product he was working on RFI-robust) he found that the 95% braid works at radio frequencies, but the foil shield does little to nothing at radio frequencies.

    But again, that serves chiefly to keep RF out. The twisted pair accepts interference, but the intended signal on the pair is out-of-phase at any point on the line, and so when one side is re-inverted on the receiving end, you get only intended signal, with the noise being now cancelled out by inversion and adding.
     
  9. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

    Messages:
    4,445
    Likes Received:
    2,845
    Location:
    Las Vegas, NV, USA
    So just to be safe, I should put a screen-wire cover over my Aluminum Foil Deflector Beanie ?:twisted:

    [​IMG]
     
  10. David Ashton

    David Ashton Active Member

    Messages:
    829
    Likes Received:
    89
    Occupation:
    truck driver
    Location:
    perth W Australia
    [

    A balanced mic line has three conductors. One crries the original audio, the second carries the audio 180-degrees out of phase with the original signal, and the third wire is the common. In a dynamic microphone, like the SM-58, this is done by using a transformer with a center-tap on the secondary side, the center-tap wire being the common or ground. Now, when outside noise gets introduced into the cable it will be in phase with itself on both audio conductors. At the other end of the line (i.e. mixing board), the audio on the second conductor is inverted 180-degrees, placing it back in phase with the original signal, and the two are combined. This causes any noise on this line to be 180-degrees out of phase with the same noise on the original audio line, and the noise cancels itself out. ;)[/QUOTE]

    Wrong, very technical and very wrong, a SM 58 and most microphones do NOT have centre tapped transformers, if they did you could expect problems with phantom powering anyway, the screen is not the common, the two wires carry the signal, nothing else r.f. is not audible so it is a mute "{moot"} point as to whether or not the screen does much, my original answer to the question answers it in an accurate way which can be readily understood by a lay person.The only conductive use of the screen is as one side of phantom powering and an earth safety path neither of which were "nailed " by Phil.All the 180 degree out of phase cancellation waffle is meaningless gobbledygook.
     
    Last edited: Oct 14, 2008
  11. philhaney

    philhaney CBMod CB Mods

    Messages:
    721
    Likes Received:
    40
    Location:
    Laguna Beach, CA
    If you hook up a dual-trace oscilloscope to pins two and three of an SM58, and place an audio source in front of the mic, I think you'll see that the signals are opposite one another (or 180-degrees out of phase).

    [​IMG]

    I know you mean well, and are convinced that you are correct. I also will defend to the death (mine) your right to state you opinion as emphatically as you wish. But with a little research I believe you'll find thast you might be off just a tad on this one. ;)
     
    Last edited: Oct 16, 2008
  12. David Ashton

    David Ashton Active Member

    Messages:
    829
    Likes Received:
    89
    Occupation:
    truck driver
    Location:
    perth W Australia
    Right I 'm sure this is a wind-up but I'll keep playing a little longer, to prove that sm58 is centre tapped you show us a circuit diagram that has no centre tap and this is "proof" there is a centre tap, lets call it a phantom centre tap.And if you put a meter across pin 1 and 2 or 3 you will read no resistance and the transformer sits in a epoxy housing not touching the wall of the mic with 2 wires in and 2 wires out.If it was centre tapped you would have 48v pin1-2 and 48v pin 1-3 feeding into the low resistance of the transformer.
    Are you a serious?
     
  13. Schniapereli

    Schniapereli Active Member

    Messages:
    334
    Likes Received:
    4
    Location:
    Provo, Utah, United States
    This image shows the 180 degrees out of phase, and how they are combined in a way that eliminates the noise from the signal.
     

    Attached Files:

  14. David Ashton

    David Ashton Active Member

    Messages:
    829
    Likes Received:
    89
    Occupation:
    truck driver
    Location:
    perth W Australia
    Yes it does, if you look at each positive pulse of interference on the one line there is an identical negative pulse on the return line to cancel it out, which is exactly what I said, an explanation which is simple can be just as accurate as an over technical one and will make much more sense to the average reader.
    No reply yet about the centre tapped earth myth with the 58's, or any other mic for that matter.
     
  15. Chris15

    Chris15 CBMod CB Mods Premium Member

    Messages:
    4,140
    Likes Received:
    417
    Location:
    Sydney, Australia
    David, I know you know what you mean, but the way you are explaining balanced is misworded. If you had an identical negative pulse for each positive pulse, then you would get double the noise of an unbalanced system. What does happen is that you get the same noise pulses injected into both hot and cold lines.

    The receiver then subtracts cold from hot via a differential amplifier, so assuming that amplifier has reasonable common mode rejection, it doesn't care if it's subtracting -0.5 from 0.5 to get one or if it's subracting 9 from 10 to get 1 (within reason and it's operating range).

    But I wholeheartedly agree that anyone claiming a 58's transformer is a] centre tapped and b] that centre is grounded, is wrong. Refer to previous comments as to why it would be unneccessary and it would mean that as noted you'd regularly blow up phantom supplies...

    Philhaney, your red in amongst a David quote makes little to no sense.

    I have 2 more uses of pin 1 and the grounding conductors :mrgreen: No 1 is to cause hum loops and No 2 is to carry voltage so that mics can shock performers:twisted: Happened a week ago, was fixed by an iso transformer

    Now as to braid vs foil. Take a leaf out of the book of our broadcast and video brothers. Use both! Most quality coaxes will have at least one layer of each, if not more. I seem to see more AES cable using a foil shield than analog cable... Perhaps someone might care to explain why? (I have ideas, but I'll give someone a chance)
     
  16. David Ashton

    David Ashton Active Member

    Messages:
    829
    Likes Received:
    89
    Occupation:
    truck driver
    Location:
    perth W Australia
    Wow! a valid answer and criticism, point taken, so I have edited to clarify my meaning.I do like to keep answers short and simple and in basic language, instead of the jargonized hyperbole which is as meaningless to the reader as the writer.e.g. "out of phase by 180 degrees" instead of "opposite" Now it is possible that Dillons explanation is true but it is so confusingly written as to be indecipherable and incoherent to any ordinary reader.I wish Americans would follow the example of one of their greatest physicists, Richard Feynman who was a genius in the art of reducing a problem to basics, the direct opposite to the long winded pieces we so often see.
     
  17. Dillon

    Dillon Active Member

    Messages:
    111
    Likes Received:
    14
    Location:
    New York
    I know how hard terms like "inverted" and "canceled" can be sometimes.
     
  18. Dillon

    Dillon Active Member

    Messages:
    111
    Likes Received:
    14
    Location:
    New York
    Allthingstheatre... here's what you edited your initial response to be:
    This is still not quite right... the whole idea of using a balanced cable is that the SAME interference shows up on both signal lines, let's call them Hot and Cold. When Hot experiences a voltage increase, so does the Cold. When Hot drops voltage, so does the Cold. The whole idea stems from the idea that if twisted tightly and occupying essentially the same physical space, the two signal wires will experience the same electromagnetic field. The field will result in identical changes to the two signal wires. Only when the signal has reached the preamp will the Cold signal be inverted (flipped, phase shifted, reversed or whatever other term you prefer) and added to the Hot. The result of that sum will be nothing but the same pure, unmolested signal produced by the source.

    Schniapereli's image is spot on for what's going on in the system.

    See this page for a nice description of the physics going on in different types of balanced outputs.
     
  19. philhaney

    philhaney CBMod CB Mods

    Messages:
    721
    Likes Received:
    40
    Location:
    Laguna Beach, CA
    First things first: [user]Allthingstheatre[/user] I responded to your post, which followed the one that said I nailed it on the head, out of emotion and not logic. This was a mistake on my part, and I ask you to please forgive me.

    My bad. I haven't been inside an SM58. I was taught, by someone I respected as an authority on the subject, that a center-tapped transformer was how dynamic balanced microphones worked. When I saw the schematic I thought the center tap was in the cartridge and it grounded itself to pin one through the cartridge case.

    I still maintain the transformer in the schematic would prevent phantom power on pins two and three from reaching the cartridge (please correct me if I am wrong, but as I understand it the positive side of phantom power is applied to pins two and three, and the negative side is applied to pin one).

    Thanks for pointing that out. I apologize again, and have edited my post to remove it and (hopefully) make more sense.


    I'm not fond of long winded overly technical pieces either. On the other hand, I'm a very technical person. My explanation was originally four times larger. I condensed it down before posting it. "Houston, we're aware of the problem and are working to correct it."

    [user]Dillon[/user]'s explanation made perfect sense and was not confusing or indecipherable.

    "Out of phase by 180 degrees" was not meaningless to the writer or most readers. I knew exactly what I meant when I wrote it.

    If you think I have posted something in too technical a fashion, please (respectfully) tell me. I will edit my post to make it easier to understand (I will al least try).

    "Phil, your post about _____ may be a bit too technical for some readers (or 'may be confusing to some readers'). Would you simplify it please?"

    I can certainly respect a request or statement like that. On the other hand, I have a hard time with, "so confusingly written as to be indecipherable and incoherent," especially from someone who claims to want to make things simple...

    One more thing, next time please say something like, "I wish people would..." instead of, "I wish Americans would..." You have every right to disagree with what a person posts, but do you really have to insult all the citizens of the country the poster comes from?
     
  20. David Ashton

    David Ashton Active Member

    Messages:
    829
    Likes Received:
    89
    Occupation:
    truck driver
    Location:
    perth W Australia
    I accept these points as they clarify the situation rather than complicate it.
    I found the posts today completely understandable and helpful
    The criticism of over complex and confusing use of language by Americans was a life long complaint of Richard Feynman, an American, Nobel prize winning physicist whose books should be compulsory reading in all high schools.
    With regard to phantom powering it is only if you would have had a centre tap to earth that you would have current flow.
    We all have ideas which are slightly wrong at times and arguing them out forces us to reevaluate them and clarify them
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice