how is noise cancelled in a balanced signal?

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travelin travis

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sorry, i know this has been explained before but i can't find it. i understand phase cancellation but i don't understand how only the noise is cancelled and not the whole signal, when running thru balanced gear. thanks.
 
in a balanced cable there are 2 hot leads instead of 1. 1st, the 2nd lead is phase reversed. then, noise is picked up by the cable. last, the 2nd lead is switched back into phase, cancelling the noise when mixed with the 1st lead.
 
thanks for the explanation but i don't understand. is the noise picked up on both hot wires? i understand how reversing the phase of one of the two wires will cancel out a signal. i don't understand how reversing the phase, then flipping it again will cancel noise. sorry, i just want to understand it.
 
this is how i understand phase cancellation: take a wave at a specific frequency, say 4khz. make a duplicate of that wave and reverse the phase. the two will cancel each other out because they are identical opposites of each other. for example: +3db + -3db = 0.
 
TravisinFlorida said:
...is the noise picked up on both hot wires?
Click to expand...
Yes for the differential circuit to "cancel" the noise it must appear on both wires - since the wires are in such close proximity (same cable) and a part of the same electrical circuit this is usually the case - in theory.
 
kylen said:
Yes for the differential circuit to "cancel" the noise it must appear on both wires - since the wires are in such close proximity (same cable) and a part of the same electrical circuit this is usually the case - in theory.
Click to expand...

it makes sense that the noise would have to be on both signals (wires) in order to cancel each other out. i don't understand how the entire signal is'nt cancelled out though.
 
If the polarity of one of the hot leads is reversed, then noise picked up is equal on both leads with respect to polarity. When the polarity is again reversed, the polarity of the noise is reversed along with the signal. As such, the noise on the two leads are of identical noise that are now directly opposite in polarity, and as with any identical signals that are opposite in polarity, they will cancel one another out. The entire signal isn't cancelled because its polarity is reversed twice, bringing it back to its original form.
 
but why is only the noise cancelled and not the rest of the signal? is the noise separated from the signal some how at some point? i don't get it.
 
FALKEN said:
in a balanced cable there are 2 hot leads instead of 1. 1st, the 2nd lead is phase reversed. then, noise is picked up by the cable. last, the 2nd lead is switched back into phase, cancelling the noise when mixed with the 1st lead.
Click to expand...

ok, so you have two signals which are identical but polar opposites, due to one wire being reverse phased. noise is picked up on both wires along with the signal. *from this point, i see all signal being cancelled out.* then one of the reverse phased wires is reversed again. *from this point I see the signals being added and amplifying the sound.*
 
Last edited:
exactly
flip the phase of the two signals...you have two signals on separate leads at the same amplitude, but opposite polarity.
the noise is added to the signal on both sides evenly....and the noise on both sides are of equal polarity.
when you flip the one lead back to + with the balancing circuit, the corresponding noise on that lead is made -....and hence canceling out the + noise when added to the other lead

but since now the GOOD signal is both +, the signal is summed.


so you got two leads:
1) +++++++++++++++++++++++++++++++++ Balancing Circuit
2) ------------------------------------------ Balancing Circuit

noise is represented as ( and is added to both sides proportionally

1) +(+(+(+(+(+(+(+(+(+(+(+(+(+(+(+(+(+(+(+ Balancing Circuit
2) -(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(-(- Balancing Circuit

now the noise is combined and one of the leads' noise is changed to )
so you get

1) +()+()+()+()+()+()+()+()+()+()+()

() = cancellation

so you're left with just

++++++++++++++++++++++++++++++
 
bennychico11 said:
exactly
flip the phase of the two signals...you have two signals on separate leads at the same amplitude, but opposite polarity.
the noise is added to the signal on both sides evenly....and the noise on both sides are of equal polarity.
when you flip the one lead back to + with the balancing circuit, the corresponding noise on that lead is made -....and hence canceling out the + noise when added to the other lead

but since now the GOOD signal is both +, the signal is summed.
Click to expand...


that makes sense. the noise is + on one wire, - on wire and the signal is + on both wires. that right?
 
TravisinFlorida said:
that makes sense. the noise is + on one wire, - on wire and the signal is + on both wires. that right?
Click to expand...

well, close
the noise is + on the wire the entire time until the two leads are summed together with the balancing circuit. And the balancing circuit reverse polarity of one lead before it sums with the other lead...so now you have a + noise and a - noise
 
here, i made a better, but quick crude drawing:

the yellow is noise added proportionally to both sides....notice the reverse polarity switch on the bottom wave too, canceling the noise out.

sorry for the bad drawing
 

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bennychico11 said:
well, close
the noise is + on the wire the entire time until the two leads are summed together with the balancing circuit. And the balancing circuit reverse polarity of one lead before it sums with the other lead...so now you have a + noise and a - noise
Click to expand...

so, coming out of the balancing circuit, the noise on each signal is opposite each other, + on one wire and - on the other.
 
TravisinFlorida said:
so, coming out of the balancing circuit, the noise on each signal is opposite each other, + on one wire and - on the other.
Click to expand...

well, remember the noise is not split when it comes out of the balancing circuit. it may be split to +/- for a quick second after it's reveresed...but then it's summed back together again (canceling the noise and aplifying the good signal) before continuing through your mixer. so now you have only ONE trace carrying the signal.
 
i think i get it now. thanks for the excellent explanation.
 
my pleasure

i just think it's an ingenious invention....whoever made it.
 
yes it is. any idea how the balancing circuitry works?
 
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