I was reading about the Twisted Pair Cables, and how they implement Balanced Line methodology to cancel out all the noise. All fine and good.
However, I just couldn't grasp this one simple doubt:
When we are twisting two wires (in a pair) close up, which are carrying equal but opposite signals, aren't they going to interfere with each other? Isn't this interference going to add noise?
I couldn't find the answer to this simple doubt anywhere. Perhaps, I am missing something very fundamental.
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Note: The original question appeared to refer to wires in multiple pairs. The OP has advised that he is asking about interference between the two wires within a single pair. The original subject line and text has been edited to reflect this.
Best Answer
Within the one pair the difference in voltage or current IS the signal. If used as a single pair carrying a signal the signal is the relative difference between the two conductors. IF the pair was used as two circuits with voltages relative to eg ground then you would get 'interference' but that is not how a twisted pair is properly used.
What you do get is series inductance and resistance and parallel capacitance (and possibly interwire leakage but that is usually negligible) which causes losses. This leads to increasing loss with frequency. This can be passively addressed with "loading coils" which flatten the frequency response across a desired range with a consequent high frequency cutoff and low pass filter action. This method was traditionally used with twisted pair telephone cables to get extended range at voice frequencies and a very sharp cutoff above the range of interest. From dim memory 3.2 kHz was considered to be the top of the speechband for residential circuits.