How balanced signal transfer operates

It is two conductors that carry the signal.


Example:


The two conductors connected to the voice coil output of a microphone.


At any instant when electron move to one direction on one conductor, say towards the preamp the electrons on the other conductor move away.


We must not forget it is not electrons that move from microphone to preamp.


They just oscillate about a point with a motion mimicking the vibration of music. Like dancing. The electrons on the other conductor do exactly the opposite as a mirror.


It is the electromagnetic wave (the cause of movement) that moves from mic to preamp at the speed of light in the material of the wires.


Same as on the sea waves. The water oscillates back and forth. It is not water that travels, it is the wave, by definition the transfer of energy. The water is not transferred from sea to shore. It stays where it is.


Back to balanced...


Electrons repel each other. The ones vibrating repel the nearby ones to vibrate too at the signal of music. The next ones near by vibrate too and so on. Until the cause (the wave) arrives at the input of the microphone preamplifier.


Electrons arriving at the live pin (2) have a mirror motion from those arriving at pin (3) of the XLR connector. If the signal goes to an input transformer when electrons on one side of the primary winding go in, electrons from the other side exit. So it is like moving a towel back and forth with our hands while drying out hair. The balanced motion aids the action.


But if there is an external interference signal arriving somewhere along the cable, it will be pickup up equally by both conductors which are next to each other. Electrons will have almost exactly the same motion, so whatever motion they have at any instant they will collide when they enter the input transformer. In fact if you think about it is the cause or 2 waves that collide.


So interference is out while music is in.


For obvious reasons the two wires are twisted, so hugging each other they have the same experience of the environment.


The shield is an extra precaution. It hugs them both. Apparently it reflects electromagnetic waves arriving externally. In reality what happens is that somewhere near or far electrons may oscillate with an unwanted noise interference signal. Jealous electrons at the screen do the same. And the electromagnetic wave is reflected like a sliver coated mirror does. Whatever happens jealous electrons do not see what happens outside. So they are not inflicted by the interference. And concentrate romantically dancing in a balanced way to music.


Reference:


Feynman Lectures in Physics


BBC notes on how balanced twisted pair signal transmition operates (exact title not recalled, internal instructions)