In article , "mc" wrote:
Now replace that battery by an AC generator. You still need only two wires.
The sine wave generator acts like a battery that is switching back and forth
periodically.

OK, I understand that....

Audio equipment designates one of the two wires "ground" because that is the
one that is shared by many different inputs and outputs. "Ground" is not an
electrical phenomenon; it just means something has been chosen to be the
common reference point.

But if you're throwing away one of the wires from the audio signal & replacing
it by a wire connected to the ground (the metal rod in the earth), how can
that metal rod be generating the rest of the signal. In DC, you have (+) and
(-). It seems to me that it's like throwing away the (-) away and replacing it
by, I don't know (X). That makes me wonder where the other half of the signal
is coming from.