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Unbalanced
An unbalanced signal has a ground signal which is the common for all other signals and the actual signal voltage is measured in reference to this ground signal.
So if there is stated that the voltage for a signal is +12 Vdc for a one and -12 Vdc for a zero, that means that those voltages are measured in reference to the ground signal.

NOTE: Normaly the ground signal is said to be 0 Vdc, but it is possible that the line carries a voltage e.g. 220 Vac. What happens is that the signal voltages will then be superimposed on the 220 Vac, so the actual measured voltage at the receiver side is still only the signal voltage.

Balanced
A balanced signal has no ground reference. The measured voltage is defined as the voltage difference between two copper lines, normaly refered to as line A and B. So for every signal that needs to be transfered you will have two leads.
The signals can be defined in two ways:

• The voltage difference between A and B can be an one when e.g. 12 volt and a zero when e.g. 6 volt.
• The voltage difference between A and B can be an one when A>B or a zero when B>A.