The term "Balun" is short for "Balance-Unbalance". It's typically (but not always) a transformer where one side is connected to a signal and a ground (such as coaxial cable), and the other side has both conductors floating relative to ground, with our signal between them. For correct operation one always needs a Balun at each end of the wire.

Why needs the balance for signal transmission, please refer to the drawing:
Coaxial Cable: You can see that a balanced signal has two matched conductors on the coaxial cable. They are matched because their paths to ground are identical, both at DC and at RF frequencies. Any interference that couples into one conductor will cause it to move in voltage and to have the interference signal problem.

Unshielded Twisted Pair (UTP) wire:, There is a interference couples into the other conductor that same as coaxial cable too. Since both conductors move, it is important that the devices at the end of the wire have absolutely identical paths to ground, and that the circuit receive only the voltage difference between the conductors, while ignoring their "Common voltage" to ground. The ability to reject this common signal (Interference), while receiving the "differential" signal is the basis of any Balun. All of these circuits perform this function, but always in a less than perfect way.

There is a measurement for how good the balance is. The method is called Common-Mode Rejection Ratio (CMRR). The measure process as below:

Step 1: On the balanced side of the Balun, one puts a zero amplitude differential signal on the two wires (they're shorted together), plus a large amplitude common signal (big voltage relative to ground).

Step 2: To measure the small voltage on the unbalanced (coaxial cable) side. Under ideally balanced conditions, the coax voltage should be zero. To the extent that there's imbalance, a small error voltage will appear. We define CMRR as the ratio of this error signal (Vdm) to the amplitude of the common (interfering) signal (Vcm):Vdm / Vcm

Step 3: After the Step 1 and 2 process and then we can start to measure the Vdm / Vcm data. We need to convert the Vdm / Vcm signal to decibels to make it more manageable, since this number is usually very small.

The closer we come to perfect balance, the better we can reject interference. A good balance circuit is tricky, and is one difference between I-View and the other competitors. I-View's Balun has 60 dB of CMRR over the range of 15KHz to 5MHz and the other competitors' Baluns have around 30 dB or less of CMRR.So our Baluns are good enough to avoid the interference when there are there are ringing telephones, data such as RS-422,24VAC, RF, or other video signals in the wire bundle.