At home one might have a few components to hookup and the distances are relatively short, perhaps 10 or 15 feet at most and everything plugs into the same outlet. As the system becomes larger and more complex with longer cables, nasty issues creep in that are very difficult to resolve. Issues such as picking up hum or the local radio station. Further, too long cables can cause loss of high frequencies.

In a commercial environment there could be dozens and dozens of sources and cable runs of more than 100 feet. The commercial guys will not hesitate to run their cables "over the river and through the woods". At home we start to worry when the length approaches 20 feet or so for audio cables. This is because the commercial guys use "balanced" cables. If you look at the balanced cable ends, there are multiple pins, not just one pin that is typical of home setups.

There is a fierce amount of hostile (to audio signals) energy floating around out there. We use "shielded" cable in an attempt to protect the signal from being polluted by noise. The shield is a flexible metal sheath enclosing the signal wire(s). The theory is that the shield will intercept the noise energy and preventing it from reaching the enclosed conductor(s) carrying the signal. Unfortunately, the shield is not 100% effective and a certain amount of noise reaches the central conductor(s) anyway. Obviously, the longer the cable run and in very noisy environments, enough noise energy reaches the center conductor(s) to cause trouble.

Balanced cables use an interesting trick to cancel most of this noise. The reason for the multiple pins is that two versions of the signal are sent down the wires inside a shield. You can think of them as (+) and (-) versions. As noise enters the cable along the way it effects the (+) and (-) wires equally. At the receiving end the two versions are subtracted. From algebra, remember that (+1)-(-1) = 2. The (2) is our signal and the noise faces (+1)-(+1)= 0 in that subtraction and the noise is cancelled.

Now to the 600 Ohms. This is a little more difficult to explain. Note that this explanation is a little crude and I'm sweeping a few things under the table that should be included, but it will help you visualize what is happening and save me from writing a book, rather than a paragraph.

If you measure the resistance of a consumer audio input jack you might measure 10K, 20K, or even 100K Ohms. In pro audio gear you'll measure something on the order of 600 Ohms. In our shielded cable only a small amount of energy leaks through the shield. Unfortunately, if this small amount of energy becomes a significant fraction of our signal energy this will cause trouble. In consumer equipment a 100K input will not dissipate much of this energy and it mixes easily with our signal. In the 600 Ohm environment the small noise energy is easily dissipated.

Why don't we use balanced at home? Simple -- balanced is much more expensive than unbalanced. First, we have three times as many conductors inside the shield and three times as many pins in the connectors. Next, we need an inverter as part of the source to generate the (+) and (-) versions. Then, we need a subtracter at the receiving end to support our cancellation trick. Finally, we need to supply more signal power from the source so that the the signal does not disappear along with the noise at the 600 Ohm receiving end. Overall, the balanced approach is several times the cost of unbalanced -- multiplied by however many inputs and outputs the equipment supports.

There is more to this story than I've indicated above. Real pros are loath to incorporate unbalanced signals into their system. The pros will use special boxes to isolate and convert the unbalanced equipment to balanced before introducing this signal into their mixer. Managing noise becomes so complicated for large systems that there are engineers who make a good living by going to these installations and eliminating the noise.