This is probably more than you wanted to know, but here you go. Balanced audio is a method of eliminating RF (Radio Frequency) noise from audio cables. This works by sending the exact same signal down two separate conductors (wires), with the polarity of one signal reversed. Both signals will pick up RF exactly the way they would if the line was unbalanced, but that is OK. It will be taken care of later. When the signal arrives at the input of the destination device, the first thing to happen is the signal is decoded. One signal (the original, unchanged signal) is left as it is. The other signal (the signal with its polarity reversed) has its polarity reversed (again) so it is once again in line with the original signal. These two signals are then combined. Both signals pickup the same RF interference in the cable. However, because one of the signals had its polarity reversed, its RF interference has a negative polarity, while the other signal has a positive polarity. The noise on the in the two signals cancels itself out, while the original signal reinforces itself.
There is no extra shielding, nor is there any unusual amount of insulation involved in balanced audio cables. In fact, as long as a cable is shield and insulated to a fairly basic level, adding more doesn't help at all. The most important factor in a balanced audio cable is the amount of copper carrying the actual signal. This is why the best mic cables (Mogami, etc.) double the number of conductors, using smaller gauge wire. Two conductors are soldered to each of the signal connections (hot and cold). This allows more copper to be encased in less insulation without affecting the diameter and stiffness of the cable. A stiff cable is more susceptible to kinks and other damage.
When dealing with 1/4" TRS phono plugs, the normal connection is:
TIP = hot (normal signal)
RING = cold (polarity reversed signal)
Sleeve = ground/shield
If you connect an unbalanced (single conductor) cable to a balanced jack, you end up connecting the cold leg of the signal to ground, which is fine, although you loose the benefits of balanced audio. You also will have 6dB less signal. This then requires more amplification to drive the destination hardware, which raises the noise floor.
The normal standard for XLR cables is:
Pin 1 = Ground
Pin 2 = Hot
Pin 3 = Cold
Unfortunately, not all manufacturers of balanced gear maintain this standard. This is particularly true amongst microphone manufactures. It is common to find gear with pins 2 and 3 reversed. Fortunately, this doesn?t matter for audio, as your ear can not tell if a sound has a negative or positive polarity, only if two versions of the same sound have a different polarity. (I work as a lighting designer, and I have run into a situation where reversing pin 2 and 3 is a major problem. There is one company, Martin, whose DMX protocol is out of polarity with the rest of the industry, and the computers which run these lights can not handle the polarity mismatch.)
It is also important to note that once the signal is inside of a device, it is not balanced. Balanced audio is ONLY for transmission between devices. Only the inputs and outputs of a device are balanced.
You will notice that I never refered to anything being "out of phase," or "reversing phase." Phase is not what is changed when a signal is balanced. Polarity is changed. The confussion comes from the fact that, when you look at a graph of a reversed polarity sine wave (see attachment), and compare it too the original signal, they look (after the first half cycle) exactly like a pair of sine waves which are 180 degrees out of phase. Natural sounds (and sine waves do not exist in nature) are far more complicated than sine waves. They change over time, so the 180 degree shift doesn't act (or look) exactly like a polarity shift. In a 180 degree phase shift, there will be slight diferences, although soniclly it will act very similar. With a polarity shift, every time there is a positive peek in the original signal, there is an exactly equal negative peek.
Too put it another way, a difference of phase is an issue of time, and a difference of polarity is an electrical issue. The two are not actually physically related, though they are soniclly related.
And one last phase nominclature confussion. There is a tendency (even among professionals) to call all phase incoherances "problems." This is not actually true. Some phase issues are problems, and sound quite bad. It is, unfortunatly, impossible to tell which sound bad and which sound good until you have tried them. With considerable amounts of experiance, you start to have some idea of which is which, but even then the best thing to is listen. It is always a good idea, if you are using many microphones, to try inverting the polarity to see which way sounds better.
And now I have (as usual) spoken at far too great of length, so I think I should shut the hell up.
Light
