A typical XLR connector has enough pins to carry a stereo signal. So is this a good idea, or is there a better solution?
The XLR connector is one of the great successes of audio. Doubtless modern science and technology could find a better solution, but the XLR doesn't have many problems, and you can use equipment with XLR connectors anywhere.
If you look at a typical XLR connector, you will see three pins or sockets, depending on whether it is a male or female connector. So you might wonder whether you could possibly run a stereo signal along one cable. After all, it would halve the number of cables, wouldn't it?
Well you can do this, but it's not ideal. The reason why it is not ideal is that the XLR connector, also the cable to which it is attached, is designed to carry a balanced signal. This requires two conductors for the signal, plus the earth. Balanced signals reject interference better than unbalanced signals.
So if you did want to use an XLR cable for stereo, then you would have to run unbalanced signals. You can share the earth between the two channels with no problem. This arrangement actually works very well for headphones - better than jacks for this purpose in the studio.
So the recommendation is really not to use XLR connectors for stereo signals, unless it is for a special one-off purpose where the lack of balancing doesn't matter, and the convenience factor outweighs anything else.
In fact there are such things as 5-pin XLR connectors. The pins are always numbered and are connected as follows:
Of course you will also need a cable with four conductors and a screen. This being provided, a 5-pin XLR cable is perfect for stereo signals.