there are two kinds of low level audio connections. Unbalanced ones are found on home hi fi, cheaper PC soundcards etc.
they are fine for short runs of cable but on longer ones they pick up noise and interference from all sorts of places – the mains electricity, computers, taxicab radios etc.
Balanced audio connections have two audio wires as well as an earth and the electronics in the equipment is arranged so that all the noise picked up on the cable does not get mixed up with the sound.
DI unit – Wikipedia, the free encyclopedia
Balanced audio – Wikipedia, the free encyclopedia