The most common type of digital audio connection is coaxial digital. A digital coaxial connector looks like an RCA connector, but it transmits digital data instead of analog signals. The digital coaxial cable is thicker than a typical audio cable, however, because it uses the same type of shielded coaxial cable you use to connect your television to your cable box. Visibly, a digital coaxial cable looks like a coaxial cable with RCA connectors on each end (see Figure 1).
Figure 1 Digital coaxial cables.
Digital coax transmits signals electrically. The electrical signal pulses through the copper wire at the heart of the coaxial cable. It's shielded from interference by an aluminum foil wrap.
One of the first things you'll notice about digital coaxial cable versus traditional audio cable is the cost. Because you're dealing with a thicker, shielded cable, digital coax costs quite a bit more than what you're used to paying for a standard RCA cable. You may pay $10 or more for a six-foot digital coaxial cable, as compared to a buck or two for an RCA cable of similar length.
Because of the cost, you might be tempted to use a standard analog RCA audio cable to make a digital connection. Not a good idea. A traditional RCA cable isn't shielded, nor does it have the bandwidth that coaxial cable provides. A digital coaxial cable offers 75-ohm impedance and a wider frequency bandwidth than the thinner, unshielded RCA cable. If you do connect a standard RCA audio cable, you'll find that you start to lose the signal over long distances, due to the lack of shielding. That lack of signal typically manifests itself in the form of stuttering audio—you're not getting all the digital bits on a consistent basis. You may also notice interference from other electrical devices, including electric lights. The longer the cable run, the worse the signal loss; you may not notice it with short cables (three foot or so).
A single digital coaxial cable carries all available audio channels. You only need a single cable to carry right and left stereo signals, or the 5.1-channel surround sound signals.