As anyone who has lived with a bad landline connection or a weak signal from the cable box can tell you, signal degradation can really ruin your day.
Copper, which still provides the core of most wiring, can suffer significant signal degradation over distance. Coaxial cable -- used to transmit television, telephone and computer network signals -- comes packed with shielding to prevent signal bleed-off, but losses due to electrical resistance still weaken the signal over distance. Shielded audio cables include some protection from outside noise, but suffer the same resistance issues [source: National Instruments].
Luckily, we have fiber optics.
Fiber-optic cables offer the kind of low signal loss and high data rates that telephone and Internet users demand by transmitting information as light through thin plastic or glass wires. These cables have total internal reflectance, meaning that light bounces along inside them without escaping. Glass fibers can transmit such "lossless" signals over great distances without needing a power boost [sources: Encyclopaedia Britannica; Wood].
By the same token, optical audio delivers higher-quality sound than older copper-wire connectors by converting electrical signals to light and piping them through optical fiber.
The age of optical audio dawned in the 1980s, when Toshiba created Toslink, the first optical audio cable. The Japanese electronics company had developed its own compact disc player and was looking for a way to output the improved digital audio quality to speakers and headphones. Appropriately, Toshiba chose an optical solution for CDs, an optical storage medium [sources: Modern Home Theater; Toshiba].
In an optical audio setup, the digital electrical signal from the source -- say, a DVD player -- is converted into light by a device called a transmission module. In Toslink, this module consists of an LED, or light-emitting diode, and a drive circuit, usually sent through plastic fiber, whereas ST Fiber Optic uses glass fiber and a red laser light at a 680-nanometer-wavelength [sources: Modern Home Theater; Toshiba]. The signal then speeds along the optical cable to the destination device, usually a television or audio receiver, where a light reception module converts it back into a digital electrical signal. From there, the device transmits it to your speakers or headphones.
Because it sends only sound, an optical audio cable is usually used with a video-only cable, such as DVI (Digital Visual Interface) or S-video.
So, what's the catch? Well, optical cables tend to be somewhat brittle, and the plastic ones are not as lossless as their glass counterparts. The reconversion of light into an electrical signal has been known to introduce errors. More to the point, however, some argue that HDMI (see sidebar) has rendered the format moot [source: Johnson].
Who is right? As with most arguments among audiophiles, the truth is in the ear of the listener.