Optical cables provide high-speed links for supercomputer networks. They carry signals from cellphone towers to the telecommunications network. They connect continents through undersea cables that carry billions of bytes of data every second---so why not put the capabilities of fiber optic cable to use in your audio system?
The first and most obvious advantage to fiber optic cable is bandwidth. Bandwidth is a measure of how rapidly signals can change and still be detected without appreciable error. Because light operates at a much higher frequency than radio waves, the bandwidth of an optical system has an inherent advantage over a radio frequency (RF) system. Bandwidth is also a function of the quality and the length of the cable. Generally, glass fiber has a higher bandwidth than plastic, and the longer the cable, the smaller the bandwidth.
Cables that carry electronic signals through conductive wires are kind of like radio antennas---they can pick up radio frequency radiation put out by television sets, dimmer switches, and power cords. Optical fiber is immune to picking up unwanted frequencies. Since the optical cable transmits no electrical signal at all, the audio will not be contaminated by humming caused by ground loops created by the connection.
Every kind of cable suffers from loss---a signal with a given magnitude is put in at one end, and it has a smaller magnitude at the other end. However, data from Submarine Networks shows that all undersea cables recently commissioned are built from optical fiber instead of copper wire because fiber optics attenuate less. Having said that, the fiber used for home audio is of substantially lower quality than that used for undersea telecommunications, so there is a finite amount of loss, in the range of 0.2 dB/m.
The Audible Difference
So, with all those advantages, surely optical audio cable is better than the electronic RF coaxial alternative, right? Not so much. Sure, it's bandwidth is 40 MHz or better, but audio signals top out about 20 kHz, so the signals don't really use all that extra capacity. They're immune to electrical interference, but well-shielded RF coax cables are protected as well. And the signal attenuation of the best RF cables is about the same as optical audio cables. Certainly, optical audio cables can be a central component of a high-end audio system, but so can quality RF cables. In the end, it's a matter of personal preference.
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