Digital Audio Cables

Digital Audio Cables

The most important factor in determining accurate digital audio signal transmission is tight control over impedance. What is impedance? Inside a digital audio cable impedance is the amount of resistance to a current as it travels through a cable and is determined by the relationship between a cable's conductor and it's ground. What most consumers in the audio market do not understand is that variations in the spatial relationship between a cable's conductor and ground causes variations in impedance and, in turn, audio signal quality. This phenomenon is known as impedance matching, and a Wikipedia definition can be found here: http://en.wikipedia.org/wiki/Impedance_matching .
Variations in an audio cable's structure or in the way the cable is joined to its connectors cause reflection loss, a situation in which audio signals "bounce" back to your audio transmitting device. A Wikipedia definition of this phenomenon can be found here: http://en.wikipedia.org/wiki/Reflection_loss. Reflection loss degrades signal integrity and ultimately effects audio quality.

For example, if your audio transmitting equipment and your audio receiving equipment use a 75-ohm signal (as in S/PDIF or digital RCA audio transmission), your receiving equipment expects to receive an "accurate" 75-ohm signal. However, as your digital audio signal departs your transmitting device at 75-ohms it will be altered by the components of your audio cable in the following in order:

connector A --> connector joint A --> audio cable --> connector joint B --> connector B.

It isn't possible to perfectly preserve 75-ohm impedance, but the goal is to minimize changes to the impedance and your digital audio signal.

Following our example, solder-style RCA connectors (originally designed for analog audio use) cause an excessive amount of back reflection because they average around 25 ohms impedance. Each cable has two connectors, so two bottlenecks exist in any cables featuring solder-style or mass-production type RCA connectors. Also, the only way to solder an RCA to coaxial cable is to destroy the spatial relationship between the cable's conductor and shield-ground. This is done by twisting the cable's "surrounding" braid shield into a thick copper wire which is soldered to a single point on the side of the RCA connector. Structural changes such as this lead to a great amount of reflection loss and signal loss. Again, there are two solder joints per cable, so this negative effect is doubled. Also, low quality cable (even if expensive) frequently has great variations throughout the cable itself, and these variations lead to further reflection loss and signal loss.

As you can see below 75-ohm style RCA connectors retain the "equidistant" relationship between conductor and ground when crimped to standard coaxial cable. Again, retaining this equidistance is the key to preserving 75-ohm impedance by reducing variations between the conductor and ground as the signal moves uniformly through the cable and connectors, without bottlenecks.