Why do digital coax cables matter?

There are differences to me. In a word, it may come down to jitter. Some cable designs adversly influence jitter more than others and all the physical elements of the cable design are in play. There are at least 3 kinds of jitter induction: optical, electrical and mechanical. Coax cables may affect both mechanical and electrical jitter. Insulation (less is more) may also influence the sound (openness) as well. Lots of people prefer silver in this application (I use copper and am very satisfied with it) and it may be due to the higher frequencies than with analog, but I'm leaving those with more of an ee background to expound on that. My cables are not shielded as I believe this also constrains the openness a bit (at least with analog IC's, IME) and I believe should ordinarily be avoided unless you have a discernable problem with interference. But, Mlsstl is right - no matter which way you go with it, there will always be someone to tell you you're wrong. Regards.

Assuming the cable is properly constructed and terminated, then in the digital domain, it is "bits in, bits out" as far as I'm concerned... Or at least until blind or ABX testing proves it to me otherwise.

I know a guy that makes and designs cables. There are some reasons known why cables in general sound different but exact reasons are not always well understood - he conjectures it has quite a bit to do with the dielectric. But sound different they do as you will find if you listen to them. If you are afraid of being fooled do a simple blind test.

Thanks
Bill

I rest my case....

;-)

Are there any engineers or physicists in the posts above?

Why is is that everyone thinks they are an expert?

Well, I'm an engineer, and I used to manufacture excellent digital and analog cables, so here are the reasons:

1) Losses that slow the risetime of the signals on the cable - this causes the receiving component to detect the edges with less certainty resulting in more jitter

2) Dielectric Absorption - this is also called "soakage" and is analogous to a sponge absorbing water. The dielectrics absorb some of the charge and then it is not discharged at a constant rate. Some cables eliminate this effect by putting a DC charge on the cable with a battery. Others minimize the effect by using air dielectrics or air-filled teflon etc.. The effect is that the energy required in the signal to make a rising or falling edge is not the same for each edge because of the charge in the dielectric. The signal must overcome this charge and it cannot, so some edges are displaced in time, causing jitter.

3) Impedance mismatches - The nominal impedance of a S/PDIF coax cable should be 75 ohms, but this varies all over the map with different cables and the connectors on the ends also affect this. Impedance discontinuities cause reflections on the cable when the signal is launched into it. These reflections can bounce from end to end until they finally dissipate with the cable losses. If they happen to hit the receiving end when it is detecting the signal edge, the edge may be pushed in time, creating jitter.

4) Metallurgical defects in the conductors - Low-jitter S/PDIF signals can have risetimes in the 1nsec range. When signals this fast are launched into a cable, the conductor metallurgy affects the signal propagation down the cable. If there are a lot of faults in the crystal lattice of the metal conductors, this causes small reflections. They are like small impedance discontinuities. These reflections can appear at the receiver at the time it is detecting the edge and cause the edge to be displaced in time, causing jitter. You can look at TDR plots of this effect on real conductors here:

http://www.empiricalaudio.com/computer-audio/technical-papers/direct-immersion-lno2-study

5) Length of the cable - All S/PDIF coax cables are imperfect and therefore cause some level of reflections, which can result in jitter if the timing of these reflections is unfortunate. By making the cable at least a certain length, one can avoid the effects of these unavoidable reflections, thereby avoiding the added jitter. This has been proven in double-blind tests by the magazine UHF in Canada. Here is a white-paper on the effect:

http://www.positive-feedback.com/Issue14/spdif.htm

Steve N.
Empirical Audio