Why do digital coax cables matter?

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

Steve, thanks for your inputs. Do you feel that the following may also be significant contributors to sonic differences between S/PDIF interconnects, at least in some systems?

6)Differences in noise-induced jitter, due to ground loop effects and/or RFI/EMI pickup, both of which may be sensitive to cable differences.

7)Differences in radiated RFI, that may affect circuitry in the system that is not directly related to the S/PDIF interface.

Concerning your no. 3, impedance mismatches, and with respect specifically to the impedance match to the components that are being connected (as opposed to mismatches between cable and connector, or impedance discontinuities within the cable) I would add the thought that what is important is not how accurately the impedance of the cable and connectors match the 75 ohm standard, but how closely they correspond to the actual output impedance of the component driving the cable, and to the actual input impedance of the component that is at the receiving end. Everything else being equal, a cable that is less accurate relative to the 75 ohm standard may therefore outperform a more accurate cable in some systems, if it happens to be a closer match to the component impedances.

Finally, I would be interested in your take on what degree of correlation can generally be expected between cable performance and cable price, for S/PDIF interconnects, given the many variables and system dependencies that are involved in the effects that have been mentioned.

Regards,
-- Al

P.S: Re your first question, I am an EE with an extensive background in digital signal transmission (not for audio).

My brain tells me no two cables that are physically different conduct electricity (or light for optical) the exact same way. So there has to be differences to some degree. The question for me is then how much and are the differences significant enough to matter in practice?

I wonder about digital ICs in general in this regard more so than analog ones. No two analog ICs usually sound the same to me. But on the several occasions where I have compared different digital cables going into my DAC(s), if there was a difference, it was not enough for me to take clear notice or even care. I know that in theory different levels of jitter is the result and that jitter level matters. But does it really in practice? It's something I have not been able to discern with my own ears so far.

So I wonder.....