Why do digital coax cables matter?

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.....

Steve, What you describe is general quality of the cable and not performance of the cable in particular system. Characteristic impedance different than 75 ohm can be very good, as Al mentioned, if it is better match for given system. Same for slowing down the edges. Uncertainty of threshold is not caused by long transitions but by the noise. Long transitions make it only more susceptible to noise induced jitter. With very little noise present longer edges might reduce impedance mismatch caused reflections, reducing jitter in effect. Making cable "at least certain length" is not precise since cable is not even considered transmission line when propagation time (one way) is shorter than 1/8 of transition time being about 0.6m for typical 25ns transitions (assuming 5ns/m).

Yes I'm also an EE with 34 years design engineering experience involved in Data Acquisition design for last 25 years - since you asked, otherwise I don't feel it would be appropriate for me to fortify my posts with it.

let me add the obvious:

take 2 audiophiles, 1 stereo system and 2 digital cables. place them in a room and have them compare the two cables.

there is a chance that they will agree on what they are hearing but they may disagree on which they prefer.

there is a chance that they will disagree on what they hear but agree on their preferences

there are two other obvious possibilities.

so what's the point.

there is no definitive answer to the question posed, because, perception and preference may differ among audiophiles.

in the empirical world, just listen and decide for yourself. it's not an original thought.

Many of us simply think that, "hey, digital is nothing more than ones and zeros, so just as long as those ones and zeros get to their destination without changing state or being corrupted, a perfect transmission will occur and no sound difference can possibly be heard, end of story."

As explained by a couple of responses above, there is one aspect of digital transmission that many audiophiles don't get. It is called [drum roll please]...timing.

Those ones and zeros must enter the DAC chip at exactly the right time to be converted into the proper analog waveform shape. If the timing is off (by mere picoseconds) the constructed waveform will be there, but not exactly the correct shape it should be. And that is where much of the sound differences of different cables come into play (and CD transports, etc., for that matter.)

I suspect that, for various reasons, all digital cables have slightly different timing characteristics. Whether your DAC chip and associated circuitry is compatible with a particular timing characteristic determines the sound outcome. The cable's timing, and whether the DAC "likes" it, is not dependant on the cost of the cable.

Stop thinking so simplistically, digital audio is not just about the ones and zeros - that is only part of the story.