Digital cable length- SPDIF vs AES/EBU- 1.5 meter for either?

Al, thanks as always for the thorough explanation. I saw the references to the comparisons between 1.0m and 1.5m cable lengths but was left with the impression that longer lengths were not desirable, all things considered and being equal. To your point, I find it difficult to believe a single length could be optimal in all circumstances given all the other variables, including the cable material and construction techniques.

Interesting....testing 1m, 1.5m and 2m AES/EBU's that test our absolutely compliant with a 110-ohm spec, there is no perceivable (i.e. audible) difference in sound quality, pacing, image placement, etc...with AES/EBU cable lengths from 1 meter on up. I cannot speak for 0.5m or 0.75m AES/EBU cables or those that I've not tested (with music where it matters) that are not 110-ohm characteristic impedance but for those I have on a high-resolving system that shows me everything out of place, there does not seem to be any difference.

Contrast that with the fairy tale that is often propogated that for 50-ohm master clocking applications there is no difference between use of a proper 50-ohm and a proper 75-ohm cable. This could not be further from the truth in that case as evidenced by some testing that several of us plus a well-known cable manufacturer are going through.  In this case, it does very much matter, i.e. the cable must be of the correct impedance from end to end.  In that camp, several manufacturers (component as well as cable) are proliferating theory that as long as the cables lengths are not long enough for the cable to become a transmission line, having 50-ohm versus 75-ohm does not matter.  Our testing/listening has proven (we think) otherwise. The tests were done with cable anonymity (i.e. a label on them, nothing more) and no fore-knowledge of whether a cable was 50-ohm or 75-ohm. All cables were 1.5 meters in length.

Is it possible that the most important part (and I've read the Positive Feedback posting) of the equation is whether the cable, be it S/PDIF, AES/EBU, etc...is properly built to the letter of the spec. and tests out completely compliant at the target characteristic impedance rather than the length of the cable (1 meter and over)???

Assuming 25ns transition time (typical transport) and about 5ns/m signal speed (typical cable) reflection in 1.5m cable will come 15ns later (after of the beginning of transition) just missing threshold point around 12.5ns (deforming transition by adding to original signal above threshold point). Engineers tried to predict it making even special tools for that (Bergeron Diagrams) but different transports have different (unknown) slew rates while speed of signal in the cable is dielectric dependent. It is all guessing game. System with very short transitions (few ns) will be less sensitive to the first reflection but will produce much stronger reflections on smaller impedance boundaries. We can only say, that there is a chance that 1.5m or 2m cable might be better than 1m cable (exactly opposite to ICs or speaker cables)