Length of SPDIF cable

I know that Nordost recommends 1.5 m but I haven’t personally heard a difference between 1.5 and 1m. I’m skeptical about these type of things that are not noticeable to your ear. I suppose you could tell a difference if you compare based on measurements alone but not actually hearing a difference. Now, the balanced vs unbalanced issue with analog interconnects and length causing hum is another story.

@jasonbourne71 

Ok, that was a good one. 👍

Beginning of transition of digital signal creates reflection from the mismatched characteristic impedance at the end of the cable. This reflection comes back and adds to original transition distorting its shape and causing possible time shift of threshold point, resulting in jitter. Typical transition is likely 25ns while signal travels at about 5ns/m. With 1.5m cable signal will come back in 2x1.5mx5ns/m=15ns, just missing half of transition (likely threshold point) at 25ns/2=12.5ns.

It assumes 25ns transition and 5ns/m signal propagation while real life values can be way different. Transitions can vary 5-50ns while signal propagation depends on cable dielectric. I would feel safer with >2m cable.
It is also possible that there are no reflections at all (good impedance matching) or that DAC has good jitter suppression.

Cable becomes transmission line (possible reflections), when signal travel time one way is longer than about 1/8 of transition time. With 25ns it comes to about 3ns equivalent to 0.6m (at 5ns/m). This value includes internal wires and PCB traces at both ends. Even 1ft wouldn’t be very safe IMO, so I would go for mentioned 6-8". In this case shorter is better.