Quality of digital cable from from source to DAC?

Like I said before, show me in operation some optical glass fiber RCA interconnects, and we can start to conflate the discussion of analog and digital transmission.

>> But my desire to have understanding of most things is strong, particularly when they effect both my happiness and my wallet. <<

Such understanding is where you will find the most divergence of opinions about cables. Manufacturers -- and their customers -- have many hypotheses about what makes certain cables special. (I use "hypothesis" in the scientific sense, to mean an unproven yet plausible idea -- what non-scientists often call a "theory." A theory in science is something supported by overwhelming evidence.) Most hypotheses about cables are not supported by unbiased science. Given that, it becomes a matter of what sounds better to each individual. For various reasons, it's easy to lose the distinction between slight differences and clear improvements.

There are strong incentives for manufacturers and dealers to steadily increase the prices of their products, as well as to offer numerous products at various price levels. Whether they do so in good faith is not for me to say.

mike_in_nc
>>But my desire to have understanding of most things is strong, particularly when they effect both my happiness and my wallet. <<

Such understanding is where you will find the most divergence of opinions about cables. Manufacturers -- and their customers -- have many hypotheses about what makes certain cables special. (I use "hypothesis" in the scientific sense, to mean an unproven yet plausible idea -- what non-scientists often call a "theory." A theory in science is something supported by overwhelming evidence.) Most hypotheses about cables are not supported by unbiased science. Given that, it becomes a matter of what sounds better to each individual. For various reasons, it’s easy to lose the distinction between slight differences and clear improvements.

>>>>I actually don’t agree with the major points of your paragraph. It all depends on who you talk to what theory is involved, what is fact, what is evidence and what breaks some sacred Law of Science. And to a large extent on whose ox is being gored. Yes, I know what you’re thinking - well, somebody must know. 🤗

These kinds of discussion will get nowhere. Anyway here are facts:
1. It’s not zero’s and one’s.
2. It’s electrical signals.
3. Once you connect A to B, A will affect B electrically regardless.

@ecolnago
All digital cables affect sound of the DAC differently.  In order to make sure DAC receives just right amount of data it has to synchronize internal D/A conversion rate with incoming data (S/Pidif) or receives data at different rate, but signals back to increase or reduce amount of incoming data (async USB, Ethernet).  In case of async USB or Ethernet D/A conversion clock is independent of incoming data timing, but it might be affected indirectly by noise injected by the cable into DAC. S/Pdif delivers data in real time and the average rate of this data adjusts rate of D/A conversion. S/Pdif cable can affect this rate two ways. Coax S/Pdif cable transitions are fast, hence susceptible to electrical reflections in the cable, that add to and modify shape of transitions affecting timing. DAC corrects most of it by adjusting to average of the frequency, but it is not perfect. In addition cable injects picked up electrical noise. Optical cable is different. It doesn't inject electrical noise and doesn't create ground loops (like coax cable can), but transitions are very slow. Slow transitions in presence of electrical noise can affect exact moment in time of level recognition (crossing threshold). It all comes to making clock of internal D/A conversion stable in time. Jittery conversion clock produces noise added to music.

In case of Ethernet or USB I would pay attention only to quality of shielding and run cable away from other cables. Same goes for coax S/Pdif, but matching characteristic impedance of the cable (to avoid reflections) is also very important. It is also desired to keep cable very short - less than foot (to avoid reflections) or longer than 1.5m (to avoid first reflection).  With optical cable, quality of the cable (clarity, etc) plays role, but the most important is to keep system electrical noise low, by using power conditioners for the source of the signal and the DAC.