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S/PDIF-how different than an IC?

I posted this on "Cables" too. One has a 75 Ohm spec and one does not. One comes in pairs and one comes as a single. Both typically have RCA's though some have BNC's. How does a cable producer change the spec on what outwardly appears to be an identical cable? If you specify from a cable producer that you want an S/PDIF how do you know they are not simply selling you one half of a pair of off-the-shelf IC's? And last but not least, how does 75 Ohm change the sound one would hear from a normal IC (when carrying digital signal rather than analogue of course)?
I know this much; a very well-respected engineer with a Ph.D. in EE who designs and produces DAC's of his own says that you are free to experiment with using a normal IC with his DAC's rather than a dedicated S/PDIF.
128x128fsonicsmith
zardozz's avatar
zardozz
11 posts
 
While they look similar because of the RCA connector (Phono Plug),it is not the same. SPDIF cables have a stereo digital connection, whereas RCA cables have a mono analog connection. There are no SPDIF to RCA cables, so you would need an RCA to S/PDIF converter to achieve that.
kijanki's avatar
kijanki
4,428 posts
 
If you don't match characteristic impedance between source, cable, and destination you might get reflections in the cable on impedance boundaries.  These reflections can modify (add to) transition edge.  Such modification, can create staircases on the signal transition.  It makes variation in exact moment of level recognition - a time jitter.  This time jitter translates to noise in frequency domain.
This characteristic impedance is pretty much SQRT(L/C).  Inductance and capacitance in the cable depend on geometry and dielectric.  Every cable has characteristic impedance.  The chances are, that that your IC have impedance that is very close especially to destination (DAC).  You often pay for high purity metals or very low absorption dielectric in analog IC, that are not important in S/Pdif cable.  In addition, reflections in the cable might miss the edge originating them (hence 1.5 min recommendation) and you might be OK even when it is mismatched.  You can experiment, but I would start with inexpensive 75 ohm coax.  Main factor here is impedance matching and inexpensive cable might be better than fancy one.
Also, recommendations/reviews posted make no sense, since it is system dependent.
fsonicsmith's avatar
fsonicsmith OP
959 posts
 
Thank you Kijanki! 
Zardozz-are you sure? What you state does not match the limited understanding I have. 
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