There was a time when I use to use a Nordost RCA Valhalla cable as a digital cable and it seem to work fine at the time. At least until I was able to save up for a real digital cable.
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in theory a digital cable may have a higher nominal impedance than a conventional rca audio cable; the extent to which you can hear the difference is another matter. i've used 75ohm digital cables as a 2ch audio interconnect (and likewise used a rca cable as a digital interconnect) with no ill effect; i do understand that the differences are more perceptible in video applications.
Nominal impedance is so because of the specific operating frequency that the measurements are taken. Obviously, a cable designed to convey a 500MHz Manchester encoded digital signal will be capable of conveying audio spectrum just as well, but noise rejection is not a strong suit of unbalanced cables to begin with. Better quality cables are made with two inner conductors (a pair) of the same wire, and an outer foil shield surrounded by 100% braided shield with a drain wire. The two inner conductors go to the signal hot and signal ground respectively on each side, and the drain wire is either connected to signal ground on the source side, or (better) to chassis ground to protect from noise infiltration on both of the signal carrying conductors. The only way to improve on this is taking the leap to XLR connectors and fully balanced differential circuits, but that’s not always practical to do.
No not at all. 75 ohm is the characteristic impedance of a coax cable. Google it for a proper technical explanation.
Nope. Not even remotely correct. Coaxial cables come in more than one characteristic impedance, and that is “Z” for impedance, not resistance, which is a static value, regardless of the operating frequency range. There are many more different coaxial cable impedance specs as there are hat sizes. Two of the more common values are 50 ohm and 75 ohm. Impedance is what describes the dynamic, frequency-affected component due to capacitive and inductive reactance.
75 ohm digital cable is no worse than garden variety coaxial audio cable for conveying analog audio signals. The downfall of coaxial cable is that only the center conductor benefits from the shielding. You may want to step up to single pair analog audio or single pair digital audio shielded cable. In this application, the signal runs on one wire of the pair, the signal ground runs on the other wire of the pair, and both are shielded by a foil and braid with a drain wire connected to (ideally) chassis earth ground or (less desirably) signal ground, in each case only at one end (usually the pre-amp) of the cable.