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.
Ca345ea6 6d98 46ab a9fe db0c7a572262fsonicsmith
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.
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.
Thank you Kijanki! 
Zardozz-are you sure? What you state does not match the limited understanding I have. 
+1 @kijanki.

Blue Jeans Cable sells an SPDIF cable that seems to be well engineered and for 3-foot length is $19 plus freight. I have a couple of their SPDIF cables that I use when I need one. I had them made with yellow cabling, so I don’t mix them up with ordinary interconnects.
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Like others have said, you can technically use a standard IC for a digital S/PDIF connection.  It will work because they are both just a wire that is connected using RCA plugs.  However, the IC is likely not going to perform as well because of the shielding and impedance requirements of the S/PDIF specification. 

The specificaion states a recommended use of BNC connector due to the 75 ohm design.  Most manufacturers just use RCA because this was a trend started a long time ago.  The problem is that RCA has an actual impedance of about 30-35 ohms.  This means it is more likely to have signal reflection problems.  This occurs when the digital pulse hits the receiver on the DAC side and actually produces a pulse reflection (like a sound wave reflected off a wall).  That reflection travels back down the cable to the source and causes distortion and inaccuracies on additional transmitted pulses.  The audible result is loss of high frequency resolution or other audio inaccuracies. 

Also, I always recommend 2 meter cable minimum to reduce reflections, even on BNC cables.
I make my own cables. SPDIF cables are made with high bandwidth 75 ohm coaxial wire and special RCA plugs with crimp on pins to maintain that bandwidth or BNC connectors.  AES/EBU cables are high bandwidth balance wire with standard XLR connectors. Regular interconnects technically do not have the bandwidth to support the higher frequencies required to pass on dense digital information such as 192/24. Only a single cable is required as both channels are contained in the signal.
Thanks all. The DAC in question (see my OP) is the SW1X III Balanced. I started out with a dedicated S/PDIF that came with BNC's but also came with BNC-RCA adaptors. My Aurender W20 offers both BNC and RCA outs among others. So I used the BNC out from the Aurender. I had to use the adaptor for the DAC since it only has an RCA input. I choose not to ID the cable maker but the cable is highly regarded though modestly priced (under $400) but in this particular application sounded dull and lifeless. 
Then, out of frustration, I pulled a 2M Analysis Plus Silver Apex RCA IC from somewhere else in my system to try. You would never believe that a simple cable swap could make so much difference. It was like the sun came out on a crappy day. Every audio descriptor or so-called attribute came out from hiding and I have been smiling ear to ear for 48 hours since the swap. Now I have ordered a custom made 1.5M S/PDIF using the Silver Apex conductor and construction from Analysis Plus. When I inquired of AP, I was told the IC would approximate but not duplicate the performance of the S/PDIF version and that it would NOT simply be a single run of Silver Apex IC. It cost me just under $800 for the single cable. 
If anyone here is interested, I will report back. Auxinput's input (pun) has me thinking I should have ordered 2M. Oh well. 

Find a set of the old A/V composite cables and strip the yellow (video) cable from it. Works fine.
BillWojo
Now I have ordered a custom made 1.5M S/PDIF using the Silver Apex conductor and construction from Analysis Plus.

I have found that BNC cables with a BNC-to-RCA adapter actually perform better than a cable with only an RCA connector on the end.
You do not want to use an adaptor. Make a cable with a BNC on one side and an RCA on the other. RCAs used for an SPDIF cable are special. You can not use just any old RCA without killing bandwidth. You can use an SPDIF cable for an analog signal but not the other way around.
Tell me, what "special" RCA exists for an SPDIF cable? 
Canare makes an RCA connector intended for 75 ohm use.
Pasternack makes an RCA connector that is very similar to the Canare.  It has the internal brass pin that is inserted into the RCA connector itself.  This is the same idea of a BNC-to-RCA adapter.  I don't know about the Canare, but the Pasternack was really crappy.  It presented a harsh/bright sound.

The best BNC-to-RCA adapter I have tested is the Black Cat BNC-to-RCA adapter.  It has the best and most transparent resolution.  The Black Cat doesn't have super grip on the plug, but it sounds better than other adapters.  The Canare adapters are soft and distorted.  Probably the next best adapters are the gold-plated cheap Monoprice type adapters.  These are the exact same adapters that Nordost includes on their cables.  The gold-plated design presents a slightly warmer sound with slightly more rolled-off highs.
I see it was mentioned above. If you think you can hear the difference between the connectors you mention above you are painfully FOS
Also you obviously know nothing about cables and connectors. What you think they sound like is less than worthless. I suggest you take you trash to some other forum.