IC vs. S/PDIF?


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
The 75 Ohm is the "characteristic impedance" and pretty much all video gear is 75 Ohms.   When the impedance of the output source and the termination source matches the characteristic impedance of the cable, there will be less overshoot and ringing of the video waveform.   The system is "properly terminated".

This means the output has a series 75 Ohm resistor and the source has a 75 Ohm resistor across the input.   This cuts the signal level in half, so one is tossing 6 dB of signal away but in return gains a purely resistive network.   The capacitance of the 75 Ohm cable impedance is rendered null and the transmission line is purely resistive.  That is wonderful stuff for RF work but for audio, tossing 6 dB is a big deal since it requires 6 dB of gain to make up for it and the signal to noise ratio degrades by 6 dB.  That plus the fact most amplifiers driving an IC can handle the little bit of capacitance without much hassle.  (If you are wondering why IC's sound different, you are on the right path here.)

Look at it this way, if you had a long trough with a water supply at one end and a vertical flat wall at the other, when you lift the door at the water end, water will flow through the trough and then hit the vertical wall at the other end.   This causes waves of reflections in the water.   If the water hitting the wall caused no reflections, then one could say the trough, source, and termination were matched in impedance.

As for knowing which IC is a real 75 Ohm and just a fake one, there are test equipment that can test that but, really, for us with mortal bank accounts, there is no need.   Buy a cable from a reputable manufacturer who states it is 75 Ohms.  

Note that it is not really possible to achieve a solid 75 Ohm impedance from an RCA jack, since the dimensions are such that do not allow that impedance.   One can do it with a BNC connector, which is why Bryston and other companies use BNC's on their 75 Ohm outputs.  However, the error is pretty small and for most applications, it won't matter.  
I believe that when source and input impedance is to be the same, as with connections between digital devices, the term is 'match' when source is to be low and input device is to be much higher, as between analog devices the term is 'bridged'. The 75ohm coax and 110ohm XLR are for matched situations. Analog ICs are for bridged.