@audio-b-dog
"Well, it was a 50 ohm cable not made for digital" isn’t scientific enough. The digital signal did pass through it, and digits are digits, and the digits are double checked to be correct
Unless I am completely mistaken, your CD connects to your DAC using S/PDIF over copper wires. S/PDIF defines the physical connection and the data link layer. These are the lowest two layers of the seven-layer OSI network model, and the data link layer is the lowest layer of the IP network model.
There is no requirement for the data link layer to detect or correct data errors. That is typically done by higher level protocols operating at higher levels in the network stack. Your assertion that "the digits are double checked to be correct" is not true for S/PDIF, any more than it is for I2S.
As others have stated, all cables carry analogue signals, so there really is no such thing as a digital cable. The confusion arises because of the characteristic impedance required. S/PDIF specifies 75 Ohms whereas most RCA cables are nominally 50 Ohms. All that happens when impedances mismatch is that more of the signal gets reflected back into the cable at termination points, and less gets through. This makes it harder to determine when the signal has transitioned between on and off, or vice versa. In turn, that makes it harder to synchronise a clock over the cable.
It does not stop a digital signal being retrieved, which is what you observed. It does not mean that all the digits are received correctly, so the sound quality may well vary, which is what you report.
You really should read up a bit more on networking, and what the various layers do.