Now, what I think you mean is that the copper in the cable picks up noise either from the device on the other end or from the surroundings and that has the possibility of affecting the DAC.
Even moderately well designed receiver should be able to galvanically isolate itself from outside sources of noise without having to resort to fiberoptics or wireless. It's just couple of dollars max for extra few components on the board.
We have a cluster with ~1000 nodes and all generations of Infiniband (DDR/QDR/EDR) equipment, and routing all these copper cables without damaging them is not easy.
The quality of digital tranmsission over copper cables is dependent on the size and clarity of "eye", which is the pattern which two sinusodial signals when run through and oscilloscope. Lower quality cables (HDMI/DP/Coax/etc.) has blurry and small eyes, which increases chances of artifacts and drives the error correction modules harder, while better cables are easier on the both sender and receiver, since they produce lower number of errors to correct.
Trivia: Amphenol / Gore has high speed digital interconnect cables called Eye Opener Plus [0], which also used some of the Infiniband cables that we use.
[0]: http://www.spectra-strip.com/Eye_Opener_Plus_Cable.cfm?Nav=E...
There is no such thing as a digital transmission. All signals are analog.
I'm just so tired of all the bullshit where it "Must be a 1 or a 0". That is patently not true. HDMI video, to name one example very relevant to this thread, has no error correction, and bit errors will indeed crop up as the "sparkling" artifacts GGGGGP reported.
We are running FPGAs (with NICs) and were getting lots of packet errors over the copper cable SFP cables. Turns out, we either needed to write out noise cleanup logic into the FPGA, or add in a noise cleanup module which would add an extra 50ns to packet processing to the FPGA. Simple solution was to convert to fiber and the noise and errors all went away.
I'd say if audiophiles truly want unmolested electricity, they ought to use optical fiber for as much of the run as possible.