What's better, one conductor or two conductors for an RCA interconnect?

My understanding is it is better for electrostatic noise rejection but there may be circumstances where shielded twisted pair is better for magnetic.

Twisted pair exposes both wires evenly to electric and magnetic fields causing identical induced noise currents that cancel. It is extremely efficient, as long as the pitch of the twist is much smaller than wavelength of the offending electrical noise. Shield works great at higher frequencies where, in spite of being non-magnetic, shields by means of skin effect (noise currents flow on the outside - shield). Shielded twisted pair is the best combination.

As for grounding the shield - I would always ground it at the source and never at the receiver end only. The reason for this is that induced common mode electrical noise (identical in both wires) normally would cancel, but since both wires have impedance and slightly different distributed capacitance to shield, grounding it at the receiver creates two lowpass filters at two different frequencies converting common mode to normal mode signal. We’re talking of few picofarad difference, but at very high frequencies it can makes a difference. Also, intuitively I wouldn’t drain electrical noise from the shield into more sensitive side (input) device.

Learning as we go:  after some googling and contemplating there is one area that needs clearing up.

   The OP was enquiring about various RCA analogue cable designs I believe.  I'm sure there can be sound differences between them, single channel coax 75 ohm unbalanced or 110 ohm balanced (3 pin) are among them.  One connector, one channel.

   A coax with RCA connectors is "probably" an s/pdif digital only cable.  The single RCA connector has a stereo digital protocol and cannot be used as a single channel analogue cable.  

    As a side note:  My Oppo 203, used as a transport, uses one of these single s/pdif to connect a Bifrost 2 DAC.  Unfortunately the s/pdif-optical output is not working correctly (verified using an optical cable) A center vocal track is slewed to the right channel, while using an HDMI cable out of the Oppo dac is perfectly centered.  Any advice to correct this would be very appreciated as for 2 ch cd output the off centered output is noticeably more resolved with lower background noise. Bummer!

Kijanki ... You need to go back and review a bit. Twisted pair is good for magnetic and perhaps low frequency EMI. Coax is superior for RFI (with proper RF shield) and is in general better for electrostatic conduction as you have no field differential between the shield and the inner conductor. Even for magnetic coax can be very good.

@jumia 

@kijanki got it right in one. @deludedaudiophile , you are ignoring in your response that an RCA cable with a twisted pair and shield is in fact a coaxial connection. If the cable is very long this doesn't work so well, but in the shorter connections that are seen in household hookups it works just fine. For longer connections of course balanced is the way to go.

I am not ignoring that it is a coaxial cable because it is not a co-axial cable -- the axis of the conductors is not consistent in a twisted pair with shield. Much of the benefit of a coax is the symmetrical arrangement of the shield w.r.t. the center conductor.

A shield around a twisted pair provide electrostatic shielding, but the question was RCA, and the two conductors (as noted by Kijanki) do not see the same electrical impedance which will defeat the benefit of twisting (at least for magnetic I/F) which works where both conductors see the same impedance hence induced voltages cancel. There is probably a reason why oscilloscopes probes, which are single ended, use coaxial cable and not shielded twisted pair.