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

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.

I suspect that reason for oscilloscope to have plain coax is capacitance. Twisting wires reduce inductance but increases capacitance - important with scope’s high input impedance and very high measured frequencies. Unfortunately this coax arrangement creates errors. When you short probe and touch tested circuit with shorted leads it will show small amount of noise (in spite of being shorted). It is because current flows from the circuit thru the shield (finding return to GND) causing voltage drop, that shows on the input as signal - exactly what we try to avoid in interconnects by using two wires inside of the shield.

Because twisted pairs provide cancellation to all external fields (magnetic or electric) it is used everywhere - in all network cables, in all audio cables etc. It would be stupid not to. Typical twisted pair provide about 40dB rejection up to about 100kHz and still 20dB rejection at 700kHz. Above that shield becomes very effective by means of skin effect. Skin depth changes with frequency squared, being roughly 2mm at 1kHz (Cu or Al). For 100kHz it will be 0.2mm. There is some info on twisted pair effectiveness here:

So what happens if you Connect the ground to the connectors on both sides of the RCA cable? If one side of the ground is not connected to the other side within the cable wouldn't the impact of a ground wire only connected on one side become an antenna?

Isn't it Better to let the ground flow back into the component and let that take care getting rid of the ground activity.