I know that a mono jack has two poles:
So I always assumed that the cables for a mono jack would have two wires. But recently I have been told that if I ever were to make my own cable, I should use a shielded single wire cable and connect ground to the shielding.
Would that even work and is that a good idea?
Doesn't this destroy the whole purpose of the shielding?
You might even get a signal through with just a single-core cable, no screen, where a cable connects two items of mains-powered equipment (and are in a country civilised enough to provide an earth connection on all power outlets.) Like a telegraph system - there was only one wire, the return was the Earth.
But we often use single-core plus shield. The shield will be grounded. It acts both as signal return and as protection from RF interference.
Where signal levels are low, like a microphone cable, we DO use a 'balanced' connection, two cores plus a seperate screen. In electrically noisy environments such as a TV studio (though they're a lot less noisy since the demise of crt monitors) Line signals are routinely balanced as well. Probably unnecessary in a home studio, though the option is there on much semi-pro equipment. You need a 3-contact connector of course, for the 3 electrical paths. Typically a TRS jack or an XLR-3.
It depends on what type of wire you are using. If you run your signal and neutral (ground) through two separate wires, then it is a good idea to also have a shield around them that is connected to ground. Often the wire comes in "twisted pair" to also help with noise reduction.
You may also use Coaxial cable for a guitar cable. In this case the second wire is the shield, being wrapped around a core wire. Coaxial tends to be used more often for instrument cables, as it is less expensive due to using less copper and one less wire.
Shielded wire is usually used inside instruments because it is more flexible and smaller, not having a plastic core around a center wire like coaxial does.
The shielding in both cases is to intercept electrical radio frequency signals that the wires will pick up (connected wire acts like an antenna). The RF interference is sent to ground and isn't picked up by your amplifier.
Entire books have been written on the subject of shielding and grounding. The unfortunate fact is that using the shield as the signal return (ground) path is a terrible idea. It greatly compromises the shield's effectiveness, though for historical reasons essentially all guitar cords work this way.
In the 1970's Gibson made a version of the Les Paul (the "Professional" IIRC) that had low-impedance, balanced pickups and wiring, with proper shielding. It had a beautifully clear and hum-free sound. AFAIK it was a flop on the market.
