Ground in audio/video cables

[Mark22]

What is the use of "ground" in SCART and RCA cables?

[davexnet]

It' s the shield, keeps the video and audio signal clean by screening out stray interference

[JVRaines]

It's also the zero voltage reference against which to measure the signal.

[jagabo]

You can't have a voltage without a reference -- ground.

[johnmeyer]

Actually, there are two ways to send signals over a wire: ground-referenced, and differential. I've never had a SCART cable in my hand, so I don't know how they are constructed, but an RCA cable uses a signal wire in the center of an insulator, with the insulator and center conductor surrounded by a wire mesh. That mesh not only provides the ground return signal, but it also provides an electrical shield which is used to block electronic noise from being induced into that center conductor via electromagnetic induction.

This works OK for short runs, but even with the shield, some noise gets through. Also, you end up with all sorts of issues when "ground" is not at the exact same level at both ends of the connection. This is very common when connecting two components that are plugged into the wall at different locations in the building, on different circuits. Because of the current flowing through the ground (neutral) wires in the building wiring, the voltage presented at the "ground" wire at the socket is not the same at each end of the connection. This is what creates ground loops.

The solution is differential wiring, such as used in "twisted pair" cables, like Cat 5 and Cat 6 Ethernet cables. These cables do not have any ground shield (although you can buy cables like this, but they aren't used much), and as a result, outside signals are free to penetrate into the conductors. The "secret" to why they actually end up with less noise than shielded wiring, and are much more immune to ground loops, is that each pair of wires that comprise each circuit are twisted together so that both wires receive exactly the same amount of induced noise. The more twists per foot, the less noise (higher category wiring has more twists). At the receiving end, you use a differential amplifier that is not referenced to ground and which looks only at the difference in the signals between each wire. The "common mode" signal that is present in both is nothing but noise, and when the signal in one wire is subtracted from the other that noise disappears and you are left with just signal. No ground is used at all, and is actually deliberately left out of the design.

Back when analog signals were all we had, differential amplifiers were relatively expensive and therefore most consumer formats used the simpler shielded cable approach in order to save cost. However, differential cables are all you find on high-end equipment. For instance, professionals and broadcasters use XLR for audio connections, not RCA, cables.

XLR is a differential-based connection scheme.

So, ground is not needed and not used in professional equipment.

Much of my video equipment is "prosumer" and it still uses RCA connections for audio. However, I have some nifty boxes that can convert analog audio and video from shielded (RCA) ground-based signal to differential, and can pass that analog signal over Cat 5 cable. I often set up my camera 100+ feet away from the soundboard in an auditorium and then run Cat 5 between my camera and the sound board. Not only is Cat 5 a lot easier to snake around, and put under gaffer tape in an auditorium, but I get zero noise (especially AC hum), even with really long runs, and even with big AC dimmers nearby that are used for stage lighting.

Just to be clear, Cat 5, when used in this manner, is not carrying digital signals, but is carrying analog audio or video signals, identical to what originated in the RCA cable, prior to the conversion to differential.