3 Simple Rules to reduce Noise Caused by Earth Loop faults in Audio Visual systems

A familiar problem for audio visual installers is the level of noise induced into the audio path created by earth or ground loops especially when using touchscreens but also projectors.

The cable screen/shield should be connected to the chassis earth only at one point (one end of the cable). This should be at the host connection for the best noise immunity.

The cable screen/shield is often connected to the chassis earth at both ends of the cable connection. Since this makes an earth to earth connection between the computer and touchscreen, it can create an “earth loop” or “ground loop” problem. Since the earths are probably at different voltages, a large current (often several amps in magnitude) can flow between the earth points if the shield has a lower impedance to the current flow than the normal earth path. For correct shielding, all cable shields should be connected together, and then earthed at a single point (at the host’s chassis earth - normally a computer in AV systems)

The signal earth must not be connected to the chassis earth

A chassis earth is intended as a safety ground for power supplies, EMI filters, voltage spike protection circuits, AC neutral returns, and all manner of AC and DC driven devices. As a result, the chassis earth can carry relatively large voltages and currents. The signal earth is not the same earth as the cable screen/shield earth which connects to the chassis earth  - it's the earth used for the audio signal

Power cable and signal cables must be segregated

The most common error when installing AV cabling is running power cables (110v or 240v  AC) in the same trunking or conduit as the audio visual cables (or running them alongside in ceiling voids) which is guaranteed to cause interference. The practice of running them together is often done to reduce the cost of cable installation but ignores the interference risks.  And if the AC cable is powering relays, motors, or fluorescent lighting with high voltage spikes then additional noise above the mains "hum" will be induced into the adjacent audio and video cables.

Systems using VGA cables are the main culprit as the audio is in analogue format and easily affected by induced hum and other noise, HDMI cables are less likely to be affected by low frequency noise - simple inline isolators can be used with VGA cables to overcome or reduce the problem

D J Edis-Bates C.Eng MIET