Dealing With Ground Loops
A ground loop occurs when two or more AV devices are connected to a common (safety) ground and to each other by signal cables.
Ground Loops and Video
Video represents an unbalanced interface with no inherent noise reduction. Because the shield of a video cable can be a return path for video signal and powerline ground current, and because the shield impedance is therefore shared by the signal and ground current, common impedance coupling is introduced.
In such situations, a voltage drop appears along the cable. Roughly half the voltage drop is added to the video signal, so the goal is to reduce that drop. Shorter cables and lower shield resistance can reduce coupling impedance. But so can putting a device in the signal path that has high common-mode impedance. Among the choices:
Common-mode chokes–aka hum suppressors, ground loop isolators, etc.–are commonly used to fight video hum bars, in part because they handle wide bandwidth (100 MHz to 1 GHz), so they work for HDTV (see "Ensure the Bandwidth You Need"). CM chokes introduce a high impedance into the ground loop to "choke" ground voltage differences and reduce hum.
Isolation transformers, which convert the video signal to an AC magnetic field then create a replica signal, are like the audio transformer for unbalanced audio interfaces. Loop current is reduced to almost nothing and hum is controlled even in situations where ground voltages differ greatly and/or you require long cable runs. One drawback: limited bandwidth (10 Hz to 10 MHz), meaning they're best suited to Composite or S-video.
Isolation amplifiers require power, unlike CM chokes and isolation transformers. This means they can have multiple outputs and adjustable gain, but you need to ensure they support the bandwidth you require for your video application.
Deciding which device to use depends on bandwidth required, the ground voltage difference in the loop, and the length and type of cable. Ground noise rejection decreases as cable length increases, and some voltage differences are too much for the device you're considering to handle.
Ensure the Bandwidth You Need
When choosing a device to put in a video signal path in order to eliminate ground loops, make sure your choice supports the video bandwidth you require.
Source: adapted in part from "understanding, finding, and eliminating av ground loops," a presentation by bill whitlock, president of jensen transformers (www.jensen-transformers.com), fellow of the audio engineering society, and senior member of the institute of electrical and electronic engineers.
A clamp-on ammeter can be a quick way to locate ground loop currents. It's able to measure low currents at 1 mA or better. Just clamp the meter around a system's cables at various locations to figure out the path and extent of ground loop current. A good one can be had for around $250.
Words of Caution
Never use a three-to-two-prong AC plug adapter (commonly known as a ground lifter) to try and remedy a ground loop. These simple devices are designed to provide a safety ground where three-prong plugs connect to two-prong outlets. By code, it's illegal to introduce such a lifter into a system and could make you liable for damages caused, for instance, if a fault occurs at a lifted component, sending current back to a properly grounded device over a signal cable.
Also, never, ever try to solve a ground loop problem by removing the grounding pin from a cord connecting the device to the outlet. Doing so risks electrocution.