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Do you have a projector that has mysterious lines traveling up the picture?  We ran into this problem at an install this week and boy did we learn a lot about a symptom called ‘Ground Loop’.  Thanks to Hefner Labs for the excellent CGLI-2 Cable Ground Loop Isolator product.

Basically, when you bring your cable signal in from the outside of the house, quite often the signal is not grounded properly.  This will cause lines to rise up on your picture when run through a receiver that will simply repeat the ground interference.  It took us a while to single out the cable box as the perpetrator of the problem, but after unplugging and plugging many different sources, it was obvious where the problem was coming from.  Here is an excellent explanation from Wikipedia:

In video, ground loops can be seen as hum bars (bands of slightly different brightness) scrolling vertically up the screen. These are frequently seen with video projectors where the display device has its case grounded via a 3-prong plug, and the other components have a floating ground connected to the CATV coax. In this situation the video cable is grounded at the projector end to the home electrical system, and at the other end to the cable TV’s ground, inducing a current through the cable which distorts the picture. As with audio ground loops, this problem can be solved by placing an isolation transformer on the cable-TV coax. Alternatively, one can use a surge protector that includes coax protection. If the cable is routed through the same surge protector as the three-prong device, both will be regrounded to the surge protector.

Luckily, this problem is easily solved with a small adapter called a Ground Loop Isolator.  It filters out the ground noise directly from the signal coming from the cable and so you get a clean signal into your cable box.  Putting this small Ground Loop Isolator on our cable signal and then directly into the cable box fixed the problem!