I’ve been building a humble living room A/V setup this summer. When I replaced my 20″ CRT TV with a 32″ HD LCD, my Tivo developed a quiet hum. I was pulling most of my TV off of an HD Antenna, and I was too busy to really look into the problem, so apart from jiggling some cables and scratching my head I let things go.
Today I got a HD Digital Cable box from Time Warner. When I hooked this up I got a “video hum”, ghost-like lines of color and brightness that slowly scroll up the screen of the TV. These distracting waves were more than I could overlook, so I started doing some research.
What I had was a ground loop problem. Wikipedia came to the rescue:
In video, ground loop 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 case 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 3-prong device, both will be regrounded to the surge protector.
I realized, the old TV had a two-prong power cord, while the new TV has a three-prong cord. The new TV was grounded separately from the incoming cable TV. I ran to Target and picked up a power strip with built in coax-protection (very much like this Power Sentry unit at Target) that provides a common ground to all of my devices. All of the visual hum is gone, and now I can be distracted by DiscoveryHD instead of video hum.