It's always nice to be able to help your parents solve a problem even though you live 2000 miles away from them. Today, I did just that and the problem and solution deserve an explanation to help benefit others.
So you have this neat new audio system or TV and everything works great except that you hear a slight humming in the background, or you see very faint wide horizontal bars that appear to go up your TV screen. This behavior isn't normal. The most common cause for this problem is called a "ground loop." A ground loop (in lay-person's terms) is what happens when the electrical shielding around cables within a system connect at different points that allows an electrical current to begin flowing in the shield (generating noise). The purpose of the shield around the cables for your TV and audio equipment is to prevent stray electrical signals that are in the air (like from your cell phone or the motor in a vacuum cleaner) from interfering with the signals you actually want passing through the cable (like sound or picture information). Ground loops are difficult to diagnose and solve because it requires identifying all the grounding sources for the different shields and isolating each one to see which makes the hum go away.
In my parents' case, we isolated the offending path to be through the cable TV wire. My father looked at the wire and, realizing that I mentioned a grounding problem, noticed that the cable TV ground outside was improperly connected to a pipe that went nowhere. Our house has a history of stray electrical signals coming into the house through the ground lines, and so we theorized that these stray electrons found a nice neat pathway through my parents new HDTV and out the cable TV line.
The way to stop ground loops from happening is to get all the involved ground connections going to the same place. In most cases, this is done for you automatically since everything goes to the same power strip, and thus, the same ground connection in your home's electrical system through the electrical outlet. However, since the cable TV line was improperly grounded at the house, it was taking a pathway to some other home down the street, which was either the source of the stray signal or where it was trying to go.
So in this case, I suggested that my parents remove the stupid ground the cable TV company connected and connect that to the common electrical ground outside that my father installed years ago (and did a great job with). That caused the cable line to ground at the same point as all the other electrical appliances in the house. It also solved the problem with the hum coming from the TV, and the hum-bars going up the screen. Mission accomplished.
As I seem to be doing more and more lately, more information can be obtained from Wikipedia at this link to electrical ground loops. It pretty much says what I just said, but has more tech info as well as other informational links.