Has this ever happened to you?
Your hi fi or computer audio setup works great... until you connect it to a tv or vcr that's also connected to cable.
Then you get an awful hum on every source -- even when the video equipment is turned off!
Try this:
Did the hum go away? Read on.
The problem is caused by a conflict between the cable company's legitimate ground and yours. It's also easily solved:
Get a couple of cheap 75-ohm to 300-ohm antenna transformers. Radio Shack has them for less than $3 each (US). I suggest one each #15-1253 and #15-1140.
Wire the two 300-ohm sides together, so you end up with a double transformer with 75-ohm connections on each side. (If you bought the part numbers suggested, this means just wiring the spade lugs up to the screw terminals. Takes about fifteen seconds with a butter knife, less with a screwdriver.)
Insert the assembly between the cable coming out of your wall, and your stereo system. Make sure the metal band or shielding around the cable or its connector doesn't touch any part of your hifi or video equipment. You can wrap it in tape if you want.
Hum will be gone.
If hum doesn't go away, chances are what you bought aren't really transformers but are autoformers: check with an ohmmeter to make sure there's absolutely no continuity between the either conductor on the 75-ohm side and either conductor on the 300-ohm side.
This is a low-cost, mostly unshielded solution. It'll work in a lot of situations, but in high rf fields may introduce ghosting between a radiated signal and a cable one. It also causes a very slight rf signal loss, which most tvs and vcrs probably won't have trouble with.
However, if the hum goes away but you can't live with ghosting and video noise: