[mythtv-users] splitting and grounding coax?

David Brodbeck gull at gull.us
Fri Mar 30 23:13:22 UTC 2007

Shane Hickey wrote:
> When I originally cabled it this way, it worked fine, but when I added an external USB hard drive to the setup, I got crazy hum out of my stereo receiver.  I'm sure I created a ground loop somehow.  So, I'm wondering what people use to eliminate that problem?

You probably have the same problem I had at one of my apartments.  Your
TV cable and your electrical power system are not grounded at the same
point.  They're at slightly different voltages, which means whenever you
connect the two together current will flow.  If that current flows
through the shield of an audio cable, it will create hum.  If it flows
through the shield of a video cable, it will create "hum bars," darker
areas that slowly walk their way up or down the picture.

You need to break the loop.  Sometimes sketching out the connections
between various components makes it easier to see where the loop is
happening.  Unplugging stuff and seeing what makes the hum go away is a
useful troubleshooting technique.  In my case it was the audio cable
between my VCR (which was grounded through the TV cable) and my computer
(which was grounded through its power supply.)  I used an audio ground
isolator to break the loop.

If removing the USB drive solves the problem, and it has a three-prong
plug, solving the problem *might* be as easy as using a three- to
two-prong "cheater" adapter to break the power ground path for the USB
drive.  Do *not* connect the ground tab on the adapter to anything. 
(This may be against your local electrical code.  The chances of getting
caught at it are exceedingly small, though. ;) )

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