Tue Aug 11th 2009 23:16:00: Virgin Media Cable TV Box Causing Audio Humm

We recently got Virgin Media cable installed in the house to get faster internet access (the ADSL was very slow). As a side effect of the weird pricing structure we got TV for free. The installation of the cable set top box had a side effect as well - it caused a 50Hz mains hum on the computer audio (which is connected to the TV).

At first this was very strange as the audio on the cable box, Wii and DVD player were all fine and it was only the computer that was affected. Eventually I realised that if you disconnected the coax cable from the cable box, the hum on the computer went away.

After asking around at work I was told it was probably a ground loop caused by no isolation on the cable. As the Wii etc are all doubly insulated, it didn''t affect them, however the computer power supply is referenced to ground and is therefore affected.

On inspection of the coax splitter this was indeed correct. I then remembered that I still had the old splitter from an (unsightly) NTL installation that I'd removed when I moved into the house. That splitter was isolated, so I swapped them over and the problem went away.

Bit of a shame that Virgin Media are using cheapo (but smaller) splitters these days :-(. Apparently if you have this problem some small capacitors (with a high impedance at low frequencies) in series with the cable will also solve this problem


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