Description
These days many more audio-visual devices in the home are connected
together. This is especially the case with the TV, which may be
connected to a DVD player, a hard disk recorder, a surround-sound
receiver and often a PC as well. This often creates a problem when earth
loops are created in the shielding of the video cables, which may cause
hum and other interference. The surround-sound receiver contains a
tuner that takes its signal from a central aerial distribution system.
The TV is also connected to this and it’s highly likely that the PC
has a TV-card, which again is connected to the same system. On top of
this, there are many analogue connections between these devices, such as
audio cables. The usual result of this is that there will be a hum in
the audio installation, but in some cases you may also see interference
on the TV screen.
The ground loop problem can be overcome by galvanically isolating
the video connections, for example at the aerial inputs of the
surround-sound receiver and the TV.
Special adaptors or filters are sold for this purpose, known as video
ground loop isolators. Good news: such a filter can also be easily made
at home by yourself. There are two ways in which you can create
galvanic isolation in a TV cable. The first is to use an isolating
transformer with two separate windings. The other is to use two coupling
capacitors in series with the cable. The latter method is easily the
simplest to implement and generally works well enough in practice. The
simplest way to produce such a ‘filter’ is as an in-line adapter, so you
can just plug it onto either end of a TV aerial cable.
Diagram and snapshoot:
The only requirements are a male and female coax plug and two
capacitors. The latter have to be suitable for high-frequency
applications, such as ceramic or MKT types. It is furthermore advisable
to choose types rated for high voltages (400 V), since the voltages
across these capacitors can be higher than you might expect (A PC that
isn’t connected to the mains Earth can have a voltage as high as 115 V
(but at a very low, safe current), caused by the filter capacitors in
its power supply.
These capacitors don’t need to be high value ones, since they only
have to pass through frequencies above about 50 MHz. Values of 1 nF or
2.2 nF are therefore sufficient. To make the isolator you should connect
one capacitor between the two earth connections of the coax plugs and
the other between the two signal connections. The mechanical
construction has to be sturdy enough such that the connections to the
capacitors won’t break whenever the inline adapter is removed forcibly.
A good way to do this is to make a cover from a piece of PVC piping
for the central part. Wrap aluminium foil round the outside and connect
it to one of the plugs, so that the internal parts are properly shielded
from external interference. Make sure that the aluminium foil doesn’t
make contact with the other plug, otherwise you lose the isolation. The
majority of earth loops will disappear when you connect these filters to
all used outputs of the central aerial distribution system where the
signal enters the house.
