DIY Video Isolator Circuit: Eliminate Hum & Interference

Description

Contemporary homes frequently feature numerous electronic devices interconnected, particularly televisions. These devices are commonly linked to DVD players, digital video recorders, surround-sound receivers, and often personal computers. This extensive connectivity can introduce problems when earth loops develop within the shielding of video cables, leading to a noticeable hum and other undesirable interference. The surround-sound receiver incorporates a tuner that receives its signal from a centralized antenna distribution network.

The television is also connected to this system, and it is highly probable that the personal computer possesses a television card, again connected to the same system. Furthermore, numerous analogue connections exist between these devices, such as audio cables. The typical outcome of this arrangement is a hum in the audio installation, though in some instances, interference may also be observed on the television screen.

The earth loop issue can be resolved by implementing galvanic isolation for the video connections, for example, at the antenna inputs of the surround-sound receiver and the television.

Specialized adaptors or filters are available for this purpose, known as video ground loop isolators. Fortunately, such a filter can be easily constructed at home by an individual. There are two methods for achieving galvanic isolation in a television cable. The initial approach involves utilizing an isolating transformer with two distinct windings. The alternative is the use of two coupling capacitors in series with the cable. The latter method is considerably simpler to implement and generally functions adequately in practice. The easiest way to produce such a ‘filter’ is as an inline adapter, which can be plugged onto either end of a television aerial cable.

Diagram and snapshot:

The sole prerequisites are a male and female coaxial plug and two capacitors. These capacitors must be suitable for high-frequency applications, such as ceramic or MKT types. It is also advisable to select types rated for high voltages (400 V), as the voltages across these capacitors can exceed expectations (A personal computer 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 permit frequencies above approximately 50 MHz. Values of 1 nF or 2.2 nF are therefore sufficient. To construct the isolator, you should connect one capacitor between the two earth connections of the coaxial plugs and the other between the two signal connections. The mechanical construction must be robust enough such that the connections to the capacitors won’t break whenever the inline adapter is removed forcefully.

A good method to accomplish this is to create a cover from a piece of PVC piping for the central portion. Wrap aluminum foil around the exterior and connect it to one of the plugs, so that the internal elements are properly shielded from external interference. Ensure that the aluminum foil doesn’t make contact with the other plug, otherwise you lose the isolation. The vast majority of earth loops will disappear when these filters are connected to all utilized outputs of the central antenna distribution system where the signal enters the house.