wiring and diagram

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.

Video Isolator Circuit

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.

This circuit is a video line driver specifically intended for use with a single-ended power supply. As a matter of fact, the synchronised outputs of a line driver for composite-video signals go negative with respect to ground. In order to be able to process these negative signals in a circuit powered from a single-ended supply, it is necessary to AC-couple the input of the opamp as well as level-shift the signal in the positive direction.

The input is terminated into a 75 Ω resistor (R1). From here, the signal passes through AC-coupling capacitor C2 and is applied to potential divider R2-R3, which provides the necessary DC-offset. The shift into the positive direction amounts to +1.7 V, with the values shown in the schematic. To avoid any misunderstandings we should add that this value is fairly critical. Deviating from the values shown can lead to distortion in the complementary input stage of the opamp that has been used here, and this of course, has to be avoided.

Because we provided the circuit with its own voltage regulator circuit (IC2), just about any mains adapter will suffice for the power supply. The current consumption is less than 20 mA. The construction of the line driver using the accompanying printed circuit board layout is no more than a simple, routine job.

Parts List :

Resistors:
R1,R7 = 75Ω
R2...R4 = 4kΩ7
R5,R6 = 1kΩ

Capacitors:
C1,C4,C5,C7C10,C12 =
100nF
C2 = 47µF 16V radial
C3,C11 = 10µF 6 V radial
C6 = 220µF 6 V radial
C8 = 1000 µF 6V radial
C9 = 100µF 16V radial

Semiconductors:
D1 = 1N4001
IC1 = OPA353UA
IC2 = 78L05

Miscellaneous:
PC1-PC6 = PCB solder pin
Case, e.g., Hammond type
1590A