Ground Loops In Analog Video Systems PCB
Ground loop (electricity)
In an electrical system, a ground loop or earth loop is an equipment and wiring configuration in which there are multiple paths for electricity to flow to ground. The multiple paths form a loop which can pick up stray current through electromagnetic induction which results in unwanted current in a conductor connecting two points that are supposed to be at the same electric potential, often, but are actually at different potentials.
Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. They do not in themselves create an electric shock hazard;however, the inappropriate connections that cause a ground loop often result in poor electrical bonding, which is explicitly required by safety regulations in certain circumstances. In any case the voltage difference between the ground terminals of each item of equipment is small. A severe risk of electric shock occurs when equipment grounds are improperly removed in an attempt to cure the problems thought to be caused by ground loops.
Ground loops in analog video systems
In analog video, mains hum can be seen as hum bars (bands of slightly different brightness) scrolling vertically up the screen. These are frequently seen with video projectors where the display device has its case grounded via a 3-prong plug, and the other components have a floating ground connected to the CATV coax. In this situation the video cable is grounded at the projector end to the home electrical system, and at the other end to the cable TV's ground, inducing a current through the cable which distorts the picture. This problem can not be solved by a simple isolating transformer in the video feed, as the video signal has a net DC component, which varies. The isolation must be put in the CATV RF feed instead. The internal design of the CATV box should have provided for this.
Ground loop issues with television coaxial cable can affect any connected audio devices such as a receiver. Even if all of the audio and video equipment in, for example, a home theatre system is plugged into the same power outlet, and thus all share the same ground, the coaxial cable entering the TV is sometimes grounded by the cable company to a different point than that of the house's electrical ground creating a ground loop, and causing undesirable mains hum in the system's speakers. Again, this problem is due entirely to incorrect design of the equipment.