When I first started to consider how to find sources for radio frequency interference (RFI) in audio systems, my natural instinct was to look for frequencies that were being transmitted on purpose. But it turned out that the most disruptive RFI sources were not designed or intended to transmit radio waves – they did it by accident! What was even worse, was the fact that most of the really bad sources of interference tended to be inside the physical system.
Anyone working in Broadcast, Recording, Live-Sound or Theatre, soon finds out that lighting control systems create a lot of wide band electrical noise and that the noise varies with the brightness of the lights. However, it is not immediately obvious that part of the noise is radiated (like radio waves) and part is coupled to the system wiring by electromagnetic induction. During the last 20 years of the 20th century, the advent of motor driven lighting equipment, power flying, stage machinery and computers, forced national standards authorities to do something about the problem. In Europe, this is called the Electro Magnetic Compatibility Directive (EMCD).
More recently, we need to be able to cope with the cell ‘phone.