An EMI filter is a device which is used to suppress electromagnetic interference on a signal or power line. Electromagnetic interference is an unwanted electrical signal and is present in the environment or generated from other equipment.
There are two types of unwanted noise created by electromagnetic interference. Conducted which travels along the electrical conductors and radiated which travels through the air as radio waves or magnetic fields.
Most electronics contain an EMI filter as there are regulations in most countries that limit the amount of noise that can be emitted. Safety standards are required with EMI filters to reduce the risk of fires and electric shock.
Applications
Electronic musical instruments
Motherboards
USB hubs
Printers
Audio and video communication equipment