Filters are familiar in everyday life.In the kitchen or garden they sort things by size.In electronics, filters almost always respond to the frequency of signals, passing low, high, or a chosen band of frequencies, and blocking others.Filters are needed because real signals contain a mixture of frequencies, not all of them useful.A radio aerial, foe example, picks up a multitude of stations, but a filter ensures that only one get thought.Other filters block everything above a given frequency, may to reduce "hiss" on old tapes.Low frequencies can be blocked too, perhaps to remove the "rumble" produced by an old record player.Filters can use either analogue or digital techniques.Analogue filters have been used since the 19th century.They rely on way inductors and capacitors respond to different frequencies.Digital filters use fast microprocessors to transform the sign marhematically into one with the unwanted frequencies.