A filter is a signal-processing component that removes unwanted parts of an input signal such as noise, extracts useful parts of an input signal lying within a certain frequency range, or reshapes the frequency spectrum of the input signal. Analog filters operate on analog or continuous signals, while digital filters operate on digital samples of a signal.
An analog filter comprises electronic components like resistors, capacitors, and op amps. These filters are widely used in noise reduction, video signal enhancement, graphic equalizers, high-fidelity audio systems, and many other areas. Design techniques for analog filters are well known. At all stages of the filter circuit, the signal undergoing filtering is a voltage or current that is the direct analog of the physical quantity involved.
Digital filters are implemented in software as an algorithm. This algorithm can be executed on a digital processor, such as a general-purpose CPU like those found in PCs, or an embedded digital-signal processing unit. Digital-signal processors are popular largely due to their use as digital filters. Field-programmable gate arrays (FPGAs) also can be configured to act as digital filters. The availability of powerful PC platforms with equally powerful graphical user environments simplifies and expedites the design of all digital filter types.