Digital Filtering

In electronics, a digital filter is any electronic filter that works by performing digital math operations on an intermediate form of a signal. This is in contrast with older analog filters which work entirely in the analog realm and must rely on physical networks of electronic components (such as resistors, capacitors, transistors, etc.) to achieve a desired filtering effect.

Digital filters can achieve virtually any filtering effect that can be expressed as a mathematical algorithm. The two primary limitations of digital filters are their speed (the filter can't operate any faster than the computer at the heart of the filter), and their cost. However as the cost of integrated circuits have continued to drop over time, digital filters have become increasingly commonplace and are now an essential element of many everyday objects such as radios, cellphones, and stereo receivers.

Digital filter advantages
Digital filters can easily achieve performance metrics far beyond what is (even theoretically) possible with analog filters. It is not particularly difficult, for example, to create a 1000Hz low-pass filter which can achieve near-perfect transmission of a 999Hz input while entirely blocking a 1001Hz signal. Analog filters cannot split apart such closely spaced signals.

http://en.wikipedia.org/wiki/Digital_filters

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