The Digital filter reference article from the English Wikipedia on 24-Apr-2004
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Digital filter

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In electronics, a digital filter is an electronic filter (usually linear), in discrete time, that is normally implemented through digital electronic computation.

Note: The parameters of digital filters are generally more stable than the parameters of analog (continuous) filters, primarily because the components of electronic filter change behaviour with temperature. Digital filters can be applied as optimal estimators. Digital filters are either finite impulse response (FIR) or infinite impulse response (IIR), though there is a third hybrid class of filters known as truncated infinite impulse response (TIIR) filters, which show finite impulse responses despite being made from IIR components.

Source: from Federal Standard 1037C and from MIL-STD-188

See also: Butterworth filter, Chebyshev filter