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Introduction to decimation.

Decimation is the process of filtering and downsampling a signal to decrease its effective sampling rate, as illustrated in . The filtering is employed to prevent aliasing that might otherwiseresult from downsampling.

To be more specific, say that x c t x l t x b t where x l t is a lowpass component bandlimited to 1 2 M T Hz and x b t is a bandpass component with energy between 1 2 M T and 1 2 T Hz . If sampling x c t with interval T yields an unaliased discrete representation x m , then decimating x m by a factor M will yield y n , an unaliased M T -sampled representation of lowpass component x l t .

We offer the following justification of the previously described decimation procedure. From the sampling theorem, we have X ω 1 T k k X l ω 2 k T 1 T k k X b ω 2 k T

The bandpass component X b Ω is the removed by M -lowpass filtering, giving V ω 1 T k k X l ω 2 k T Finally, downsampling yields

Y ω 1 M T p 0 M 1 k k X l ω 2 p M 2 k T 1 M T p 0 M 1 k k X l ω 2 k M p M T 1 M T l l X l ω 2 l M T
which is clearly a M T -sampled version of x l t . A frequency-domain illustration for M 2 appears in .

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Source:  OpenStax, Digital signal processing (ohio state ee700). OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10144/1.8
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