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Digital Signal Processing
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Anti-aliasing Filters
In the general case, we do not know the frequency of any of the oscillations that might be present in the signal being measured. But, as was just shown, we do know those of half or more the sampling rate will produce aliases during acquisition. Therefore, to ensure that aliases are prevented, it is necessary to remove all components of the signal and noise with frequencies of half or more the sampling rate with a low-pass anti-aliasing filter. This is an analog circuit through which the input signal must pass on its way to the analog-to-digital converter. Of course, the filter not only eliminates any aliasing in the digital data, but also attenuates any true signals -- wanted or unwanted -- above the stopband of the filter. Thus, when the data is subsequently analyzed with digital signal processing, we need not worry about any false lower-frequency signals being left in the data by the analog-to-digital conversion process. 
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