Real physical signals rarely satisfy the conditions of the sampling theorem because often they are not band limited and noise is present. This lesson introduces you to a technique that is used in every practical sampling system. An analog or continuous-time low pass filter is applied to the signal before sampling. This insures the signal to be sampled is indeed band limited. Realistic analog filters have frequency responses that transition gradually from pass band to stop band. You will also explore the impact of this transition band on the sampling frequency.
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