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Signals and Systems Fall 2003 Lecture #13 21 October 2003 1. The Concept and Representation of Periodic Sampling of a CT Signal 2. Analysis of Sampling in the Frequency Domain 3. The Sampling Theorem the Nyquist Rate 4. In the Time Domain: Interpolation 5. Undersampling and Aliasing

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SAMPLING We live in a continuous-time world: most of the signals we encounter are CT signals, e.g. x(t). How do we convert them into DT signals x[n]? Sampling, taking snap shots of x(t) every T seconds. T –sampling period x[n] x(nT), n=..., -1, 0, 1, 2,... regularly spaced samples Applications and Examples Digital Processing of Signals Strobe Images in Newspapers Sampling Oscilloscope … How do we perform sampling?

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Why/When Would a Set of Samples Be Adequate? Observation: Lots of signals have the same samples By sampling we throw out lots of information –all values of x(t) between sampling points are lost. Key Question for Sampling: Under what conditions can we reconstructthe original CT signal x(t) from its samples?

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Impulse SamplingMultiplying x(t) by the sampling function

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Analysis of Sampling in the Frequency Domain Multiplication Property => =Sampling Frequency Important to note:

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Illustration of sampling in the frequency-domain for a band-limited (X(jω)=0 for |ω| > ω M ) signal

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Reconstruction of x(t) from sampled signals If there is no overlap between shifted spectra, a LPF can reproduce x(t) from xp(t)

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Suppose x(t) is bandlimited, so that X(jω)=0 for |ω| > ω M Then x(t) is uniquely determined by its samples {x(nT)} if where ω s = 2π/T ω s > 2ω M = The Nyquist rate

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Observations on Sampling (1) In practice, we obviously dont sample with impulses or implement ideal lowpass filters. One practical example:The Zero-Order Hold

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Observations (Continued) (2) Sampling is fundamentally a time varyingoperation, since we multiply x(t) with a time-varying function p(t). However, is the identity system (which is TI) for bandlimited x(t) satisfying the sampling theorem (ω s > 2ω M ). (3) What if ω s <= 2ω M ? Something different: more later.

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Time-Domain Interpretation of Reconstruction of Sampled SignalsBand-Limited Interpolation The lowpass filter interpolates the samples assuming x(t) containsno energy at frequencies >= ω c

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Graphic Illustration of Time-Domain Interpolation Original CT signal After Sampling After passing the LPF

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Interpolation Methods Bandlimited Interpolation Zero-Order Hold First-Order Hold Linear interpolation

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Undersampling and Aliasing When ω s 2ω M => Undersampling

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Undersampling and Aliasing (continued) X r (jω)X(jω) Distortion because of aliasing Higher frequencies of x(t) are folded back and take on the aliases of lower frequencies Note that at the sample times, x r (nT) = x(nT)

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A Simple Example X(t) = cos(w o t + Φ) Picturewould be Modified… Demo: Sampling and reconstruction of cosw o t

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