Sampling of Continuous-Time Signals. Reference chapter 4 in Oppenheim and Schafer.

Transcription

1 Sampling of Continuous-Time Signals Reference chapter 4 in Oppenheim and Schafer.

2 Periodic Sampling of Continuous Signals T = sampling period fs = sampling frequency when expressing frequencies in radians per second.

3 Mathematical Model for Periodic Sampling Impulse train modulator followed by conversion of impulse train to sequence.

4 Mathematical Model for Periodic Sampling

5 Mathematical Model for Periodic Sampling

6 Frequency-Domain Representation of Sampling

7 Figure 4.3 Frequency-domain representation of sampling in the time domain. (a) Spectrum of the original signal. (b) Fourier transform of the sampling function. (c) Fourier transform of the sampled signal with Ω s > 2Ω N. (d) Fourier transform of the sampled signal with Ω s < 2Ω N. aliasing (distortion)

8 Recovery of Signal

9 Figure 4.4 Exact recovery of a continuous-time signal from its samples using an ideal lowpass filter.

10 Aliasing Example with Cosine no aliasing reconstruction: with aliasing reconstruction: (taken on identity, alias, of lower frequency signal)

11 Sampling Theorem

12 General Case for Discrete-time Fourier Transform (DTFT)

13 General Case for Discrete-time Fourier Transform (DTFT)

14 Examples of ambiguity due to sampling.

15 Figure 4.6 (a) Continuous-time and (b) discrete-time Fourier transforms for sampled cosine signal with frequency Ω 0 = 4000π and sampling period T = 1/6000.

16 Examples of ambiguity due to sampling. First case with no aliasing. Second case with aliasing.

17 Reconstruction of a band limited signal from its samples.

18 Figure 4.7 (a) Block diagram of an ideal bandlimited signal reconstruction system. (b) Frequency response of an ideal reconstruction filter. (c) Impulse response of an ideal reconstruction filter.

19 Figure 4.8 Ideal bandlimited interpolation.

20 Figure 4.9 (a) Ideal bandlimited signal reconstruction. (b) Equivalent representation as an ideal D/C converter.

21 Discrete-Time Processing of Continuous-Time Signals

22 Discrete-Time Processing of Continuous-Time Signals

23 Discrete-Time LTI Processing of Continuous-Time Signals

24 Discrete-Time LTI Processing of Continuous-Time Signals

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26 Figure 4.11 (a) Frequency response of discrete-time system in Figure (b) Corresponding effective continuoustime frequency response for bandlimited inputs.

27 Figure 4.12 (a) Fourier transform of a bandlimited input signal. (b) Fourier transform of sampled input plotted as a function of continuous-time frequency Ω. (c) Fourier transform X (e jω ) of sequence of samples and frequency response H(e jω ) of discrete-time system plotted versus ω. (d) Fourier transform of output of discrete-time system. (e) Fourier transform of output of discrete-time system and frequency response of ideal reconstruction filter plotted versus Ω. (f) Fourier transform of output.

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30 Figure 4.13 (a) Frequency response of a continuous-time ideal bandlimited differentiator H c (jω) = jω, Ω < π/t. (b) Frequency response of a discrete-time filter to implement a continuous-time bandlimited differentiator.

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32 Figure 4.14 (a) Continuous-time LTI system. (b) Equivalent system for bandlimited inputs.

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34 Continuous-Time Processing of Discrete-Time Signals

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38 Figure 4.16 (a) Continuous-time processing of the discrete-time sequence (b) can produce a new sequence with a half-sample delay.

39 Moving-Average Example Figure 4.17 The moving-average system represented as a cascade of two systems.

40 M = 5

41 In this case works out to delay of 5/2 or 2.5 samples.

42 Figure 4.18 Illustration of moving-average filtering. (a) Input signal x[n] = cos(0.25πn). (b) Corresponding output of six-point moving-average filter.

43 Changing the Sampling Rate with Discrete-Time Processing resampling

44 downsampling (sampling rate) compressor

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47 Figure 4.20 Frequency-domain illustration of downsampling.

48 Figure 4.21 (a) (c) Downsampling with aliasing. (d) (f) Downsampling with prefiltering to avoid aliasing.

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52 interpolator

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55 Figure 4.24 Frequency-domain illustration of interpolation.

56 Figure 4.25 Impulse response for linear interpolation.

57 Figure 4.26 (a) Illustration of linear interpolation by filtering. (b) Frequency response of linear interpolator compared with ideal lowpass interpolation filter.

58 Figure 4.27 Illustration of interpolation involving 2K = 4 samples when L = 5.

59 Figure 4.28 Impulse responses and frequency responses for linear and cubic interpolation.

60 Figure 4.29 (a) System for changing the sampling rate by a noninteger factor. (b) Simplified system in which the decimation and interpolation filters are combined.

61 Figure 4.30 Illustration of changing the sampling rate by a noninteger factor.

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