Ideal reconstruction system
Figure 1 consists of a sampling device which produces a time-discrete sequence xs(n). The reconstruction filter, h(t), is an ideal analog sinc filter, with h(t)=sinc(
t |
Ts |
∞ |
∑ |
n=−∞ |
Ideal system including anti-aliasing
To be sure that the reconstructed signal is free of aliasing it is customary to apply a lowpass filter, an anti-aliasing filter, before sampling as shown in Figure 2.
But if the anti-aliasing filter removes the "higher" frequencies, (which in fact is the job of the anti-aliasing filter), we will never be able to exactly reconstruct the original signal, s(t). If we sample fast enough we can reconstruct x(t), which in most cases is satisfying.
The reconstructed signal, x(t), will not have aliased frequencies. This is essential for further use of the signal.
Reconstruction with hold operation
To make our reconstruction system realizable there are many things to look into. Among them are the fact that any practical reconstruction system must input finite length pulses into the reconstruction filter. This can be accomplished by the hold operation. To alleviate the distortion caused by the hold opeator we apply the output from the hold device to a compensator. The compensation can be as accurate as we wish, this is cost and application consideration.
• Introduction; • Proof; • Illustrations; • Matlab example; • Hold operation; • Aliasing applet; • Exercises
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