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We will now look at aliasing and its effect on the sampled signal. As you know, aliasing exists whenever signalfrequencies greater than Fs/2 are sampled using a sampling frequency of Fs. To eliminate aliasing, most sound cards and DSPboards have some sort of built-in analog anti-aliasing filter that removes all input signals greater than a certain frequency prior tosampling. It is important to remember that anti-aliasing filters must do the filtering prior to sampling–otherwise, the high-frequency signals would have already aliased to lowerfrequencies by the sampling process.
Some boards (and most Soundcards) have anti-aliasing filters with variable cutoff frequencies that removeall frequencies>Fs/2. As Fs changes, the cutoff frequency of the anti-aliasing filter changes as well. If a boardhas a variable frequency anti-aliasing filter that is always set to Fs/2, there will never be aliasing (at least in theory). Althoughthis type of operation is ideal from a performance point of view, it doesn’t allow the user to examine the effect of aliasing. The SPEEDY-33 does not have an anti-aliasing filter. Any frequencygreater than 24kHz can cause aliasing if the sampling frequency is set to 48kHz.
Describe what you hear and see (especially the frequency spectrum) when you sweep the sinusoid frequency from 100 Hz. to 50 kHz. Take special note of what happens to both the amplitude and frequency of the reconstructed sinusoid once you get past 24 kHz.
What happens to the amplitude of the reconstructed signal once the input signal frequency hits 24 kHz? Explain.
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