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Describe what you observe in the frequency domain when you sweep a square wave from 100 Hz. to 50 kHz. Is this expected?
We will now look at aliasing and the operation of the anti-aliasing filter on the DSP board. As youknow, aliasing exists whenever signal frequencies greater than Fs/2 are sampled using a sampling frequency of Fs. To eliminatealiasing, most sound cards and DSP boards have some sort of built-in analog anti-aliasing filter that removes all input signalsgreater than a certain frequency prior to sampling. It is important to remember that anti-aliasing filters must do the filtering priorto sampling–otherwise, the high-frequency signals would have already aliased to lower frequencies by the samplingprocess.
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 board has a variablefrequency anti-aliasing filter that is always set to Fs/2, there will never be aliasing (at least in theory). Although this type ofoperation is ideal from a performance point of view, it doesn’t allow the user to examine the effect of aliasing. Fortunately forus, our particular DSP board (or more precisely, the Audio Daughter Card attached to the DSP board) has a fixed-frequency anti-aliasingfilter with a cutoff frequency of 24 kHz. If Fs = 48 kHz, this filter will prevent aliasing completely. However, if Fs is chosento be some value<48 kHz, the anti-aliasing filter won’t remove all frequencies>Fs/2, so some aliasing will result. The fact that our anti-aliasing filter is fixed rather than variable allowsus to examine what happens when we have aliasing. We will examine this now.
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.
Explain the difference in the results of the experiment when we change the sampling frequency from 48000 Hz to 8000 Hz. Pay particular attention to what happens once the signal hits 4000 Hz. and when it hits 24000 Hz.
What happens to the amplitude of the reconstructed signal once the input signal frequency hits 24 kHz? Explain.
Describe what you observe in the frequency domain when you sweep a square wave from 100 Hz. to 50 kHz. Is this expected?
When you are done with each lab, exit LabVIEW, log off the computer, turn off the function generator andthe headphone volume booster, and unplug the DSP board from the A/C adapter. Turn in Answered Questions to the TA.
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