We will now look at aliasing and its effect on the sampled signal. As you know, aliasing exists whenever signal frequencies greater than Fs/2 are sampled using a sampling frequency of Fs. To eliminate aliasing, most sound cards and DSP boards have some sort of built-in analog anti-aliasing filter that removes all input signals greater than a certain frequency prior to sampling. 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 lower frequencies by the sampling process.

Some boards (and most Soundcards) have anti-aliasing filters with variable cutoff frequencies that remove all frequencies > Fs/2. As Fs changes, the cutoff frequency of the anti-aliasing filter changes as well. If a board has a variable frequency anti-aliasing filter that is always set to Fs/2, there will never be aliasing (at least in theory). Although this 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 frequency greater than 24kHz can cause aliasing if the sampling frequency is set to 48kHz.

We will now look at aliasing and the operation of the anti-aliasing filter on the DSP board. As you know, aliasing exists whenever signal frequencies greater than Fs/2 are sampled using a sampling frequency of Fs. To eliminate aliasing, most sound cards and DSP boards have some sort of built-in analog anti-aliasing filter that removes all input signals greater than a certain frequency prior to sampling. 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 lower frequencies by the sampling process.

Some boards (and most Soundcards) have anti-aliasing filters with variable cutoff frequencies that remove all frequencies > Fs/2. As Fs changes, the cutoff frequency of the anti-aliasing filter changes as well. If a board has a variable frequency anti-aliasing filter that is always set to Fs/2, there will never be aliasing (at least in theory). Although this type of operation is ideal from a performance point of view, it doesn’t allow the user to examine the effect of aliasing. Fortunately for us, our particular DSP board (or more precisely, the Audio Daughter Card attached to the DSP board) has a fixed-frequency anti-aliasing filter with a cutoff frequency of 24 kHz. If Fs = 48 kHz, this filter will prevent aliasing completely. However, if Fs is chosen to 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 allows us to examine what happens when we have aliasing. We will examine this now.

When you are done with each lab, exit LabVIEW, log off the computer, turn off the function generator and the headphone volume booster, and unplug the DSP board from the A/C adapter. Turn in Answered Questions to the TA.