You can demonstrate the effects of sampled data aliasing using the Daqarta Generator. Note that this demonstration works on the raw output data, before it is sent to the DAC. It is thus a simulation of the case where analog input data is sampled without an anti-alias filter. Or, it is what you might hear in the Generator output if there were no output filter present. (All sound cards have input and output filters.)

Toggle Spectrum mode on, and make sure that X-Axis (eXpand) is off. (If you have just done the Sine Wave Multiplication Experiment, make sure you toggle AM off as well.) To allow simpler frequency scrolling, change the frequency step to 1 kHz: Click on the Tone Freq button to bring up its dialog, then click on the button under the Main Freq control (Step Direct default) to pop up the frequency step dialog. Toggle the Hertz button, and set its step size to 1000. (You could also use Direct if you set its step to 1000.)

Now go back and scroll Main Freq up. The spectrum display shows the peak at the Main Freq value, until it goes beyond half the sample rate (the Nyquist frequency, just past the right end of the unexpanded spectrum display). After that, it seems to "bounce" off the right end of the display and move down as you continue to raise the frequency. Eventually, the response frequency will go down to zero and bounce back in the proper direction, but the indicated frequency will be lower by the amount of the sampling frequency. The bouncing continues for ever-higher test frequencies.

See also Spectrum (Fourier Transform) Theory

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