DADiSP WebWorksheet™

Digital Aliasing Example

This WebWorksheet demonstrates the principle of digital aliasing. A sinusoid is sampled at a constant rate, defaulted to 100 Hz. The slider controls the frequency of the sinewave.

W1 displays the sampled sinewave.

W2 compares the sampled sinewave to the original continuous waveform. The samples are plotted as solid red circles.

W3 computes the Discrete Fourier Transform of the sampled sinewave using the FFT function. The transform magnitude is displayed. The location of the peak of the transform represents the measured frequency value of the sinusoid.

As long as F, the frequency of the sinewave is less than one half the sample frequency, 1/2 F_s (50 Hz), the calculated frequency compares well with the actual frequency. However, once the frequency of the sinewave exceeds this value, the measured frequency reports a value less than the actual frequency. The measured frequency aliases the actual value to a lower frequency. This result can also be seen in W2, where the sampled waveform appears as a slower sinewave than the original continuous waveform.

The frequency value 1/2 F_s is referred to as the Nyquist Frequency. In general, the sample rate of a digital system must be at least twice as high as the highest frequency produced by the system to prevent digital aliasing.