One way of viewing aliasing is illustrated in the applet below for a triangular spectrum. When the Highest Frequency/Sampling Frequency is less than ½ then the green line shows the analog spectrum and also the spectrum of the recovered signal after filtering by a LPF with upper cutoff uqual to ½ the sampling frequency. When the Highest Frequency/Sampling Frequency is greater than ½ then the spectrum centered at the sampling frequency now extends to frequencies less than ½ the sampling frequency as shown by the orange curve. The shaded orange area shows the portion of the spectrum centered at the sampling frequency which gets added to the part of the spectrum centered at zero.

If there were no aliasing, then the spectrum of the recovered filtered signal would be shown by the magenta line i.e. spectrum of the analog signal cutoff at ½ the sampling frequency. In fact the spectrum of the recovered signal is shown by the red lines(shifted slightly upwards) and includes the aliased shaded orange contribution. The shaded green region shows the portion of the spectrum centered at zero which gets added to the spectrum centered at the sampling frequency.

© 1999 Cuthbert A. Nyack.