Author:Graham Sexton of Northumbria University
The applet demonstrates graphically how an undersampled sinusoidal signal appears as though it has a lower "alias" frequency.
According to the sampling theorem, a sinusoidal signal (or component of a more complex signal) can be correctly reconstructed from values sampled at discrete, uniform intervals as long as the signal frequency is less than half the sampling frequency. Any component of a sampled signal with a frequency above this limit, often referred to as the folding frequency, is subject to aliasing.
The applet is based on a fixed sampling rate of 8000 samples per second (one sample every 0.125 milliseconds). The folding frequency is thus half of 8000Hz or 4000Hz.
The input signal frequency, in Hz, is first typed into the text box to the left. With the checkbox for "Input Signal" ticked, clicking on "Plot" will show the input signal plotted against time. The checkbox labelled "Grid" swiches on and off vertical gridlines indicating the instants at which the signal is sampled. The checkbox for "Sample points" switches the display of sampled values of the input signaly. The checkbox for "Alias frequency" is only displayed when the input signal frequency is above the 4000 Hz folding frequency. This checkbox controls the plotting of a sinusoidal signal at the alias frequency - in other words, the input signal as it appears after the sampling process.
If the applet doesn't appear to work on your PC, it is possible that Java
has been disabled in the browser setting. In Internet Explorer it is in Tools,
Internet Options, Advanced tab, then scroll down to Microsoft VM and check
that Java Console is enabled.
updated 15.03.07 RA
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