INTERACTIVE SIGNAL DEMO
You are seeing discrete samples of a periodic waveform (above) and the
absolute value of its discrete Fourier transform (DFT), obtained using
a fast Fourier transform (FFT ) algorithm (below).
In the lower plot, frequencies from 0 to 100 Hertz are displayed.
The DFT at negative frequencies is a mirror image of the DFT at positive
frequencies. The sampling rate is 200 Hertz which means the "Nyquist
frequency" is 100 Hertz. The DFT at frequencies above the Nyquist
frequency is the same as the DFT at lower (negative) frequencies.
Click and drag a point on the waveform displayed in the upper plot
to move that point to a new location, thereby setting a new fundamental
frequency and amplitude.
Use the pop-up menu in the bottom left of the figure window to change
the shape of the waveform. The possible wave shapes are sinusoidal,
square, and sawtooth.
The fundamental frequency of the waveform is given in the editable
text box in the middle of the bottom row. You can change this
fundamental frequency by clicking in the text box and editing
the number there, and then pressing RETURN . The fundamental is also
changed when the waveform is altered by clicking and dragging.
If the Signal Processing Toolbox is installed, then the menu entitled
"Window" allows you to select a window function. This window is
multiplied by the time waveform prior to taking the DFT. To display
the current window function in another figure window, select the menu
item "Show window...".