Last modified 14 years ago Last modified on 02/05/2009 11:59:52 AM

Menu entry Spectrogram

Computes spectrogram of selected trace part, i.e. the trace within the drag window selected by the right mouse button, and displays it in a separate window. Each call of this menu entry opens a new display window. When working with unfiltered traces remember to remove the trace offset (Menu entry Demean) otherwise the energy at frequency zero dominates everything.

The spectrogram is computed as follows: a subwindow of N samples of the selected trace window is transformed to frequency domain using the FFT algorithm (see Numerical Recipes, Press et al., Cambridge University Press, 1988). N is determined by the configuration parameter spectrogram_width and must be a power of 2. Reasonable values are e.g. 512 or 1024.

Before FFT the input trace is tapered in time domain using a cosine taper

(2*cos(x)-1, -pi < x < pi)

The squared frequency amplitudes are displayed in a colour-coded vertical bar/line in the output window. The next subwindow for FFT is found by shifting the last one by K samples to the right. K is determined by the configuration parameter spectrogram_step. The first subwindow starts with the first sample of the selected trace part, the last subwindow ends between 0 and K-1 samples before the end of the selected trace part.

The output matrix shows frequency over time, the frequency amplitudes are indicated by colour. In the default display (blue-red-yellow), blue colours mark low amplitudes, red colours mark medium amplitudes and yellow colours mark high amplitudes.

All spectrograms are normalised independently, amplitudes cannot be compared between two figures. The frequency axis is logarithmic. When moving the cursor into the colour matrix the numerical values of the current position in time and frequency are specified in the bottom text line.

  • Plot: Redraws content of window, e.g. after changing the size of the window.
  • Pow ++: by default the colour coded amplitudes are scaled linearly. The amplitudes may be computed to the Nth power. This button increments N.
  • Pow --: see above. This button decrements N.
  • G-R: Show amplitudes in Green-Red colours.
  • B-R: Show amplitudes in Blue-Red colours.
  • B-W: Show amplitudes in Black-White colours.
  • B-Y: Show amplitudes in Blue-Red-Yellow colours (default).
  • Quit: closes spectrogram window.

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