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Example: tracker


This program produces an animated display which tracks the amplitude and phase of selected line features in the spectrum. It has a number of user-settable options which determine how the line is tracked. To run this program, type
tracker | xmgr -pipe
and an animated display will start up. In normal use, the parameters should be set as follows:

num_points: a power of two. A single phase/amplitude point is printed for each set of num_points samples.
padding_factor: a power of two. This determines the amount of padding done on the data set, and thus the ultimate frequency resolution of the line discrimination.
fpreset: your best guess for the frequency that you want to track. If the actual frequency of the spectral line differs from this value, then the phase will slowly drift as a linear function of time. (The tracker program does a robust best linear fit to this slope, and uses it to report a best frequency estimate.)
estimate: if set to zero, then the phase of the line found is always compared with the frequency preset above. If set non-zero, then tracker will make a ``best estimate" of the true frequency, and compare the phase of the line found with the phase appropriate to that sinusoid.
nbins: the number of (padded) frequency bins adjacent to the one of interest in which the line will be searched for. The frequency range covered is thus given by
\Delta f = \pm {{\tt nbins} \over \Delta t ({\tt num\_points} \times {\tt padding\_factor}+2)}
\end{displaymath} (16.29.334)

num_display: the number of points displayed by tracker. The total amount of time covered by the output is num_display $\times$ num_points where $\Delta t$ is the sample interval.
num_win,nwdt: these parameters are described in the section on multi-taper methods.
maxpass: the number of passes to make within remove_spectral_lines(). This number should be set as small as possible, provided that you still ``catch" the line of interest.

Figure: Output of the example program tracker, making use of remove_spectral_lines() to track the amplitude and phase of a selected ``spectral line" features from the (whitened) output of the Caltech 40-meter interferometer. The upper two graphs show the approximately exponential decay of the 582.396 Hz violin mode; the lower two graphs show the amplitude and phase of the third harmonic of the 60Hz line noise (note the remarkable amplitude stability).

Authors: Bruce Allen ( and Adrian Ottewill (
Comments: None.

next up previous contents
Next: Example: trackerF Up: GRASP Routines: General purpose Previous: Example: ifo_clean   Contents
Bruce Allen 2000-11-19