SAC Command Reference Manual

SPECTROGRAM

SUMMARY

Calculate a spectrogram using all of the data in memory.

SYNTAX

SPECTROGRAM options

where options are one or more of the following:

WINDOW v
SLICE v
ORDER n
CBAR ON | OFF
SQRT | NLOG | LOG10 | NOSCALING
YMIN v
YMAX v
METHOD PDS | MEM | MLM
COLOR | GRAY
PRINT pname

INPUT

WINDOW v:Set the sliding data window length in seconds to v. This window length determines the size of the fft.
SLICE v:Set the data slice interval in seconds to v. A single spectrogram line is produced for each slice interval.
ORDER n:Specifies the number of points in the autocorrelation function used to compute the spectral estimate.
CBAR {ON|OFF}:Turn reference color bar on or off.
{SQRT|NLOG|LOG10|NOSCALING}:
 Specify natural log, log base 10, or square root scaling of amplitudes.
YMIN v:Specifies the minimum frequency to plot.
YMAX v:Specifies the maximum frequency to plot.
METHOD {PDS|MEM|MLM}:
 Specifies the type of spectral estimator used. MLM stands for maximum likelihood and MEM stands for maximum entropy spectral estimators, respectively. See description and references below.
{COLOR|GRAY}:Specifies a color or grayscale image.
PRINT {pname}:Prints the resulting plot to the printer named in pname, or to the default printer if pname is not used. (This makes use of the SGF capability.)

DEFAULT VALUES

SPECTROGRAM WINDOW 2 SLICE 1 METHOD MEM ORDER 100 NOSCALING YMIN 0 YMAX FNYQUIST COLOR

DESCRIPTION

A spectrogram is computed by calculating power spectra of consecutive, possibly overlapping time windows of data and plotting the spectra side by side along a time axis. The spectra are calculated from a truncated autocorrelation function using either the maximum likelihood method (MLM), maximum entropy method (MEM), or Power Density Spectral method (PDS). In general, the high resolution, maximum likelihood and maximum entropy methods are preferred because they improve resolution and because they do not produce artifacts (sidelobes) in the spectra due leakage of energy between different frequencies. Descriptions of these techniques can be found in Kanasewich (1981) and Lacoss (1971) and the references therein. The length of the truncated autocorrelation function is determined by the order parameter. To maintain consistency with the spe subroutines we have set the defaults order to 200 for the power density spectra (pds) and 100 for the maximum entropy and maximum likelihood spectral estimates. In sac the length of each data window is determined by the window parameter. The spacing between spectra along the spectrograms time axis is determined by the slice parameter. The difference between these two parameters determines the amount of overlap between adjacent time window as indicated in the diagram below.:

Time --->
0     1     2     3     4     5     6     7     8     9    10    11
|.....|.....|.....|.....|.....|.....|.....|.....|.....|.....|.....|
|__^__| window 1, First time will be at the center of this window.
      |__^__| window 2
            |__^__| window 3

|.....| Slice: Difference between the start times of adjacent windows.

The start and end points on the spectrograms time axis depend on the length of the time series being analysed and the window and slice parameters. The spectrogram's start time is one-half a window later than the time series start time because it is defined as the center of time of the first window. SAC doesn't pad the start of the data with zeros.

Kanasewich, E. R., "Time Sequence Analysis in Geophysics", The University of Alberta Press, Edmonton, 1981.

Lacoss, R. T., Data Adaptive Spectral Analysis Methods", Geophysics, Vol. 36, 661-675, 1971.

LIMITATIONS

The size of the image in the frequency direction is 512.

PROBLEMS

There is currently very little error checking of the headers to make sure that they are from the same component and are contiguous in time. This will be corrected in the future.

HEADER VARIABLES

REQUIRED:DELTA
CHANGED:NPTS, DELTA, B, E, IFTYPE, DEPMIN, DEPMAX, DEPMEN
CREATED:NXSIZE, XMINIMUM, XMAXIMUM, ,BREAK NYSIZE, YMINIMUM, YMAXIMUM

LATEST REVISION

May 26, 1995 (Version 00.31)