SUMMARY
Performs a discrete Fourier transform.
SYNTAX
FFT {WOMEAN|WMEAN},{RLIM|AMPH}
INPUT
WOMEAN: Remove mean before transform. WMEAN: Leave mean in transform. RLIM: Output should be in real-imaginary format. AMPH: Output should be in amplitude-phase format.
ALTERNATE FORMS
Purists may use DFT in place of FFT.
DEFAULT VALUES
FFT WMEAN AMPH
DESCRIPTION
Before the transform is performed, each data file is padded with zeros to the next power of two. SAC data files, on disk and in memory, can contain either time-series data or spectral data. The spectral data may be in either amplitude-phase format or real-imaginary format. The IFTYPE field in the header tells you which kind of data is stored in a particular file and what its format is. Most commands work on only one type, either time-series or spectal. Certain commands such as FFT, IFFT, UNWRAP, etc. change data in memory from one data type or format to another. The spectral files that result from this command can be plotted using the PLOTSP command or saved on disk with the WRITE or WRITESP command. If one has more than one data file in memory, PLOT2 can be used to plot the amplitudes or real part. The SAC sign convention is such that the phase for a causal function decreases with increasing frequency. This is the same convention as in program EVALRESP and RDSEED.
HEADER CHANGES
B, E, and DELTA are changed to the beginning, ending and sampling frequencies of the transform respectively. The original values of B, E, NPTS and DELTA are saved as SB, SE, NSNPTS and SDELTA and are restored if an inverse transform is done.
ERROR MESSAGES
- 1301: No data files read in.
- 1306: Illegal operation on unevenly spaced file
- 1307: Illegal operation on spectral file
- 1606: Maximum allowable DFT is
LATEST REVISION
May 6, 2010 (Version 101.4)