./guide
gfft.pro

Last modification date:

Wed Sep 28 13:27:09 2016

gfft

procedure gfft[, real_buffer][, imag_buffer], [/inverse], [bdrop=integer], [edrop=integer]

Do an FFT or an inverse FFT on the data container(s) indicated by the arguments. This always overwrites the data array in those data containers with the result.

In the case of the forward FFT (the default), if a buffer number for the imaginary part is omitted, it is assumed that the input array is a pure real array (the imaginary part is all 0 and any data at the imag_buffer location is ignored on input (and overwritten on output).

The units of the x-axis and the data are not changed here. The user needs to keep track of the state of their data containers. If a non-zero bdrop or edrop are used, the resulting data containers will be shortened by that many elements using dcextract. Consequently, it may not be appropriate to use the same bdrop and edrop on an inverse FFT as it was when the FFT was first done.

See the discussion in dcfft on how inverse is used for spectral-line data vs continuum data. For spectral-line data, an FFT is a transformation from frequency to time and an inverse FFT is a transformation in the other direction. For continuum data, an FFT is a transformation from time to frequency and an inverse FFT is a transformation in the other direction.

Examples

   getps, 34    ; get some data
   gfft         ; uses buffer 0, result in 0 (real) and 1 (imag)
   oshow, 1
   gfft, 0, 1, /inverse  ; the other direction
 

Uses

dcfft dcextract

Version

$Id$

Parameters
real_buffer in/out, optional variable (def. 0)	The global buffer number from which the real values going in to the FFT are obtained. The real values from the result are overwritten to this location. Defaults to buffer 0.
imag_buffer in/out, optional variable (def. 1)	The global buffer number from which the imaginary values going in to the FFT are obtained. The imaginary values from the result are overwritten to this location. If this parameter is omitted on the forward transformation (inverse is not set) then the input values are assumed to be all real (the imaginary part is all 0) and any data at this location is ignored and overwritten on output. Defaults to buffer 1.

Keywords
inverse in, optional boolean	When set, the inverse FFT is performed as described above.
bdrop in, optional integer (def. 0)	The number of channels to exclude from the FFT at the beginning.
edrop in, optional integer (def. 0)	The number of channels to exclude from the FFT at the end.