;+
; NAME:
;	HILBERT
;
; PURPOSE:
;	Return a series that has all periodic terms shifted by 90 degrees. 
;
; CATEGORY:
;	G2 - Correlation and regression analysis
;	A1 - Real arithmetic, number theory.
;
; CALLING SEQUENCE:
;	Result = HILBERT(X [, D])
;
; INPUT:
;	X:	A floating- or complex-valued vector containing any number 
;		of elements.
;
; OPTIONAL INPUT:
;	D:	A flag for rotation direction.  Set D to +1 for a 
;		positive rotation.  Set D to -1 for a negative rotation.
;		If D is not provided, a positive rotation results.
;
; OUTPUTS:
;	Returns the Hilbert transform of the data vector, X.  The output is 
;	a complex-valued vector with the same size as the input vector.
;
; COMMON BLOCKS:
;	None.
;
; SIDE EFFECTS:
;	HILBERT uses FFT() so this procedure exhibits the same side 
;	effects with respect to input arguments as that function.
;
; PROCEDURE:
;	A Hilbert transform is a series	that has had all periodic components 
;	phase-shifted by 90 degrees.  It has the interesting property that the 
;	correlation between a series and its own Hilbert transform is 
;	mathematically zero.
;
;	The method consists of generating the fast Fourier transform using 
;	the FFT() function and shifting the first half of the transform 
;	products by +90 degrees and the second half by -90 degrees.  The 
;	constant elements in the transform are not changed.
;
;	Angle shifting is accomplished by multiplying or dividing by the 
;	complex number, I=(0.0000, 1.0000).  The shifted vector is then
;	submitted to FFT() for transformation back to the "time" domain and the
;	output is divided by the number elements in the vector to correct for
;	multiplication effect peculiar to the FFT algorithm.  
;
; REVISION HISTORY:
;	JUNE, 1985,	Written, Leonard Kramer, IPST (U. of Maryland) on site
;			contractor to NASA(Goddard Sp. Flgt. Cntr.)
;-
	FUNCTION HILBERT,X,D   ; performs the Hilbert transform of some data.
	ON_ERROR,2           ; Return to caller if an error occurs
	Y=FFT(X,-1)	     ; go to freq. domain.
	N=N_ELEMENTS(Y)
	I=COMPLEX(0.0,1.0)
	IF N_PARAMS(X) EQ 2 THEN I=I*D
	N2=N/2-1	     ; effect of odd and even # of elements 
			     ; considered here.
	Y(1)=Y(1:N2)*I       ; multiplying by I rotates counter c.w. 90 deg.
	N2=N-N2
	Y(N2)=Y(N2:N-1)/I
	Y=FFT(Y,1)  ; go back to time domain
	RETURN,Y
END