pro difference,indata,outdata
;Subroutine to difference (whiten) input data. Expected input is a (4,*)
;array, where (0,*) is the time data.

;===========================================================================
;INITIALIZE
array_size = size(indata)      ;check size of the array
npts       = array_size(2)     ;and specify number of data points for later use
outdata    = dblarr(2,npts)


;for i=1,npts-1 do begin
;   outdata(1,i) = (indata(1,i)-indata(1,i-1))/(indata(0,i)-indata(0,i-1))
;   outdata(2,i) = (indata(2,i)-indata(2,i-1))/(indata(0,i)-indata(0,i-1))
;   outdata(3,i) = (indata(3,i)-indata(3,i-1))/(indata(0,i)-indata(0,i-1))
   outdata(1,*) = deriv(indata(1,*))
   outdata(2,*) = deriv(indata(2,*))
   outdata(3,*) = deriv(indata(3,*))
;endfor

outdata(0,*) = indata(0,*)
;outdata(1,0) = outdata(1,1)    ;to remove initial spike
;outdata(2,0) = outdata(2,1)
;outdata(3,0) = outdata(3,1)

return
end