;  f channel compression algorythm 16 bits to 8 bits
;  taken from Functional Requirements GLL-4-2034, Rev A
;  written by Scott Lasica, 1/10/95

pro comp_f,data_in,data_out

; since IDL does not have a log base 2 routine, I am forced to use
; the conversion:  log base(b) X = ln X / ln b
; so: log base 2 of X is equivalent to ln X / ln 2 (where ln = natural log)

X = fix( ( alog(data_in)/alog(2) ) +1 )
; X in this case is the most significant bits ( 4-7 )

C = fix( (data_in * 2.^(5-X)) -16. )
; C in this case is the least significant bits ( 0-3 )

; now I need to reassemble these bits into a number

X = X * 16  	; this will change this number to the upper 4 bits 
		; of an 8 bit byte

data_out = byte (X + C)	; this should re-assemble these numbers into
			; an 8 bit byte
return
end