loop subtraction

["Mike McGary" <mcgary@metronet.com>]

Just a thought....

Long ago I worked with an FFT that would show you a
graphical depiction of the current sound source (SoundForge
has this in their 4.5 version).  One of the features of the
hardware FFT was the ability to 'subtract' sounds.  If you
had a sound that had an overpowering 600hz section, you could
put it in subtract mode...play a 600hz sound....and the 
resulting sound was the original minus the 600hz sound
(kind of a follow-me EQ...)

Wouldn't it be slick to have a looping device that allowed
this?  You start a base pattern....play some parts on top of it
and build it's thickness....and then 'subtract' out the base pattern
by playing it again.  

(excuse my ignorance if there is already a device like this and everybody
already knows about it....slap me if my echoplex already does it)

            -Mike McGary