Digital Watermarks for copy protection in recent Billbo

[Alan Horowitz]

> However, what is somewhat less 
> well-known is the fact that in order to keep higher frequencies from being 
> "aliased" (reflected to lower frequencies by heterodyne processes) it is 
> necessary to remove (by filtering) any frequency content above that maximum, 
> before sampling is done.

   Well, fudge sticks. That sounds like this thing called an "image" in 
heterodyne analog RF receivers. I know how those work.

What is the physical basis for "aliasing" as you describe, in the 
sampling theater of operations?