A question about aliasing and sampling
One formula is 


  fx = |fh - N fs|


or if you prefer


  fx = abs(fh - N fs)


where N is whatever integer you need to get a result between 0 and fs.

If fh &lt; fs, then N is 0 and there's no aliasing.  If fs/2 &lt; fh &lt; fs, then N is 1 and you get "folding" of the frequency.