Phil Mendelsohn wrote:
      If you think about this for a couple of seconds,
you may
     decide to reformulate your opinion.  In a low-level signal,
     you say the "top" (Most Significant) bits will be zero.  In
     a high-level signal, though, the bottom 12-bits are all ones. 
</lurk>
Sorry - but this is just untrue.  A high-level signal has a larger
peak-to-peak voltage/value and hence uses more of the available bits.
The least significant bits will not be pinned at '1' as you appear to
suggest.
[Apologies if I got the quoting wrong.]
cheers
R