4.28. Signature frame
This frame enables a group of frames, grouped with the 'Group
identification registration', to be signed. Although signatures can
reside inside the registration frame, it might be desired to store
the signature elsewhere, e.g. in watermarks. There may be more than
one 'signature frame' in a tag, but no two may be identical.
<Header for 'Signature frame', ID: "SIGN">
Group symbol $xx
Signature <binary data>
@author Eric Farng
@version $Revision: 1.4 $